Introduction to the Science of Hazards

Natural Disasters Patrick L. Abbott

N i n t h E d i t i o n

G A R R E T T , M E G A N 1 3 2 4 T S

Natural Disasters

Ninth Edition

Patrick L. Abbott San Diego State University

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G A R R E T T , M E G A N 1 3 2 4 T S

NATURAL DISASTERS: NINTH EDITION

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Abbott, Patrick L. Natural disasters / Patrick L. Abbott. -- Ninth editon. pages cm Includes index. ISBN 978-0-07-802287-6–ISBN 0-07-802287-8 1. Natural disasters. I. Title. GB5014.A24 2012 363.34–dc23 2012040469

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P o

pu la

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n

“Mankind was destined to live on the edge of perpetual disaster. We are mankind because we survive.”

—James A. Michener, 1978, Chesapeake

The world population of humans continues to increase exponentially. Photo of shopping area in New Delhi, India, by Dr. Parvinder Sethi. Photo © Dr. Parvinder Sethi RF.

LEARNING OUTCOMES The human population is growing rapidly. Natural disasters are causing great numbers of deaths and economic losses. After studying this chapter you should

• recognize the differences between a natural hazard, a natural disaster and a great natural disaster.

• be familiar with the processes that cause the deadliest natural disasters.

• understand the relationship between frequency and magnitude of natural disasters.

• know the size of the human population.

• understand the significance of exponential growth.

• recognize the demographic transition of human populations.

• be able to explain the concept of carrying capacity.

OUTLINE • Natural Disasters in 2011

• Great Natural Disasters

• Human Fatalities in Natural Disasters

• Economic Losses from Natural Disasters

• Natural Hazards

• Overview of Human Population History

• The Human Population Today

• Future World Population

• Carrying Capacity

1 CHAPTER

Natural Disasters and the Human Population

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8 Chapter 1 Natural Disasters and the Human Population

Year 2011 brought terrible disasters. Japan was hit hard. At 2:46 p.m. on Friday, 11 March 2011, a powerful earthquake broke loose 70 km (43 mi) off- shore from northeastern Japan. The seafloor thrust up- ward 5 to 8 m (16 to 26 ft), loading the seawater with energy that drove tsunami ashore beginning less than 30 minutes later. At 69 minutes after the earthquake, as people around the world watched in horror, an NHK News helicopter broadcast live video of the tsunami as a massive sheet of water rolling over the fields and through the city of Sendai, catching and overwhelming fleeing cars and their passengers. The overpowering one–two punches of a great earthquake and a massive tsunami killed 19,184 people, damaged or destroyed 330,000 buildings, ruined roads and railways ( figure 1.1 ), caused one dam to fail, triggered numerous fires, and caused nuclear-power plants to fail. The radioactivity released by the Fukushima Daichi reactors likely surpassed the level of the Chernobyl incident (see page __). Three reactors un- derwent total meltdowns. The radioactivity event was ranked at level 7, the maximum scale value on the Inter- national Nuclear Event Scale. Towns up to 80 km (50 mi) away were evacuated. The final effects remain to be seen.

The Japanese are calling this seism the Great Eastern Ja- pan Earthquake. At magnitude 9.0, it ties for fourth largest in the past 108 years (Table 4.1). The most violent earth move- ments lasted for 6 minutes. Tsunami height reached 40.5 m (133 ft) in Miyako, and travel distance extended inland 10 km (6 mi) in Sendai. Causes of deaths were tsunami (93%), build- ing collapses (4%), burns (1%), and unknown (2%).

In the United States, 2011 was the second deadliest tornado year on record, with 559 people killed. By con- trast, the rest of the world had a total of 24 tornado deaths. Outbreaks of powerful tornadoes swept across several U.S. states in mid-April, late April, and late May. Included in the hundreds of twisters were six EF5 torna- does, the maximum category, in which rotating wind speeds exceed 200 mph. EF5s cut paths of devastation through Alabama (two), Mississippi (two), Oklahoma, and Missouri. The Joplin, Missouri, tornado was the seventh deadliest individual tornado in U.S. history (f igure 1.2 ).

Natural Disasters in 2011 In 2011, there were 175 natural disasters . They were caused by earthquakes, tsunami, hurricanes (� cyclones � typhoons) , floods, tornadoes, landslides, winter storms, heat waves, and wildfires; they killed almost 29,000 people. The 14 deadliest events are listed in Table 1.1 . As horrible as the 2011 death total is, it is markedly less than in 2010, when about 286,000 people were killed in two events alone (Haiti earthquake: 230,000; Russian heat wave: 56,000). All these disasters were the result of natural processes operating at high energy levels for brief times in restricted areas.

Great Natural Disasters The Japan earthquake and tsunami in 2011, the Haiti earth- quake in 2010 and the Myanmar cyclone and China earth- quake in 2008 combined to kill almost 500,000 people. They are examples of great natural disasters: these events so overwhelm regions that international assistance is needed to rescue and care for people, clean up the destruction, and begin the process of reconstruction. Great natural disasters

Figure 1.1 Tsunami destroyed the fishing port of Kesennuma, Miyagi Prefecture, Japan. A railroad runs through the center of this image, but is covered with debris and difficult to distinguish.

Figure 1.2 Joplin, Missouri, was hit by the 7th deadliest single tornado in U.S. history on 22 May 2011. U.S. Navy sailors assist in cleaning up the devastated city. St. Johns Hospital is in the upper left.

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Human Fatalities and Economic Losses in Natural Disasters 9

1.3 ); the increase is partly due to the human population more than doubling in size since 1960. The annual occur- rence of great natural disasters ranges from zero (1952, 2009) to 15 (1993), with an average of nearly five.

Today, in earthquake-active areas of the world, several hundred million people live in buildings that will collapse during a strong earthquake. An earthquake killing more than 100,000 people could happen any day in Teheran, Iran; in Istanbul, Turkey; or in other large cities. Today, people by the millions are moving to the ocean shores, where they can be hit by tsunami, hurricanes, and floods. We need to learn how to build disaster-resistant communities to lessen the human fatalities and economic losses resulting from natural disasters.

Human Fatalities and Economic Losses in Natural Disasters The 40 deadliest disasters in the 42-year period from 1970 to 2011 are shown in table 1.2 . The most frequent mega-killers were earthquakes (25) and hurricanes (8). Notice that 26 of the 40 worst natural disasters occurred in a belt running from China and Bangladesh through India and Iran to Turkey. Nine happened in Latin America. Only three mega-killer disasters happened in western Europe, and none in the United States and Canada.

What is the correlation between human population den- sity and the number of natural-disaster deaths? The data of table 1.2 paint a clear picture: densely populated Asia domi- nates the list of fatalities. The Asian experience offers a sobering view of what may befall the global population of humans if we continue our rapid growth. Where humans are

The 14 Deadliest Natural Disasters in 2011

TABLE 1.1

Fatalities Date Event Country 19,184 11 Mar Earthquake and

tsunami Japan

1,149 16 Dec Typhoon Washi Philippines

902 11 Jan Floods and landslides

Brazil

813 27 Jul Floods Thailand

644 23 Oct Earthquake Turkey

456 1 Aug Floods Pakistan

354 22 Apr Tornadoes USA

350 3 Jun Floods China

320 13 Aug Floods Cambodia

181 22 Feb Earthquake New Zealand

178 1 Apr Floods Colombia

155 20 May Tornadoes USA

155 30 Jun Floods and landslides

Nepal

151 19 Oct Cyclone O2B Myanmar

24,992 Total deaths

2000 2005 2010

14

16

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10

8

6

4

2

0 1995199019851980197519701965196019551950

Earthquake, tsunami, volcano Storm

Flood Heat wave, drought, wildfire

Figure 1.3 Great natural disasters, 1950–2010. Source: Munich Reinsurance Company.

commonly kill thousands of people, leave hundreds of thou- sands homeless, and overwhelm the regional economy.

From 1950 to 2010, 290 great natural disasters occurred, and the yearly trend is crudely upward ( figure

Source: Data from Swiss Reinsurance Company (2012).

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10 Chapter 1 Natural Disasters and the Human Population

The 40 Deadliest Natural Disasters, 1970–March 2012

TABLE 1.2

Fatalities Date/Start Event Country 300,000 14 Nov 1970 Hurricane (Bhola) Bangladesh

255,000 28 Jul 1976 Earthquake (Tangshan) China

245,000 26 Dec 2004 Earthquake and tsunami Indonesia, Sri Lanka, India, Thailand

230,000 12 Jan 2010 Earthquake Haiti

140,000 2 May 2008 Hurricane Nargis Myanmar

140,000 29 Apr 1991 Hurricane Gorky Bangladesh

88,000 8 Oct 2005 Earthquake Pakistan

87,500 12 May 2008 Earthquake China

66,000 31 May 1970 Earthquake and debris flow (Nevados Huascaran) Peru

55,630 15 Jun 2010 Heat wave and fire Russia

50,000 21 Jun 1990 Earthquake (Gilan) Iran

35,000 Aug 2003 Heat wave Europe

27,000 26 Dec 2003 Earthquake (Bam) Iran

25,000 7 Dec 1988 Earthquake Armenia

25,000 16 Sep 1978 Earthquake (Tabas) Iran

23,000 13 Nov 1985 Volcanic eruption and mudflows (Nevado del Ruiz) Colombia

22,000 4 Feb 1976 Earthquake Guatemala

20,103 26 Jan 2001 Earthquake (Gujarat) India

19,184 11 Mar 2011 Earthquake and tsunami Japan

19,118 17 Aug 1999 Earthquake (Izmit) Turkey

18,000 15 Dec 1999 Flooding and debris flows Venezuela

15,000 19 Sep 1985 Earthquake (Mexico City) Mexico

15,000 1 Sep 1978 Flood (monsoon rains in north) India

15,000 29 Oct 1999 Hurricane (Orissa) India

11,000 22 Oct 1998 Hurricane Mitch Honduras

11,000 25 May 1985 Hurricane Bangladesh

10,800 31 Oct 1971 Flood India

10,000 20 Nov 1977 Hurricane (Andhra Pradesh) India

9,500 30 Sep 1993 Earthquake (Marashtra state) India

8,000 16 Aug 1976 Earthquake (Mindanao) Philippines

6,425 17 Jan 1995 Earthquake (Kobe) Japan

6,304 5 Nov 1991 Typhoons Thelma and Uring Philippines

5,778 21 May 2006 Earthquake Indonesia

5,422 30 Jun 1976 Earthquake (West Irian) Indonesia

5,374 10 Apr 1972 Earthquake (Fars) Iran

5,300 28 Dec 1974 Earthquake Pakistan

5,112 15 Nov 2001 Floods and debris flows Brazil

5,000 23 Dec 1972 Earthquake (Managua) Nicaragua

5,000 5 Mar 1987 Earthquake Ecuador

4,800 23 Nov 1980 Earthquake (Campagna) Italy

2,050,350 Total deaths

Source: Data after Swiss Reinsurance Company (2012).

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Natural Hazards 11

concentrated, disasters can kill many more people during each high-energy event.

THE ROLE OF GOVERNMENT IN NATURALDISASTER DEATH TOTALS As the global population of humans increases, the number of deaths by natural disasters is expected to rise, but the relationship has complexities. Analyses by Gregory van der Vink and students at Princeton University show that between 1964 and 1968, about 1 person in 10,000 was killed by a natural disaster. Between 2000 and 2004, even though the population of humans doubled, the death rate by natural disaster dropped to about 1 person in 100,000. Yet, great natural disasters still result in horrific death totals in some countries. What relationships, in addition to population size, explain the locations of great natural disasters? Van der Vink and students compared natural-disaster deaths to the levels of democracy and economic development within 133 nations with populations greater than 1 million that experi- enced five or more natural disasters between 1964 and 2004. Democracy is assessed by the World Bank’s Democ- racy Index, and economic development by gross domestic product (GDP).

The Princeton researchers state that more than 80% of deaths by natural disasters between 1964 and 2004 took place in 15 nations, including China, Bangladesh, and Indonesia. For these 15 countries, 87% are below the median democracy index and 73% are below the median GDP. The correlation between high GDP and low death totals shows exceptions in Iran and Venezuela, two oil-rich nations with significant GDP but low democracy indices. These excep- tions suggest a greater importance for democracy than GDP: the stronger the democracy index, the lower the death totals from natural disasters. The mega-killer natural disasters of the last seven years fit this trend also: Pakistan earthquake in 2005 (88,000 dead), Myanmar cyclone in 2008 (140,000 dead), China earthquake in 2008 (87,500 dead), and Haiti earthquake in 2010 (230,000 dead).

In a thought-provoking paragraph in their conclusion, van der Vink and students state: “Deaths from natural disas- ters can no longer be dismissed as random acts of nature. They are a direct and inevitable consequence of high-risk land use and the failures of government to adapt or respond to such known risks.”

HUMAN RESPONSES TO DISASTER Decades of social science research help us understand how most human beings react to natural disasters, and the news is good. Our behavior in ordinary times changes following disasters. In day-to-day life, most people are primarily con- cerned with their own needs and those of their immediate families; other relationships tend to be more superficial. After a natural disaster, many people change from inward- directed concerns to outward-directed actions. After an

initial response of shock and disbelief, our emotions of sym- pathy and empathy tend to dominate. Personal priorities may be set aside and humanitarian and community-oriented actions take over. People reach out to others; they give aid and comfort to strangers; they make great efforts to provide help. Following a natural disaster, people become better con- nected and cohesive; they experience a heightened and com- pelling desire to add to the common good.

ECONOMIC LOSSES FROM NATURAL DISASTERS The deaths and injuries caused by natural disasters grab our attention and squeeze our emotions, but in addition, there are economic losses. The destruction and disabling of buildings, bridges, roads, power-generation plants, and transmission systems for electricity, natural gas, and water, plus all the other built works of our societies, add up to a huge dollar cost. But the economic losses are greater than just damaged structures; industries and businesses are knocked out of oper- ation, causing losses in productivity and wages for employ- ees left without places to work.

In 2011, 325 catastrophic events occurred globally. The number of disasters was not unusual, but the economic losses of more than US$370 billion were the highest amount ever. The Japan earthquake losses alone came to at least US$210 billion. Insured losses were US$110 billion of the total.

Insured Portion of Economic Losses The 40 greatest disasters between 1970 and 2011 from the insurance company perspective of dollar losses are listed in table 1.3 . Notice that 39 of the 40 most expensive disasters were due to natural processes. The list of most expensive events is dominated by storms (32 of 40), whereas earth- quakes contributed seven. Compare the events on the 40 deadliest disasters list (see table 1.2 ) with table 1.3 .

The locations of the worst dollar-loss disasters for the insurance industry ( table 1.3 ) are different from the worst locations for fatalities (see table 1.2 ). The highest insurance dollar losses occurred in the United States (23 of 40), Europe (8), and Japan (5). Wealthy countries are better insured and their people live in safer buildings.

The extent of economic and insured losses may take years to become known. For example, the insured losses from the January 1994 Northridge earthquake were listed at $2.8 billion in February 1994, but they grew to $10.4 billion in January 1995 and increased to $15.3 billion in April 1998.

Natural Hazards Many sites on Earth have not had a natural disaster in recent time, but are hazardous nonetheless. Natural hazards may be assessed as the probability of a dangerous event occur- ring. For example, people migrate and build next to rivers that are likely to flood, on the shoreline of the sea awaiting a

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12 Chapter 1 Natural Disasters and the Human Population

The 40 Costliest Insurance Disasters, 1970–2011

TABLE 1.3

Losses in Millions of 2011 US$ Fatalities Date/Start Event Country

74,686 1,836 29 Aug 2005 Hurricane Katrina USA

35,000 19,184 11 Mar 2011 Earthquake and tsunami Japan

25,641 43 24 Aug 1992 Hurricane Andrew USA

23,848 2,982 11 Sep 2001 Terrorist attack USA

21,239 57 17 Jan 1994 Earthquake (Northridge) USA

21,141 136 6 Sep 2008 Hurricane Ike USA

15,350 124 2 Sep 2004 Hurricane Ivan USA

14,468 35 16 Oct 2005 Hurricane Wilma USA

12,000 813 27 Jul 2011 Floods (monsoon) Thailand

12,000 181 22 Feb 2011 Earthquake New Zealand

11,625 34 20 Sep 2005 Hurricane Rita USA

9,583 24 11 Aug 2004 Hurricane Charley USA

9,322 51 27 Sep 1991 Typhoon Mireille Japan

8,292 71 15 Sep 1989 Hurricane Hugo USA

8,248 562 27 Feb 2010 Earthquake Chile

8,036 95 25 Jan 1990 Winter Storm Daria Europe

7,830 110 25 Dec 1999 Winter Storm Lothar Europe

7,300 354 22 Apr 2011 Tornadoes (Alabama) USA

7,050 155 20 May 2011 Tornadoes (Missouri) USA

6,609 54 18 Jan 2007 Winter Storm Kyrill Europe

6,135 22 15 Oct 1987 Storm Europe

6,127 38 26 Aug 2004 Hurricane Frances USA

6,060 63 17 Oct 1989 Earthquake (Loma Prieta) USA

5,491 64 26 Feb 1990 Winter Storm Vivian Europe

5,454 26 22 Sep 1999 Typhoon Bart Japan

5,300 55 22 Aug 2011 Hurricane Irene USA

5,155 — 4 Sep 2010 Earthquake New Zealand

4,870 600 20 Sep 1998 Hurricane Georges USA, Caribbean

4,577 41 5 Jun 2001 Tropical Storm Allison USA

4,527 3,034 13 Sep 2004 Hurricane Jeanne USA, Haiti

4,268 45 6 Sep 2004 Typhoon Songda Japan

4,095 135 26 Aug 2008 Hurricane Gustav USA

3,918 45 2 May 2003 Tornadoes USA

3,810 70 10 Sep 1999 Hurricane Floyd USA, Bahamas

3,697 59 4 Oct 1995 Hurricane Opal USA

3,648 6,425 17 Jan 1995 Earthquake (Kobe) Japan

3,418 25 24 Jan 2009 Winter Storm Klaus France, Spain

3,240 45 27 Dec 1999 Winter Storm Martin Spain, France

3,055 246 10 Mar 1993 Storm (East Coast) USA

2,886 38 6 Aug 2002 Floods Europe

$428,999 Billion 38,793 Total deaths

Source: Data after Swiss Reinsurance Company (2012).

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Natural Hazards 13

powerful storm, and on the slopes of volcanoes that will eventually erupt. Decades, or even centuries, may pass with no great disasters, but the hazard remains.

Sites with natural hazards must be studied and under- stood. Their risks must be evaluated. Then we can try to prevent natural hazards from causing natural disasters. Remember: Natural hazards are inevitable, but natural disasters are not .

In the process of mitigation, we make plans and take actions to eliminate or reduce the threat of future death and destruction when natural hazards suddenly become great threats. The mitigating actions taken to protect us may be engineering, physical, social, or political.

Another need for mitigation occurs after great disasters, because people around the world tend to reoccupy the same site after a disastrous event is done. Earthquakes knock cities down, and then the survivors may use the same bricks and stones to rebuild on the same site. Floods and hurricanes inun- date towns, but people return to refurbish and again inhabit the same buildings. Volcanic eruptions pour huge volumes of magma and rock debris onto the land, burying cities and kill- ing thousands of people, yet survivors and new arrivals build new towns and cities on top of their buried ancestors. Why do people return to a devastated site and rebuild? What are their thoughts and plans for the future? For a case history of a natu- ral hazard, let’s visit Popocatépetl in Mexico.

POPOCATÉPETL VOLCANO, MEXICO Popocatépetl is a 5,452 m (17,883 ft) high volcano that lies between the huge populations of Mexico City (largest city in Mexico) and Puebla (fourth largest city in Mexico) ( figure 1.4 ). The volcano has had numerous small eruptions over thousands of years; thus its Nahuatl name, Popocatépetl, or Popo as it is affectionately called, means smoking mountain. But some- times Popo blasts forth with huge eruptions that destroy cities and alter the course of civilizations. Around the year 822 ce (common era), Popo’s large eruptions buried sig nificant cities. Even its smaller eruptions have affected the course of human affairs. In 1519, Popo was in an eruptive sequence as Hernán Cortéz and about 500 Spanish conquistadors marched west- ward toward Tenochtitlan, the Aztec capital city. The supersti- tious Aztec priest-king Montezuma interpreted the eruptions as omens, and they affected his thinking on how to deal with the invasion.

Popocatépetl has helped change the path of history, but what is the situation now? Today, about 100,000 people live at the base of the volcano; they have been attracted by the rich volcanic soil, lots of sunshine, and fairly reliable rains. Millions more people live in the danger zone extending 40 km (25 mi) away. The Nahuatl people consider Popo to be divine—a living, breathing being. In their ancient religion, God, rain, and volcano are intertwined. Most do not fear the volcano; rather, they believe that God decides events and that with faith, things will work out. Thus, good opportunities for farming, coupled with faith and fatalism, bring people back.

Volcanic activity on Popo resumed on 21 December 1994 with eruptions of ash and gases. The sequence of inter- mittent eruptions continues today. How do we evaluate this hazard? Is this just one of the common multiyear sequences of small eruptions that gave the volcano its name? Or are these little eruptions the forewarnings of a giant killing erup- tion that will soon blast forth? We cannot answer these ques- tions for sure. How would you handle the situation? Would you order the evacuation of 100,000 people to protect them, and in so doing, have them abandon their homes, sell their livestock, and leave their independent way of life for an unknown length of time that could be several years? Or would you explain the consequences of an unlikely but pos- sible large eruption and let them decide whether to stay or go? If they decide to stay and then die during a huge volcanic blast, would this be your fault?

It is relatively easy to identify natural hazards, but as the Popocatépetl case history shows, it is not easy to decide how to answer the questions presented by this volcanic hazard.

Figure 1.4 Popocatépetl in eruption on 19 December 2000. The cathedral was built by the Spanish on top of the great pyramid at Cholula, an important religious site in a large city that was mostly buried by an eruption around 822 CE . Photo © Wesley Bocxe/The Image Works.

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14 Chapter 1 Natural Disasters and the Human Population

Source: US Geological Survey Fact Sheet (unnumbered).

We are faced with the same types of questions again and again, for earthquakes, landslides, tornadoes, hurricanes, floods, and fire.

MAGNITUDE, FREQUENCY, AND RETURN PERIOD Earth is not a quiet and stable body. Our planet is dynamic, with major flows of energy. Every day, Earth experiences earthquakes, volcanic eruptions, landslides, storms, floods, fires, meteorite impacts, and extinctions. These energy- fueled events are common, but their magnitudes vary mark- edly over space and time.

Natural hazards and disasters are not spaced evenly about Earth. Some areas experience gigantic earthquakes and some areas are hit by powerful hurricanes; some are hit by both, while other areas receive neither.

During a period of several years or even several decades, a given area may experience no natural disasters. But given enough time, powerful, high-energy events will occur in every area. It is the concentrated pulses of energy that concern us here, for they are the cause of natural disasters—but how fre- quent are the big ones? In general, there is an inverse correla- tion between the frequency and the magnitude of a process. The frequent occurrences are low in magnitude, involving little energy in each event. As the magnitude of an event increases, its frequency of occurrence decreases. For all hazards, small- scale activity is common, but big events are rarer. For example, clouds and rain are common, hurricanes are uncommon; streams overflow frequently, large floods are infrequent.

Another way of understanding how frequently the truly large events occur is to match a given magnitude event with its return period, or recurrence interval, which is the num- ber of years between same-sized events. In general, the larger and more energetic the event, the longer the return period.

A U.S. Geological Survey mathematical analysis of natural-disaster fatalities in the United States assesses the likeliness of killer events. Table 1.4 shows the probabilities of 10- and 1,000-fatality events for earthquakes, hurricanes, floods, and tornadoes for 1-, 10-, and 20-year intervals, and estimates the return times for these killer events. On a yearly basis, most low-fatality events are due to floods and torna- does, and their return times are brief, less than one year. High-fatality events are dominantly hurricanes and earth- quakes, and their return times for mega-killer events are much shorter than for floods and tornadoes.

Knowing the magnitude, frequency, and return period for a given event in a given area provides useful information, but it does not answer all our questions. There are still the cost- benefit ratios of economics to consider. For example, given an area with a natural hazard that puts forth a dangerous pulse of energy with a return period of about 600 years, how much money should you spend constructing a building that will be used about 50 years before being torn down and replaced? Will your building be affected by a once-in-600-year disas- trous event during its 50 years? Should you spend the added

money necessary to guarantee that your building will with- stand the rare destructive event? Or do economic consider- ations suggest that your building be constructed to the same standards as similar buildings in nearby nonhazardous areas?

ROLE OF POPULATION GROWTH The world experiences significant numbers of great natural disasters and increasing economic losses from these events. The losses of life and dollars are occurring at the same time the global population of humans is increasing ( figure 1.5 ). Population

Probability Estimates for 10- and 1,000-Death Natural Disasters in the United States

TABLE 1.4

Likeliness of a 10-Fatality Event

During 1 Year

During 10 Years

During 20 Years

Return Time

(in years) Earthquake 11% 67% 89% 9

Hurricane 39 99 � 99 2

Flood 86 �99 �99 0.5

Tornado 96 �99 �99 0.3

Likeliness of a 1,000-Fatality Event

During 1 Year

During 10 Years

During 20 Years

Return Time

(in years) Earthquake 1% 14% 26% 67

Hurricane 6 46 71 16

Flood 0.4 4 8 250

Tornado 0.6 6 11 167

Figure 1.5 The number of people on Earth continues to grow rapidly. Photo courtesy of Pat Abbott.

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Overview of Human Population 15

growth places increasing numbers of people in hazardous set- tings. They live and farm on the slopes of active volcanoes, build homes and industries in the lowlands of river floodplains, and move to hurricane-prone coastlines. How have the numbers of people grown so large? The present situation can best be appre- ciated by examining the record of population history.

Overview of Human Population The most difficult part of human history to assess is the begin- ning, because there are no historic documents and the fossil record is scanty. In 2003, modern human fossils discovered in Ethiopia were dated as 160,000 years old. Our species appears to have began in Africa about 200,000 years ago. The rate of population growth and the number of people alive early in human history were so small that they cannot be plotted accu- rately on the scale of figure 1.6 . The growth from a few thou- sand people 160,000 years ago to more than 7 billion people in the year 2011 did not occur in a steadily increasing, linear fashion. The growth rate is exponential.

THE POWER OF AN EXPONENT ON GROWTH The most stunning aspect of figure 1.6 is the peculiar shape of the human population curve; it is nearly flat for most of human time and then abruptly becomes nearly vertical. The marked upswing in the curve shows the result of exponential growth of the human population. Possibly the least appreciated

concept of present times is what a growth-rate exponent does to the size of a population over time. Exponential growth moves continuously in ever-increasing increments; it leads to shockingly large numbers in surprisingly short times. Probably our most familiar example of exponential growth occurs when interest is paid on money.

It can be difficult to visualize the results of exponential growth when it is expressed only as a percentage over time, such as the very small growth rate of the human population in 160,000 years or as 7% interest on your money for 50 years. It is easier to think of exponential growth in terms of doubling time—the number of years required for a popu- lation to double in size given an annual percentage growth rate. A simple formula, commonly called the rule of 70, allows approximation of doubling times:

70 Doubling time (in years) �

% growth rate/year

Learning to visualize annual percentage growth rates in doubling times is useful whether you are growing your money in investments or spending it by paying interest on debts (especially at the high rates found with credit-card debt). Table 1.5 shows how interest rates affect how quickly your money will grow.

THE PAST 10,000 YEARS OF HUMAN HISTORY The long, nearly flat portion of the population curve in f igure 1.6 certainly masks a number of small-scale trends, both upward and downward. The fossil record is not rich

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Years before present Today

8

7

6

5

4

3

2

1

160,000 80,000120,000 10,00040,000

Figure 1.6 Human population growth since its start about 160,000 years ago.

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16 Chapter 1 Natural Disasters and the Human Population

and water supplies, the world population grew at faster rates. From about 1 ce to 1750, world population grew to about 800 million. Growth occurred at an average rate of 0.056% per year, meaning that another 560 people were added per million per year.

Throughout the history of the human race, high rates of birth were required to offset high rates of infant mortality and thus maintain a viable-sized human population. The 18th century saw many of the intellectual advances that set the stage for the present phase of cultural change. At long last, the causes of many diseases were being recognized, and the principles of public health were being established. Advances in the medical world greatly improved the odds for the sur- vival of individual humans through their reproductive years. No longer were many mothers and great numbers of children dying during childbirth and infancy.

The 18th century saw death rates drop dramatically, but birth rates remained high and population doubling times dropped dramatically; thus population size soared. About 1804, the human population reached 1 billion; by 1922, it had grown to 2 billion; in 1959, it reached 3 billion; by 1974, it was 4 billion; by early 1987, it was 5 billion; in 1999, it reached 6 billion; it passed 7 billion in October 2011 ( figure 1.9 ). Notice the continuing decline in the number of years it takes for a net gain of another 1 billion people on Earth.

The 20th-century growth of the human population is unprecedented and breathtaking. The number of humans doubled twice—from about 1.5 billion to 3 billion and again to more than 6 billion. The increased population used 16 times more energy, increased industrial output 40 times, used 7 times more water, caught 35 times more fish, and expanded the cattle population to 1.4 billion. The effect of exponential growth is racing ahead. In his book Wealth of Nations, published in 1776, Adam Smith said, “Men, like all other animals, naturally multiply in proportion to the means of their subsistence.”

THE HUMAN POPULATION TODAY At present, the world population is growing at about 1.2% per year for a doubling time of 58 years ( Table 1.6 ). The 1.2% gain is a net figure derived by measuring the birth rate ( fertility rate) and subtracting the death rate ( mortality rate). Even after subtracting all the human lives lost each year to accidents, diseases, wars, and epidemics such as AIDS, the human population still grows by more than 80 million people per year. Each year, the world population increases by about the total population of Germany.

The net growth of the human population can be grasped by viewing it on short timescales ( figure 1.10 ). There is a net addition of 2.6 people every second, a rate comparable to a full jetliner landing a load of new people every minute. The monthly net growth of people is greater than the population of Massachusetts.

Doubling Times at Some Common Percentage Rates

TABLE 1.5

Growth Rate (% per year)

Doubling Time (years)

0.02 3,500

0.5 140

1 70

1.2 58

2 35

5 14

7 10

10 7

17 4

Time

P op

ul at

io n

Harsh weather Scarce food Disease

Good weather Abundant food

Figure 1.7 Good weather and plentiful food cause upsurges in population; bad weather, disease, and scarce food cause downswings in population.

enough to plot a detailed record, but surely at times when weather was pleasant and food from plants and animals was abundant, the human population must have risen ( figure 1.7 ). Conversely, when weather was harsh, food was scarce, and diseases were rampant, the human population must have fallen.

The nearly flat population growth curve began to rise about 8,000 years ago, when agriculture became established and numerous species of animals were domesticated. The world population is estimated to have been about 8 million people by 10,000 years ago. After the development of agri- culture and the taming of animals removed much of the hard- ship from human existence, the population growth rate is likely to have increased to 0.036% per year, yielding a net gain of 360 people per million per year. This increased rate of population growth probably caused the human population to reach 200 million people by 2,000 years ago.

As humans continued to improve their ability to modify the environment with better shelter and more reliable food

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Overview of Human Population 17

P op

ul at

io n

(b ill

io ns

)

10

9

8

7

6

5

4

3

2

1

0 1800 1850 1900

Year

1804

118 years

1922

1959

1974

1987

1999

2011

2023

2039

37 years

15 years

13 years

12 years

12 years

12 years

16 years

1950 2000 2050

Figure 1.9 Growth of the world population of humans. Notice how the time to add another billion people has decreased to date but is projected to start increasing in the future. Source: US Census Bureau.

Interest Paid on Money: An Example of Exponential Growth Compare the growth of money in different situations ( figure 1.8 ). If $1,000 is stashed away and another $100 is added to it each year, a linear growth process is in operation. Many of the processes around us can be described as linear, such as the growth of our hair or fingernails.

If, in contrast, another $1,000 is stashed away but this time earns interest at 7% per year and the interest is allowed to accu- mulate, then an exponential growth-rate condition exists. Not only does the $1,000 earn interest, but the interest from prior years remains to earn its own interest in compound fashion.

Notice that an exponential growth curve has a pronounced upswing, or J shape. A comparison of the linear and exponential curves in figure 1.8 shows that they are fairly similar in their early years, but as time goes on, they become remarkably different. The personal lesson here is to invest money now. Smaller amounts of money invested during one’s youth will become far more important than larger amounts of money invested later in life. Individuals who are disciplined enough to delay some gratifica- tion and invest money while they are young will be wealthy in their later years. Albert Einstein described compound interest, the exponential growth of money, as one of the most powerful forces in the world.

Here is a riddle that illustrates the incredible rate of exponential growth; it shows the significance of doubling times in the later stages of a system. Suppose you own a pond and add a beautiful water lily plant that doubles in size each day. If the lily is allowed to

grow unchecked, it will cover the pond in 30 days and choke out all other life-forms. During the first several days, the lily plant seems small, so you decide not to worry about cutting it back until it cov- ers half the pond. On what day will that be?

Side Note

D ol

la rs

Years

32,000

30,000

25,000

20,000

15,000

10,000

5,000

1,000

0 10 20 30 40 50

Hide $1,000 in basement

Depos it $1,00

0;

add $1 00 eac

h year

Deposit $1,000, one time, at 7% interest

Figure 1.8 Amounts of money versus time. Compound interest (exponential growth) produces truly remarkable sums if given enough time.

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18 Chapter 1 Natural Disasters and the Human Population

TABLE 1.6

World Population Data, Mid-2012

Population (millions)

Birth Rate (per 1,000)

Death Rate (per 1,000)

Yearly Growth %

Doubling Time (in years)

Projected Population in 2050 (millions)

World 7,058 20 8 1.2 58 9,624

More-developed countries

1,243 11 10 0.1 700 1,338

Less-developed countries

5,814 22 8 1.4 50 8,286

Least-developed countries *

876 35 10 2.4 29 1,899

Africa 1,072 36 11 2.5 28 2,339

Asia 4,260 18 7 1.1 64 5,284

Europe 740 11 11 0 — 732

Northern America 349 13 8 0.5 140 471

Latin America 599 19 6 1.3 54 740

Oceania 37 18 7 1.1 64 57

†Subset of less-developed countries Source: World Population Data Sheet (2012).

Per minute 158 people

In 2009, world population grew:

A Boeing 737 airplane

United States and Mexico 417 million people

Per year 83 million people

Germany 82 million people

Per second 2.6 people Per day

227,030 people

Two extra-large sports stadiums

Per month 6,906,000 people

Massachusetts 6.45 million people

Projected over the next 5 years: 415 million people

Figure 1.10 Growth of world population over differing lengths of time. Source: Modified from US Census Bureau.

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Future World Population 19

Future World Population Today, most of the more-developed countries have gone through demographic transitions; they have gone from high death rates and high birth rates to low death rates and low birth rates. But many less-developed countries have low to moderate death rates and high birth rates; will they go through demographic transitions? In demographic transition theory, both mortality and fertility decline from high to low levels because of economic and social development. Yet even without significant economic development, Population Ref- erence Bureau estimates of the rates of world population growth are dropping: from 1.8% in 1990, to 1.6% in 1997, to 1.4% in 2000, and to 1.2% in 2010. What is causing this decrease in fertility? It appears to be due largely to urbaniza- tion and increased opportunities for women. At the begin- ning of the 20th century, less than 5% of people in less-developed countries lived in cities, but by the year 2010, the majority of people were living in urban areas ( table 1.7 ). This is a change from farmer parents wanting many children to work in the fields and create surplus food, to city parents wanting fewer children to feed, clothe, and educate. Urban women have greater access to education, health care, higher incomes, and family-planning materials. When presented with choices, many women choose to have fewer children and to bear them later. Both of these choices lower the rate of population growth.

In the last 50 years of the 20th century, population grew from about 2.5 billion to over 6 billion, an increase of 3.5 billion people. Even with the recent decreases in fertility rates, the population explosion is not over. A growth rate of

1.2% per year will cause the world population of humans to approach 9.5 billion by the year 2050 (see table 1.6 ), an increase of another 3.5 billion people within 51 years. Popu- lation growth is not evenly distributed around the world. In general, wealthy countries have low or even negative rates of population growth. Many poor nations have high rates of population growth ( figure 1.11 ).

An important factor in estimating future growth is the age distribution of the population ( table 1.7 ). A significant percentage of the population today is less than 15 years old, meaning their prime years for childbearing lie ahead. The century from 1950 to 2050 will see the world population grow from 2.5 billion to about 9.6 billion people.

The number of births per woman has a dramatic effect on human population growth. Starting in the year 2000 with a world population in excess of 6 billion people, look at three scenarios for population size in the year 2150 based on births per woman: (1) if women average 1.6 children, world popu- lation drops to 3.6 billion; (2) if women average 2 children, population grows to 10.8 billion; (3) if women average 2.6 children, population grows to 27 billion. The difference between a world population of 3.6 billion or 27 billion rests on a difference of only one child per woman.

DEMOGRAPHIC TRANSITION The demographic transition model is based on the popula- tion experiences of economically wealthy countries in the past few centuries. Up through the 17th century, a woman had to bear several children to have a few survive to adult- hood and replace the prior generation. Births had to be

TABLE 1.7 Data Influencing Future Population, Mid-2012

Percent of Population of Age

Average Number of Children Born

per Woman

Percent Urban (cities

>2,000 people)

Percent of Married Women Using Modern Contraception <15 65+

World 26 8 2.4 51 56

More-developed countries

16 16 1.6 75 63

Less-developed countries

29 6 2.6 46 54

Least-developed countries

41 3 4.4 28 27

Africa 41 3 4.7 39 26

Asia 25 7 2.2 45 59

Europe 16 16 1.6 71 62

Northern America 19 13 1.9 79 73

Latin America 28 7 2.2 78 67

Oceania 24 11 2.5 66 60

Source: World Population Data Sheet (2012).

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20 Chapter 1 Natural Disasters and the Human Population

2050

China India Rest of

Asia

Near East

Sub- Saharan

Africa

E as

te rn

E ur

op e

Ja pa

n W

es te

rn E

ur op

e N

or th

A m

er ic

a

N or

th A

fr ic

a

La tin

A m

er ic

a

9.62 billion

2002 6.22 billion

1950 2.56 billion people

Figure 1.11 World population by region: 1950, 2002, 2050. Source: US Census Bureau.

numerous to compensate for the high rates of infant mortal- ity. Beginning in the 18th century, discoveries in public health, medicine, and immunization caused the death rate to drop dramatically. During this time, birth rates stayed high, so overall population grew rapidly. As time passed and peo- ple realized that most of their children would survive to adulthood, birth rates dropped and population stabilized at a new and higher level.

The demographic transition takes place in phases:

1. Before the transition: high death rates are offset by high birth rates to maintain a population.

2. During the transition: low death rates coupled with con- tinuing high birth rates cause population to soar

3. After the transition: low death rates combine with low birth rates to achieve a stable population at a signifi- cantly higher level.

Today the transition is taking place at different rates in different countries ( figure 1.12 ). Most of the population growth is occurring in the poorest areas of the poorest coun- tries. Some of the wealthiest countries now have more deaths than births each year.

URBANIZATION AND EARTHQUAKE FATALITIES During the past 500 years, global earthquakes killed about 5 million people. Average numbers of deaths were about 1 million per century, or 100,000 per decade. These simple averages are misleading because they hide the effects of the deadliest earthquakes, such as the 250,000 people killed by the Tangshan, China, event in 1976. The mega-killer earth- quakes of the past 500 years occurred in China, Indonesia,

Very early

Uganda (46:12) Afghanistan (44:16) Nigeria (41:16)

Iraq (35:6) Guatemala (30:6) Ghana (31:8)

Gabon (27:9) India (23:7) Malaysia (21:5)

China (12:7) Japan (8:9) Germany (8:10)

Early Late After

Demographic transition

H ig

h Lo

w

D eath rate

Birth rate

Figure 1.12 Demographic transition. Today, the shifts in birth rates and death rates vary markedly between countries. Birth and death rates are both expressed in number of people per 1,000 each year. For example, Uganda has 46 births and 12 deaths per 1,000 people each year (46:12). Data from Population Reference Bureau.

Pakistan, Iran, Turkey, Italy, Japan, and Haiti—and they may occur there again.

An analysis of the past 500 years by Roger Bilham shows that, with an average population of about 1.5 billion people, there was one earthquake that killed nearly a million people. But with population becoming five times larger at 7.5 billion people about the year 2018, million-death earth- quakes may occur five times as frequently, or about one per century. Most of the human population growth, by birth and by migration, is occurring in cities in less-developed coun- tries. Many of these people are living in poorly constructed

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Future World Population 21

A Classic Disaster: Influenza (FLU) Pandemic of 1918 In July 1914, a major war, eventually known as World War I, broke out in Europe. The countries and empires involved contained more than half the people in the world. When the war ceased in November 1918, almost 7 million soldiers had been killed in battle, along with about 1 million civilians.

As bad as 8 million war deaths sounds, a far more deadly natu- ral disaster began during that time: the influenza pandemic of 1918–1919. The flu pandemic killed about 50 million people; this was 3% of the world’s population. Estimates of total deaths range up to 100 million people. The influenza migrated around the world in waves. In the United Kingdom, the first wave arrived in the spring of 1918. In the fall of 1918, a longer-lasting, deadlier wave of flu swept the world, followed in 1919 by yet a third wave. Most flu victims were healthy young adults rather than the more typical elderly or juvenile victims of influenza ( figure 1.13 ).

World War I did not cause the flu, but the global movements of millions of troops, weakened by stress and battle, increased the spread and deadly effects of the virus . Another 3 million soldiers died, not from World War I battles, but from influenza. In 1918, children skipped rope to this rhyme:

I had a little bird Its name was Enza I opened the window And in-flu-enza.

Side Note

Figure 1.13 A typical scene during the 1918 flu pandemic. The Oakland Municipal Auditorium was used as a temporary hospital, allowing volunteer nurses to tend to the sick. Photo by Edward A. “Doc” Rogers. From The Joseph R. Knowland Collection at the Oakland History Room, Courtesy Oakland Public Library.

buildings in mega-cities. Million-death earthquakes are pos- sible in a growing number of mega-cities.

DISEASE PANDEMICS Throughout recorded history, deadly diseases have swept throughout the world, killing millions of people in pandem- ics. For example, the bacterium Yersinia pestis, transmitted to humans by fleas, caused the bubonic plague—the Black Death that killed about 75 million people in Europe in the 14th century.

Viruses have also caused pandemics via smallpox, HIV, polio, influenza, and other diseases. For example, in 1918– 1919, the influenza virus A (H1N1) spread around the world, killing about 50 million people. With the human population now exceeding 7 billion people, with more than 50% of people now living in cities, and with the rapid movement of people worldwide via jet airplanes, the potential exists for a new pandemic disease.

Viruses Viruses are life in the simplest form. They are genetic mate- rial (DNA or RNA) coated by fat and protein. A virus might have only 4 genes, whereas a bacterium might have 4,000 genes, and a human 24,000 genes. Viruses cannot reproduce by themselves; they must invade a host cell and cause the host to reproduce the virus.

Viruses infect many forms of life, including animals, plants, and even bacteria. The same viruses commonly exist in humans, pigs, and birds, and move easily between them. Because humans commonly live and interact with birds and pigs, the transfer of viruses between them is especially likely. When two different viruses enter a single cell, their genes can form new combinations, creating a new type of virus. On the surface of a virus are molecules shaped into unique configu- rations that might match a living cell and allow entry, much like a unique key will open a specific lock.

Influenza A Viruses Influenza A viruses cause recurrent epidemics and pandemics, as in 1918–1919. Type A viruses examined on the basis of their haemagglutinin (HA) and neuraminidase (NA) molecules are divided into 16 HA sub- types (H1 to H16) and 9 NA subtypes (N1 to N9). In 2005, researchers reported the results of a study of the 1918–1919 influenza virus collected from samples preserved from World War I flu-victim soldiers and from historic individuals buried in Arctic permafrost (frozen soil). The 1918–1919 influenza was type A (H1N1), a subtype with an early history in birds.

Early in 2009, a flu epidemic broke out near La Gloria in the state of Veracruz, Mexico. By 23 April 2009, 23,000 cases had been reported. By 7 May 2009, the flu had spread to become a pandemic, with cases identified in 21 countries on five continents. Laboratory analyses showed

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22 Chapter 1 Natural Disasters and the Human Population

that this new virus was type A (H1N1) and was made up of genes from four different flu viruses: from North American pigs (30.6%), Eurasian pigs (17.5%), North American birds (34.4%), and humans (17.5%). People were worried. Could this virus evolve into as big a killer as the one in 1918–1919?

Analysis of H1N1 deaths in 2009 from 214 countries showed 44,100 deaths—a significant total, but far less severe than in 1918. Like the 1918 influenza, though, most of the deaths occurred in young people; 73% of deaths were people 29 years old and younger. The death percentages by age groups include:

• 37% were 10 to 19 years old • 22% were less than 9 years old • 14% were 20 to 29 years old

In the reverse of a typical flu year, people 60 years old and older suffered only 3% of the deaths. If one views the 2009 H1N1 figures as deaths only, then it was not as bad a year as had been feared. But if one considers the number of years of life lost by the young victims, then the 2009 pandemic would be more equivalent to 250,000 deaths in a typical flu year.

CARRYING CAPACITY How many people can Earth support? At this time, the ques- tion is unanswerable. Nonetheless, many people worry about dangers resulting from the unprecedented growth of the human population, such as more and greater natural disasters, increased global warming, decreasing supplies of fresh water, depletion of fossil fuels, increased pollution, increased desertification, and the increased rate of extinction of spe- cies. Other people see no big problems and point out that humans have already increased the carrying capacity of Earth for us via agriculture, water storage and purifi- cation, and advances in public health; they feel that any upcoming problems will be solved just like oth- ers have been in the past.

In the natural world, biologists studying carrying capacity of the environment for indi- vidual species of mammals, birds, frogs, and other animals find that population size is regu- lated by the resources available. For example, when a resource such as available food increases, a feeding population grows in size. If that food resource decreases due to drought, disease, or other causes, the population dependent on that food dies back and decreases in size.

Ireland in the 1840s Ireland in the 1840s provides a human example of carrying capacity. The European explorers of the 1500s brought the potato back from South America. The potato is a highly nutri- tious food. A diet of potatoes, milk (from animals fed potatoes), and greens constitute a nutritionally complete diet. An acre of potatoes could feed an Irish family of six for a year. In Ireland, the potato was the wonder crop that allowed a child-loving

population to grow explosively. By 1841, Ireland’s population had grown well past 8 million, with nearly half the people surviving wholely or mostly on potatoes. In 1845, heavy spring rains aided growth and spread of a fungal infestation, the potato blight, which caused potatoes to rot during storage. But when the potatoes rotted there was no substitute food. Malnourish- ment became common. Then the winter of 1846–1847 hit with unusual severity, causing weakened people to suffer even more. The toll was severe: a million people died from disease, and another 1.5 million people emigrated. During their travel to the United States and Canada, 1 in 7 emigrés died.

The carrying capacity of Irish land increased for humans when the potato arrived. Potato plants covered the lands, even extending into bogs and up steep mountain slopes. The human population fed by the increased food supply grew rapidly. But when the potato supply dropped suddenly, so did the human population.

Easter Island (Rapa Nui) Easter Island (Rapa Nui) is a triangular-shaped, volcanic mass with an area of about 165 km 2 (64 mi 2 ). It lies over 2,000 km (more than 1,200 mi) east of Pitcairn Island and over 3,700 km (more than 2,200 mi) west of Chile ( figure 1.14 ). Easter Island is isolated; it has high tempera- tures and humidity, poorly drained and marginal soils, no permanent streams, no terrestrial mammals, about 30 native

Hawaiian Islands

40°

40°

60° 160° 140° 120° 100° 80°

20°

20°

0°

Kiritimati Island (Christmas Island)

Marquesas Islands

Equator

Pacific Ocean

Ocean Galápagos Islands

Easter Island (Rapa Nui) 3,701 km

(2,300 mi) C hi

le

TahitiSociety

Islands

Pitcairn Island

Chatham Islands

Samoa Islands

Tokelau Islands

Kermadec Islands

Tonga Islands

P O

L Y

N E

S

I A

2,052 km (1,275 mi)

Figure 1.14 Easter Island (Rapa Nui) is an isolated outpost of Polynesian civilization nearly lost in the vast Pacific Ocean.

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Future World Population 23

Figure 1.15 Rapa Nui inhabitants spent much of their energy creating giant statues (moai). Photo © Adalberto Rios Szalay/Sexto Sol/Getty Images RF.

plant species including trees in locally dense growths, and few varieties of fish in the surrounding sea. Year-round water is available only in little lakes within the volcano caldera.

About 1,000 years ago, seafaring Polynesian people arrived on Rapa Nui with 25 to 50 settlers. They were part of the great Polynesian expansion outward from southeast Asia that led them to discover and inhabit islands from Hawaii in the north to New Zealand in the southwest and to Rapa Nui in the southeast. The wide-ranging voyagers colo- nized islands over a Pacific Ocean area more than twice the size of the United States.

The colonizers of Rapa Nui brought chickens and rats, along with several of their food plants. The climate was too severe for most of their plants except the yam. Their resulting diet was based on easily grown chickens and yams, and hous- ing was fashioned using wood from native trees; the people had lots of free time.

The islanders used their free time to develop a complex social system divided into clans that practiced elaborate ritu- als and ceremonies. Their customs included competition between the clans in shaping and erecting mammoth statues. The statues were carved out of volcanic rock using obsidian (volcanic glass) tools. The statues (moai) were more than 6 m (20 ft) high, weighed about 15 tons apiece, and were erected on ceremonial platforms (ahu) ( figure 1.15 ).

The peak of the civilization occurred about 1550 ce , when the human population had risen to about 7,000; statues numbered more than 600, with half as many more being

shaped in the quarries. But from its peak, the civilization declined rapidly and savagely, as first witnessed by the crew of a Dutch ship on Easter Sunday, 5 April 1722. The Europeans found about 2,000 people living in caves in a primitive society engaged in almost constant warfare and practicing cannibalism. What caused this cultural collapse? It appears that human activities so overwhelmed the environ- ment that it was no longer able to support the greatly enlarged human population. The customs of society dissolved in the fight of individuals and clans to survive.

Carving the giant statues had not been particularly dif- ficult, but transporting them was physically and environmen- tally strenuous. Trees were cut down and placed under statues as rollers. Islanders pushed the heavy statues from the quarry and levered them onto their ceremonial platforms. The com- petition between clans to create the most statues helped destroy the forests. Without trees, houses could not be built, and people had to move to caves. There was little fuel for cooking or to ward off the chill of colder times. Soil erosion increased and agricultural production dropped. Without trees, there were no canoes, and so islanders caught fewer fish. Without canoes, there was no escape from the remote and isolated island. As food resources declined, the social system collapsed, and the statue-based religion disintegrated. Clans were reduced to warfare and cannibalism in the strug- gle for food and survival.

Competition between clans was so consuming that they did not consider the health of the environment and thus paid a price: the human population on the island collapsed. Easter Island is one of the most remote inhabited areas on Earth, a tiny island virtually lost in the vast Pacific Ocean. When problems set in faster than the Rapa Nui customs could solve them, there was no place to turn for help, no place to escape. The carrying capacity of the land had been exceeded, and the human popula- tion suffered terribly. What lesson does Easter Island have for the whole world? Earth is but a tiny island lost in the vast ocean of the universe ( figure 1.16 ); there is no realistic chance of the human population escaping to another hospitable planet.

Figure 1.16 Earth is an isolated outpost nearly lost in the vast “ocean” of the universe. Photo courtesy of Pat Abbott.

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24 Chapter 1 Natural Disasters and the Human Population

Terms to Remember carrying capacity 22 CE 13 cyclone 8 demographic transition 19 earthquake 8 energy 8 epidemic 21 exponential growth 15 fertility 16 frequency 14 great natural disaster 8 hurricane 8 influenza 21

The Easter Island example raises interesting philo- sophical questions. If climate change decreases global food production, causing the human population to exceed Earth’s

Great natural disasters killed almost 500,000 people in four recent events: 2011 Japan earthquake and tsunami, 2010 Haiti earthquake, 2008 hurricane in Myanmar and earth- quake in China. Over time, the two deadliest events are tropical storms (hurricanes) and earthquakes. In 2011, the known economic losses from natural disasters were about US$375 billion. The long-term trend is for economic losses to increase.

Natural hazards exist in areas of obvious danger, such as cities built on the slopes of active volcanoes or on the flood- plains of rivers. For these sites, it is only a matter of time before the hazard is realized as a disaster. At any one site, the greater the magnitude of a disaster, the less frequently it occurs. Large disasters have longer return periods.

The curve describing the history of human population growth is flat to gently inclined for 160,000 years, and then it rises rapidly in the last three centuries. In the past, women bore numerous children, but many died, so overall popula- tion growth was slow. With the arrival of the scientific- medical revolution and the implementation of the principles of public health, the human population has soared. Birth rates remain high in much of the world, even though death rates have plummeted. The population reached 1 billion in about 1804, 2 billion in 1922, 3 billion in 1959, 4 billion in 1974, 5 billion in 1987, 6 billion in 1999, and it passed 7 bil- lion in 2011.

A steeply rising growth curve is exponential; in terms of population, more people beget ever more people. One way to visualize exponential growth is by using doubling time, the length of time needed for a population to double in size. Doubling times can be approximated by the rule of 70:

70 Doubling time (in years) �

% growth rate/year

At present, after subtracting deaths from births, world population increases 1.2% per year for a doubling time of 58 years.

Much hope is placed in the demographic transition model, which holds that economic wealth, combined with knowing that one’s children will survive, leads to dramatic drops in birth rates. This model holds for some more- developed countries. Now some less-developed countries are experiencing drops in birth rates, presumably due to urban- ization and more choices for women. Even at lower rates of growth, human population is projected to exceed 9 billion in

the year 2039. The rapid growth in human population sets the stage for mega-death earthquakes and hurricanes.

New flu viruses are commonly created where people live closely with birds and pigs. These new viruses which have the potential to kill millions of people, can rapidly spread around the world.

Carrying capacity is an estimate of how many individu- als of a species the environment can support. How many people can Earth support? The answer is not known, but it is the subject of much debate.

Summary

Questions for Review 1. What types of natural disasters killed the most people in the

past 40 years? Where in the world are deaths from natural disasters the highest? Where in the world are insurance losses from natural disasters the highest?

2. What is a great natural disaster? 3. What is the difference between a natural disaster and a natural

hazard? How do economic losses differ from insured losses? 4. For nations, what is the relationship between natural-disaster

deaths, gross domestic product, and level of democracy? 5. What is the relationship between the magnitude of a given

disaster and its frequency of occurrence? 6. Draw a curve showing the world population of humans in the

past 100,000 years. Why has the curve changed shape so dramatically?

7. Explain the concept of exponential growth. 8. What is the size of the world population of humans today?

Extrapolating the current growth rate, what will the population be in 100 years? In 200 years? Are these large numbers environmentally realistic?

carrying capacity, could human value systems change fast enough to solve the problem? If all the people on Earth had to face the Easter Island situation, how would we fare?

magnitude 14 mitigation 13 mortality 16 natural disaster 8 natural hazard 11 pandemic 21 return period 14 tornado 8 tsunami 8 typhoon 8 virus 21 volcano 13

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Questions for Further Thought 25

9. What are the population doubling times given these annual growth rates: Africa, 2.4%; world, 1.2%?

10. For nations, what are demographic transitions? 11. How much time does it take for a flu pandemic to infect

people all around the world? 12. What is the relationship between earthquake fatalities and

cities? 13. Explain the concept of carrying capacity for a species. What

processes might limit the numbers of a species?

Questions for Further Thought 1. Would we call a large earthquake or major volcanic eruption

a natural disaster if no humans were killed or buildings destroyed?

2. Which single disaster could kill the most people—a flu pandemic, an earthquake, or a hurricane?

3. Could global building designs be made disaster-proof, thus reducing the large number of fatalities?

4. What is the carrying capacity of Earth for humans—that is, how many humans can Earth support? What factors are most likely to slow human population growth?

5. Compare the rate of change of human populations to the rate of change in religious and cultural institutions. Can religious and cultural institutions change fast enough to deal with world population growth?

6. Evaluate the suggestion that the overpopulation problem on Earth can be solved by colonizing other planets.

7. Is a nation’s destiny determined by its demographics?

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CHAPTER Internal Energy and Plate Tectonics 2

In te

r n

al E

n er

g y

Such superficial parts of the globe seemed to me unlikely to happen if the Earth were solid to the centre. I therefore imagined that the internal parts might be a fluid more dense, and of greater specific gravity than any of the solids we are acquainted with; which therefore might swim in or upon that fluid. Thus the surface of the globe would be a shell, capable of being broken and disordered by the violent movements of the fluid on which it rested.

—Benjamin Franklin , 1780

LEARNING OUTCOMES Internal energy has caused the Earth to differentiate into layers. Throughout the Earth, materials move vertically and horizontally. After studying this chapter you should

• know the layering of the Earth and how it formed.

• be familiar with the sources of energy inside the Earth.

• understand the behavior of materials.

• be able to explain how plate tectonics operates.

• comprehend Earth’s magnetic field and the evidence it provides for plate tectonics.

• know the age of the Earth and how it is determined.

• appreciate the thought processes used to understand the Earth.

OUTLINE • Origin of the Sun and Planets

• Earth History

• The Layered Earth

• Internal Sources of Energy

• Plate Tectonics

• The Grand Unifying Theory

• How We Understand Earth

Satellite view of Arabia moving northeast away from Africa. Photo from NOAA.

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Origin of the Sun and Planets 27

together and grew in size, resulting in greater gravitational attraction to nearby particles and thus more collisions. As matter drew inward and the size of the cloud decreased, the speed of rotation increased and the mass began flattening into a disk ( Figure 2.1 b ). The greatest accumulation of matter occurred in the center of the disk, building toward today’s Sun ( Figure 2.1 c ). The two main constituents of the Sun are the lightweight elements hydrogen (H) and helium (He). As the central mass grew larger, its internal temperature increased to about 1,000,000 degrees centigrade (°C), or 1,800,000 degrees Fahrenheit (°F), and the process of nuclear fusion began. In nuclear fusion, the smaller hydrogen atoms com- bine (fuse) to form helium, with some mass converted to energy. We Earthlings feel this energy as solar radiation (sunshine).

The remaining rings of matter in the revolving Solar System formed into large bodies as particles continued col- liding and fusing together to create the planets ( Figure 2.1 d ). Late-stage impacts between ever-larger objects would have been powerful enough to melt large volumes of rock, with some volatile elements escaping into space. The inner plan- ets (Mercury, Venus, Earth, Mars) formed so close to the Sun that solar radiation drove away most of their volatile gases and easily vaporized liquids, leaving behind rocky planets. The next four planets outward (Jupiter, Saturn, Uranus, Neptune) are giant icy bodies of hydrogen, helium, and other frozen materials from the beginning of the Solar System.

At 3 a.m., on 12 April 1977, a 1.4 kg (3 lb) meteor-ite tore through the roof of a parked car in the city of Chambery in the French Alps. Friction created while speeding through the atmosphere made the exterior of the meteorite so hot that it set the car on fire. The car’s owner was awakened by the impact and fire but refused to believe it was caused by a meteorite. He filed an arson complaint with the police. But sometimes things are ex- actly what they seem to be—the fire was caused by a meteorite.

Origin of the Sun and Planets Impacts of material are not rare and insignificant events in the history of our Solar System; they probably were respon- sible for its formation. The most widely accepted hypothesis of the origin of the Solar System was stated by the German philosopher Immanuel Kant in 1755. He thought the Solar System had formed by growth of the Sun and planets through collisions of matter within a rotating cloud of gas and dust.

The early stage of growth began within a rotating spheri- cal cloud of gas, ice, dust, and other solid debris Figure 2.1 ( a ). Gravity acting upon matter within the cloud attracted parti- cles, bringing them closer together. Small particles stuck

(a) (b)

(c) (d)

Figure 2.1 Hypothesis of the origin of the Solar System. (a) Initially, a huge, rotating spherical cloud of ice, gas, and other debris forms. (b) The spinning mass contracts into a flattened disk with most of its mass in the center. (c) Planets grow as masses collide and stick together. (d) The ignited Sun is surrounded by planets. Earth is the third planet from the Sun.

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28 Chapter 2 Internal Energy and Plate Tectonics

enlarged, the gravitational force may have pulled more of the metallic pieces toward the center, while some of the lighter- weight materials may have concentrated near the exterior. Nevertheless, Earth in its infancy probably grew from ran- dom collisions of debris that formed a more or less homoge- neous mixture of materials.

But Earth did not remain homogeneous. The very pro- cesses of planet formation ( Figure 2.2 ) created tremendous quantities of heat, which fundamentally changed the young planet. The heat that transformed Earth came primarily from (1) impact energy, (2) decay of radioactive isotopes, (3) gravitational energy, and (4) differentiation into layers ( Figure 2.2 ).

As the internal temperature of Earth rose beyond 1,000°C (1,800°F), it passed the melting points of iron at various depths below the surface. Iron forms about one-third of Earth’s mass, and although it is much denser than ordinary rock, it melts at a much lower temperature. The buildup of heat caused immense masses of iron-rich meteorites to melt. The high- density liquid iron was pulled by gravity toward Earth’s cen- ter. As these gigantic volumes of liquid iron moved inward to form Earth’s core, they released a tremendous amount of gravitational energy that converted to heat and probably raised Earth’s internal temperature by another 2,000°C (3,600°F). The release of this massive amount of heat would have pro- duced widespread melting likely to have caused low-density materials to rise and form: (1) a primitive crust of low-density rocks at the surface of Earth; (2) large oceans; and (3) a denser atmosphere. The formation of the iron-rich core was a unique event in the history of Earth. The planet was changed from a somewhat homogeneous ball into a density-stratified mass with the denser materials in the center and progressively less- dense materials outward to the atmosphere.

The low-density materials (magmas, waters, and gases), freed by the melting, rose and accumulated on Earth’s exterior

IMPACT ORIGIN OF THE MOON Large impacts can generate enough heat to vaporize and melt rock; they can produce amazing results. For example, the dominant hypothesis on the origin of Earth’s Moon involves an early impact of the young Earth with a Mars-size body, a mass about 10 times larger than the Moon. The resultant impact generated a massive cloud of dust and vapor, part of which condensed and accumulated to form the Moon. This theory suggests that the Moon is made mostly from Earth’s rocky mantle. The theory accounts for the lesser abundance of iron on the Moon (iron on Earth is mostly in the central core ) and the Moon’s near lack of lightweight materials (such as gases and water), which would have been lost to space.

Earth History To understand the origin and structure of Earth, we must know the flows of energy throughout the history of our planet. Studying early history is difficult because Earth is a dynamic planet; it recycles its rocks and thus removes much of the record of its early history. The older the rocks, the more time and opportunities there have been for their destruc- tion. Nonetheless, the remaining early Earth rocks, along with our growing knowledge of the processes in Earth’s interior and in the Solar System, allow us to build an increas- ingly sophisticated approximation of early Earth history.

Earth appears to have begun as an aggregating mass of particles and gases from a rotating cloud about 4.6 billion years ago. During a 30- to 100-million-year period, bits and pieces of metal-rich particles (similar to iron-rich meteor- ites), rocks (similar to stony meteorites), and ices (composed of water, carbon dioxide, and other compounds), accumu- lated to form Earth. As the ball of coalescing particles

Impact of asteroids and comets

Decay of radioactive isotopes

Gravitational energy

Differentiation into layers

Figure 2.2 Heat-generating processes during the formative years of Earth include (1) impact energy, (2) decay of radioactive isotopes, and (3) gravitational energy. Increasing heat caused Earth to differentiate into layers.

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The Layered Earth 29

as continents, oceans, and atmosphere. It seems that oceans and small continents existed by 4.4 billion years ago, life probably was present as photosynthetic bacteria 3.5 billion years ago, large continents were present at least 2.5 billion years ago, and the outer layers of Earth were active in the process of plate tectonics by at least 1.5 billion years ago.

The Layered Earth Earth today is differentiated into layers of varying densities. As we have noted, much of the densest material was pulled toward the center, and some of the least dense substances escaped to the surface ( Figure 2.3 ). At the center of Earth is a dense, iron- rich core measuring about 7,000 km (4,350 mi) in diameter. The inner core is a solid mass 2,450 km (1,520 mi) in diameter with temperatures up to 4,300°C (7,770°F). The outer core is mostly liquid, and the viscous movements of convection cur- rents within it are responsible for generating Earth’s magnetic field. The entire iron-rich core is roughly analogous in compo- sition to a melted mass of metallic meteorites.

Surrounding the core is a rocky mantle nearly 2,900 km (1,800 mi) thick, with a composition similar to that of stony meteorites. The mantle comprises 83% of Earth’s volume and 67% of its mass. The rocks of the mantle can be approximated by melting a stony meteorite in the laboratory; this produces a separation in which an upper froth rich in low-density ele- ments rises above a residue of denser minerals/elements. The low-density material is similar to continental crust that by

melting and separation has risen above the uppermost mantle. All the years of heat flow toward Earth’s surface have “sweated out” many low-density elements to form a conti- nental crust. Today, the continents make up only 0.1% of Earth’s volume. Floating above the rocky layers of Earth are the oceans and the atmosphere.

Earth’s layering can be described as based on either (1) different strengths or (2) different densities due to vary- ing chemical and mineral compositions ( Figure 2.3 ). Both temperature and pressure increase continuously from Earth’s surface to the core, yet their effects on materials are differ- ent. Increasing temperature causes rock to expand in volume and become less dense and more capable of flowing under pressure and in response to gravity. Increasing pressure causes rock to decrease in volume and become more dense and more rigid. Visualize tar at Earth’s surface. On a cold day, it is solid and brittle, but on a hot day, it can flow as a viscous fluid. Similar sorts of changes in physical behavior mark different layers of Earth. In fact, from a perspective of geological disasters, the crust-mantle boundary is not as important as the boundary between the rigid lithosphere (from the Greek word lithos, meaning “rock”) and the “soft plastic” asthenosphere (from the Greek word asthenes, meaning “weak”) ( Figure 2.4 ). The mesosphere, the mantle below the asthenosphere (see Figure 2.3 ), is solid; it is a “stiff plastic,” but it is not brittle like the lithosphere. The differences in strength and mechanical behavior between solid, “plastic,” and fluid states are partly responsible for earthquakes and volcanoes.

Asthenosphere (“soft plastic”)

Mesosphere (“stiff plastic”)

Lithosphere (solid)

Hydrosphere (liquid)

Atmosphere (gas)

Density (grams/cm3)

1.03 (Ocean)

Crust (light color, low-density rock)Mantle

(dark color, heavy rock)

2.7 (Continent)

3.3 3.6

4.3

5.7

9.7

11.8

14 Core

(metallic)

6,371 km

Outer core (liquid)

Inner core

(solid)

100 km

350 km

16

2,89 0 km

5, 15

0 km

Figure 2.3 Density stratification within Earth—that is, lower-density materials float atop higher-density materials. Pressure and temperature both increase from the surface to the center of Earth. Layers illustrated on the left show the differences in physical properties and strengths. Layers on the right emphasize different mineral and chemical compositions.

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30 Chapter 2 Internal Energy and Plate Tectonics

describe behavior at an instant in time—but how do the sub- stances behave when viewed over a longer timescale? Spe- cifically, some solids yield to long-term pressure such that at any given moment, they are solid, yet internally they are deforming and flowing—that is, behaving as a fluid. A famil- iar example is the ice in a glacier. When a glacier is hit with a rock hammer, solid chunks of brittle ice break off. Yet, inside the glacier, atoms are changing positions within the

BEHAVIOR OF MATERIALS The concepts of gas, liquid, and solid are familiar. Gases and liquids are both fluids, but a gas is capable of indefinite expansion, while a liquid is a substance that flows readily and has a definite volume but no definite shape. A solid is firm; it offers resistance to pressure and does not easily change shape. What is not stated but is implicit in these definitions is the effect of time. All of these definitions

Mother Earth The history of the 4.6-billion-year-old Earth has been metaphorically contrasted with the life history of a 46-year-old woman by Nigel Calder in his book The Restless Earth. In this metaphor, each of “Mother” Earth’s years equals 100 million years of geologic time. The first seven of her years are mostly lost to the biographer. Like human memory, the early rock record on Earth is distorted; it emphasizes the more recent events in both number and clarity. Most of what we know of “Mother” Earth happened in the past six years of her life. Her continents had little

life until she was 42. Flowering plants did not appear until her 45th year. Her pet dinosaurs died out eight months ago. In the middle of last week, some ancestors of present apes evolved into human ancestors. Yesterday, modern humans ( Homo sapiens ) evolved and began hunting other animals, and in the last hour, humans discovered agriculture and settled down. Fifteen minutes ago, Moses led his people to safety; 5 minutes later, Jesus was preaching along the same fault line; and after another minute, Muhammad taught in the same region. In the last minute, the Industrial Revolution began, and the number of humans increased enormously.

Side Note

0 (Sea level)

20

40

60

80

100

120

Continental crust Crust

Atmosphere (gas)

Mantle

Asthenosphere (weak solid)

Lithosphere (strong solid)

Hydrosphere (liquid)

Ocean

Oceanic crust

D ep

th (

km )

Figure 2.4 Upper layers of Earth may be recognized (1) compositionally, as lower-density crust separated from the underlying higher-density mantle, or (2) on the basis of strength, as rigid lithosphere riding atop “soft plastic” asthenosphere. Notice that the lithosphere includes both the crust and the uppermost mantle.

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The Layered Earth 31

reached and permanent strain occurs. Most rocks are brittle at the low temperatures and low pressures at Earth’s surface. Most rocks are ductile at the high temperatures and high pres- sures at depth inside Earth. In the asthenosphere, rock deforms in a “soft plastic” fashion. Most of the deeper mantle rock is solid but not brittle; it behaves as a “stiff plastic”—it deforms.

The top of the asthenosphere comes to the surface at the ocean’s volcanic mountain chains but lies more than 100 km (about 60 mi) below the surface in other areas. It has gradational upper and lower boundaries and is about 250 km (155 mi) thick. What are the effects of having this “soft plastic” ductile zone so near Earth’s exterior? Within the asthenosphere, there is a lot of flowage of rock that helps cause Earth’s surface to rise and fall. For example, Earth is commonly described as a sphere, but it is not. Earth may be more properly described as an oblate ellipsoid that is flattened at the poles (nearly 30 km, or 19 mi) and bulged at the equator (nearly 15 km, or 9 mi). Earth is neither solid enough nor even strong enough to spin and maintain a spherical shape. Rather, Earth deforms its shape in response to the spin force. The flattening of Earth during rotation is analogous to the flattening of the early Solar System from a sphere to a disk (see Figure 2.1 ).

ISOSTASY From a broad perspective, Earth is not a homogeneous, solid ball but rather a series of floating layers where less dense materials successively rest upon layers of more dense materi- als. The core, with densities up to 16 gm/cm3, supports the mantle, with densities ranging from 5.7 to 3.3 gm/cm3. A top the denser mantle float the continents, with densities around 2.7 gm/cm3, which in turn support the salty oceans, with densities of about 1.03 gm/cm3, and then the least dense layer of them all—the atmosphere. The concept of floating layers holds true on smaller scales as well. For example, the oceans are made of layered masses of water of differing densities. Very cold, dense Antarctic waters flow along the ocean bot- toms and are overlain by cold Arctic water, which is overlain by extra-salty waters, which in turn are overlain by warmer,

ice and dominantly moving to downhill positions of lower stress. At no instant in time does the glacier fit our everyday concept of a liquid, yet over time, the glacier is flowing downhill as an ultra-high-viscosity fluid.

When materials are subjected to sufficient stress, or force, they deform or undergo strain in different ways ( figure 2.5 ). Stress may produce elastic (or recoverable) deformation, as when you pull on a spring. The spring deforms while you pull or stress it, but when you let go, it recovers and returns to its original shape.

If greater stress is applied for a longer time or at higher temperatures, ductile (or plastic) deformation may occur, and the change is permanent. You can visualize this with a wad of chewing gum or Silly Putty. If you squeeze them in your hands, they deform. Set them down and they stay in the deformed shape; this is ductile deformation. Another exam- ple occurs deep within glaciers where the ice deforms and moves with ductile flow.

If stress is applied rapidly to a material, it may abruptly fracture or break into pieces, called brittle deformation. Take a chunk of ice from your refrigerator and drop it or hit it; it will shatter with brittle failure. Notice that the ice in a glacier exhibits both brittle and ductile behavior. Near the surface, there is little pressure on the rigid ice and it abruptly fractures when stressed. Deep within the glacier, where the weight of overlying ice creates a lot of pressure, the ice deforms and moves by ductile flow. The style of ice behavior depends on the amount of pressure confining it.

The type of mechanical behavior illustrated by ice deep within a glacier typifies that of the rock within the Earth’s mantle. This rock is plastic in the sense used by William James in his 1890 Principles of Psychology. He defined plas- tic as “possession of a structure weak enough to yield to an influence, but strong enough not to yield all at once.”

When a material such as rock is subjected to the same large amounts of stress on all sides, it compresses. When stresses coming from different directions vary, strain can occur. When the differences in stress are low, strain is elastic and reversible. As stress differences increase, the yield stress is

Volcanoes and the Origin of the Ocean, Atmosphere, and Life The elements in volcanic gases are predominantly hydrogen (H), oxygen (O), carbon (C), sulfur (S), chlorine (Cl), and nitrogen (N). These gaseous elements combine at Earth’s surface to make water (H 2 O), carbon dioxide (CO 2 ), sulfur dioxide (SO 2 ), hydrogen sulfide (H 2 S) with its rotten egg smell, carbon monoxide (CO), nitrogen (N 2 ), hydrogen (H 2 ), hydrochloric acid (HCl), methane (CH 4 ), and numerous other gases. The dominant volcanic gas is water vapor; it commonly makes up more than 90% of total gases.

Side Note The elements of volcanic gases (C, H, O, N, S, Cl) differ from

the elements of volcanic rocks: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K). The elements of volcanic gases make up the oceans, the atmosphere, and life on Earth, but they are rare in rocks. The 4.5 billion years of heat flow from Earth’s interior have “sweated” out many lightweight elements and brought them to the surface via volcanism. Billions of years of volcanism on Earth go a long way toward explaining the origin of the continents, the oceans, the present atmosphere, and the surface concentration of the CHON elements (carbon, hydrogen, oxygen, nitrogen) of which all life on Earth is composed and on which it depends.

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32 Chapter 2 Internal Energy and Plate Tectonics

thick “root” beneath it (see figure 2.4 ). Visualize a boat float- ing in water: Add a load onto the boat and it sinks downward; remove the load and the boat rises upward. So it is with a continent. Add a load onto the land, such as a large glacial ice mass, and the land will sink downward as rock flows outward at depth in the asthenosphere; remove the load (the ice melts), and the land rises or rebounds upward as rock flows inward in the asthenosphere ( figure 2.6 ). An example of this buoy- ancy effect, or isostatic equilibrium, was defined by carefully surveying the landscape before and after the construction of Hoover Dam across the Colorado River east of Las Vegas, Nevada. On 1 February 1935, the impoundment of Lake Mead began. By 1941, about 24 million acre feet of water had been detained, placing a weight of 40,000 million tons over an area of 232 square miles. Although this is an impres- sive reservoir on a human scale, what effect can you imagine it having on the whole Earth? In fact, during the 15 years from 1935 to 1950, the central region beneath the reservoir sank up to 175 mm (7 in) ( figure 2.7 ). The relatively simple act of impounding water behind the dam triggered an isostatic adjustment as asthenosphere rock flowed away from the pres- sure of the overlying reservoir, causing the area to subside.

Just how solid and firm is the surface of the earth we live on? Larger-scale examples are provided by the great ice sheets of the recent geologic past. The continental glacier that buried the Finland-Sweden region was up to 3 km (2 mi)

less dense seawater. Earth is composed basically, from core through atmosphere, of density-stratified layers.

The concept of isostasy was developed in the 19th cen- tury. It applies a principle of buoyancy to explain how the low-density continents and mountain ranges literally float on the denser mantle below. Just as an iceberg juts up out of the ocean while most of its floating mass is beneath sea level, so does a floating continent jut upward at the same time it has a

(a) Elastic – recovers 1

2

3

1

2

3

1

2

(b) Ductile – deforms

(c) Brittle – breaks

Figure 2.5 Behavior of materials. (a) Elastic: bend a thin board; let it go and the board recovers its original shape. (b) Ductile: squeeze a wad of bubblegum or Silly Putty; let it go and the mass stays in the deformed shape. (c) Brittle: bend a thin board sharply and it breaks.

Ice

Rock Rock flows outward

(a)

Ice

Atmosphere

Land Si

nks

Land rebounds

Rock flows inward

(b)

Figure 2.6 Isostatic equilibrium. (a) Land sinks as weight of ice causes rock at depth to flow outward. (b) Land rebounds as ice melts and removes weight, causing rock at depth to flow inward.

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Internal Sources of Energy 33

mass on Earth. The surface of Earth clearly is in a delicate vertical balance. Do major adjustments and movements also occur horizontally? Yes, there are horizontal movements between lithosphere and asthenosphere, which will bring us into the realm of plate tectonics (described later in this chapter).

Internal Sources of Energy The flow of energy from Earth’s interior to its surface comes mainly from three sources: impact energy, gravitational energy, and the ongoing decay of radioactive isotopes.

IMPACT ENERGY AND GRAVITATIONAL ENERGY The impact energy of masses colliding with the growing Earth produced heat. Tremendous numbers of large and small asteroids, meteorites, and comets hit the early Earth, their energy of motion being converted to heat on impact.

Gravitational energy was released as Earth pulled into an increasingly dense mass during its first tens of millions of years. The ever-deeper burial of material within the growing mass of Earth caused an increasingly greater gravitational pull that further compacted the interior. This gravitational energy was converted to heat.

The immense amount of heat generated during the for- mation of Earth did not readily escape because rock conducts

thick less than 20,000 years ago. The land was depressed beneath this great weight. By 10,000 years ago, the ice sheet had retreated and melted, and the water returned to the ocean. The long-depressed landmass, now freed from its heavy load, is rebounding upward via isostatic adjustment. In the past 10,000 years, northeastern coastal Sweden and western Finland have risen about 200 m (650 ft). This upward move- ment was vividly shown during excavation for a building foundation in Stockholm, Sweden. Workers uncovered a Viking ship that had sunk in the harbor and been buried with mud. The ship had been lifted above sea level, encased in its mud shroud, as the harbor area rose during the ongoing iso- static rebound. Gravity measurements of this region show a negative anomaly, indicating that another 200 m (650 ft) of isostatic uplift is yet to come. The uplift will add to the land of Sweden and Finland and reduce the size of the Gulf of Bothnia between them.

Some of the early uplifting of land after ice-sheet removal occurred in rapid movements that ruptured the ground sur- face, generating powerful earthquakes. In northern Sweden, there are ground ruptures up to 160 km (100 mi) long with parallel cliffs up to 15 m (50 ft) high. The rocks in the region are ancient and rigid, suggesting that ruptures may go 40 km (25 mi) deep and that they generated truly large earthquakes.

Vertical movements of the rigid lithosphere floating on the flexible asthenosphere are well documented. If we add a load on the surface of Earth, we can measure the downward movement. For example, Antarctica is buried beneath ice up to 4,470 m (2.8 mi) thick. A 100-meter-thick ice mass will cause the land to sink about 27.5 m (90 ft). Thus, Antarctica is depressed up to 1,230 m (4,000 ft), placing most of the conti- nent below sea level ( figure 2.8 ). If the ice is removed, Antarctica will slowly rise up and become the fifth largest land

Lake Mead

Hoover Dam

U S 93

US 95

Boulder City

Las Vegas

0 15 30 mi

0 25

N

50 km

Pa ci

fic

U ni

on

–120

US 91

+20

–20

–40

–40–60–80 –100

�100

–120

–100

–140 –160

–17 0

0

R R

Figure 2.7 Isostatic downwarping caused by the weight of Lake Mead, from 1935 to 1950. Black circular lines (contour lines) define the depressed land surface. In the center is a –170 line where land sank 170 mm (7 in). Source: Smith, W. O., et al., Comprehensive Survey of Sedimentation in Lake Mead, 1948–49, in US Geological Survey Professional Paper 295, 1960. Figure 2.8 Antarctica with its ice cover removed. Green

and yellow areas are above sea level; blue areas are below sea level. When isostatic rebound finishes, the blue area will be above sea level and Antarctica will be the fifth largest land mass on Earth.

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34 Chapter 2 Internal Energy and Plate Tectonics

4 0

2

4

6

8

10

3 2 1 0 Billions of years ago

10 20

c al

or ie

s/ ye

ar

Figure 2.9 The rate of heat production from decay of radioactive atoms has declined throughout the history of Earth.

Increasing time measured in half-lives

P er

ce nt

ag e

of p

ar en

t a to

m s

re m

ai ni

ng

100

50

25

12.5

6.25 3.125 1.5625

0 1 2 3 4 5 6 7

Figure 2.10 Negative exponential curve showing decay of radioactive parent atoms to stable daughter atoms over time. Each half-life witnesses the disintegration of half the remaining radioactive parent atoms.

TABLE 2.1

Some Radioactive Isotopes in Earth

Parent Decay Product Half-Life

(billion years) Aluminum-26 Magnesium-26 0.00072 (720,000

years)

Uranium-235 Lead-207 0.71

Potassium-40 Argon-40 1.3

Uranium-238 Lead-206 4.5

Thorium-232 Lead-208 14

Rubidium-87 Strontium-87 47

Samarium-147 Neodymium-147 106

radioactive isotopes and a much greater heat production from them than it does now ( figure 2.9 ). With a declining output of radioactive heat inside the Earth, the flow of energy from Earth’s interior is on a slow decline heading toward zero.

The radioactive-decay process is measured by the half-life, which is the length of time needed for half the present number of atoms of a radioactive isotope (parent) to disintegrate to a decay (daughter) product. As the curve in figure 2.10 shows, during the first half-life, one-half of the atoms of the radioactive isotopes decay. During the second half-life, one-half of the

heat very slowly. Some of this early heat is still flowing to the surface today.

RADIOACTIVE ISOTOPES Energy is released from radioactive isotopes as they decay. Radioactive isotopes are unstable and must kick out sub- atomic particles to attain stability. As radioactive isotopes decay, heat is released.

In the beginning of Earth, there were abundant, short- lived radioactive isotopes, such as aluminum-26, that are now effectively extinct, as well as long-lived radioactive isotopes, many of which have now expended much of their energy ( table 2.1 ). Young Earth had a much larger complement of

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Radioactive Isotopes Each chemical element has a unique number of positively charged protons that define it. However, the number of neutrons varies, giving rise to different forms of the same element, known as iso- topes. Some isotopes are radioactive and release energy during their decay processes. In radioactive decay, unstable parent atoms shed excess subatomic particles, reducing their weight and becom- ing smaller daughter atoms ( figure 2.11 ). The overly heavy radioac- tive isotopes slim down to a stable weight by splitting apart, as in emitting alpha particles consisting of two protons and two neu- trons (effectively, the nucleus of a helium atom). Beta particles are electrons freed upon a neutron's splitting. Gamma radiation, which is similar to X-rays but with shorter wavelength, is emitted, lower- ing the energy level of a nucleus. As the rapidly expelled particles are slowed and absorbed by surrounding matter, their energy of motion is transformed into heat.

Dating the Events of History The same decaying radioactive isotopes producing heat inside Earth, Moon, and meteorites also may be read as clocks that date events in history. For example, uranium-238 decays to lead-206 through numerous steps involving different isotopes and new elements ( figure 2.12 ). By emitting alpha and beta particles, 32 of the 238 subatomic particles in the U-238 nucleus are lost, leaving the 206 particles of the Pb-206 nucleus. Laboratory measurements of the rate of the decay process have given us the U-238-to-Pb-206 half- life of 4.5 billion years. These facts may be applied to quantifying history by reading the radiometric clocks preserved in some miner- als. For example, some igneous rocks (crystallized from magma ) can be crushed, and the very hard mineral zircon (from which zirco- nium, the diamond substitute in jewelry, is synthesized) separated from it. Zircon crystals contain uranium-238 that was locked into their atomic structure when they crystallized from magma, but they originally contained virtually no lead-206. Thus, the lead-206 pres- ent in the crystal must have come from decay of uranium-238.

The collected zircon crystals are crushed into a powder and dissolved with acid under ultraclean conditions. The sample is placed in a mass spectrometer to measure the amounts of parent uranium-238 and daughter lead-206 present. Then, with three known values—(1) the amount of U-238, (2) the amount of Pb-206, and (3) the half-life of 4.5 billion years for the decay process—it is easy to calculate how long the U-238 has been decaying into Pb-206 within the zircon crystal. In other words, the calculation tells us how long ago the zircon crystal formed and consequently the time of formation of the igneous rock.

In Greater Depth

Protons

Neutrons

Alpha

Parent Daughter

Beta Gamma

decays to

(α) (β) (γ)

Electron

Figure 2.11 A radioactive parent atom decays to a smaller daughter atom by emitting alpha particles (such as the nucleus of a heli- um atom, i.e., two protons and two neutrons), beta particles (electrons), and gamma radiation (such as X-rays).

Pb206 Po210

Bi210

81 82 83 84 85 86 87 88 89 90 91 92 Atomic number

Uranium

Thorium

Actinium

Radium

Francium

Radon

Astatine

Polonium

Bismuth

Lead Thallium

Protactinium

Po214

Bi214TI210

Pb214 Po218 Rn222 Ra226

Pa234

Th234 U238

α

ααααα

α

α

α

β

β

ββ

β

β

β

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Pb210

Figure 2.12 Radioactive uranium-238 (U 238 ) decays to stable lead-206 (Pb 206 ) by steps involving many intermediate radioactive atoms. The atomic number is the number of protons (positively charged particles) in the nucleus.

remaining radioactive atoms decay (equivalent to 25% of the original parent atoms). The third half-life witnesses the third halving of radioactive atoms present (12.5% of the original parent atom population), and so forth. Half-lives plotted against time produce a negative exponential curve; this is the opposite direction of a positive exponential curve, such as interest being paid on money in a savings account.

The sum of the internal energy from impacts, gravity, and radioactive isotopes, plus additional energy produced by tidal

friction, is very large. The greater abundance of radioactive isotopes at Earth’s beginning combined with the early gravi- tational compaction and more frequent meteorite impacts to elevate Earth’s internal temperature during its early history. It is noteworthy that this heat buildup reached a maximum early in Earth’s history and has declined significantly since then. Nonetheless, the flow of internal heat toward Earth’s surface today is still great enough to provide the energy for continents to drift, volcanoes to erupt, and earthquakes to shake.

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Radioactivity Disasters The term radioactivity disasters brings to mind the meltdown of the uranium-rich core of a nuclear-power plant, as happened at Chernobyl in Ukraine, part of the former Soviet Union, on 26 April 1986. This human-caused disaster occurred when the night-shift workers made a series of mistakes that unleashed a power surge so great that the resultant explosions knocked off the 1,000-ton lid atop the nuclear reactor core, blew out the building's side and roof, triggered a partial meltdown of the reactor core's radioactive fuel, and expelled several tons of uranium dioxide fuel and fission prod- ucts, including cesium-137 and iodine-131, in a 5 km (3 mi) high plume. As many as 185 million curies of radioactivity were released. (The worst U.S. incident released 17 curies from the Three Mile Island nuclear-power plant in Pennsylvania in 1979.) After the 1:24 a.m. explosion, people near Chernobyl were at least fortunate that they were indoors and thus somewhat sheltered, there was no rain in the area, and the contaminant plume rose high instead of hugging the ground. The cloud of radioactive contaminants affected people, livestock, and agriculture from Scandinavia to Greece. In the Cher- nobyl power plant area, about 50 people died directly. Most of the deaths will come later from cancer and other diseases. The worst contaminant is radioactive iodine-131, which lodges in the thyroid. Cancer of the thyroid is expected to be common in the area begin- ning about 2010; it is estimated that it will shorten the lives of about 8,000 people.

An earthquake may have helped trigger this disaster. The Chernobyl power-plant workers were having difficulties in the early morning hours of 26 April, and then a magnitude 3 earthquake occurred 12 km (7 mi) away. The panicked supervisor thought the shaking meant the power plant was losing control, and he quickly implemented emergency maneuvers, but they jammed the internal

In Greater Depth works of the reactor, leading to the fateful explosion 22 seconds after the earthquake.

Chernobyl was a human-caused disaster. What can happen under natural conditions? Today, on Earth and Moon, uranium is present mostly as the heavier U-238 isotope, which has a combined total of 238 protons and neutrons in each uranium atom nucleus. The lighter-weight uranium isotope, U-235, makes up only 0.7202% of all uranium atoms. In nuclear-power plants, the uranium ore fed to nuclear reactors is enriched to 2–4% U-235 to promote more potent reactions. Remember from table 2.1 that U-235 has a half-life of 0.71 billion years, whereas the half-life of U-238 is 4.5 billion years. Because U-235 decays more rapidly, it would have been relatively more abundant in the geologic past. In fact, at some past time, the U-235 natural percentage relative to U-238 would have been like the U-235 percentage added to U-238 and fed as ore to nuclear reactors today.

Have natural nuclear reactors operated in the geologic past? Yes. A well-documented example has been exposed in the Oklo uranium mine near Franceville in southeastern Gabon, a coastal country in equatorial West Africa. At Oklo, 2.1 billion years ago, sands and muds accumulated along with organic carbon from the remains of fossil bacteria. These carbon-bearing sediments were enriched in uranium; U-235 was then 3.16% of total uranium. The sand and mud sediments were buried to shallow depths, and at least 800 m3 (1,050 yd3) of uranium ore sustained nuclear fission reactions that generated temperatures of about 400°C (750°F) regionally and much higher temperatures locally. At Oklo, 17 sites started up as natural nuclear reactors about 1.85 billion years ago; they ran for at least 500,000 years (and maybe as long as 2 million years). Nine of the natural reactors that have been carefully studied are estimated to have produced at least 17,800 megawatt years of energy.

AGE OF EARTH The oldest Solar System materials are about 4.57 billion (4,570 million) years old. The 4.57-billion-year age has been measured using radioactive isotopes and their decay prod- ucts collected from Moon rocks and meteorites. The oldest Earth rocks found to date are in northwest Canada, they are 4.055 billion years old. These rocks are of crustal composi- tion, implying that they were recycled and formed from even older rocks. The oldest ages obtained on Earth materials are 4.37 billion years, measured on sand grains of the mineral zircon collected from within a 3.1-billion-year-old sandstone in western Australia.

Our understanding of the age of Earth is improving rap- idly as new technologies allow measurement of more types of radioactive isotopes. It now seems that Earth has existed as a coherent mass for about 4.54 billion years. Earth must be younger than the 4.57-billion-year-old materials that col- lided and clumped together to form the planet. The time it took to build Earth is possibly as short as 30 million years. The collision of Earth with the Mars-size body that formed our Moon seems to have occurred between 4.537 and 4.533 billion years ago, suggesting that Earth was already a large, coherent mass at that time. Coming from the other direction,

Earth must be older than the 4.37-billion-year-old zircon grains collected from sandstone in Australia. In sum, our planet has existed for about 4.5 billion years.

The work to exactly determine the age and early his- tory of Earth continues today. It is challenging to try and find the oldest minerals and rocks because Earth is such an energetic planet that surface rocks are continually being formed and destroyed. Because of these active earth processes, truly old materials are rarely preserved; there have been too many events over too many years that destroy rocks.

Plate Tectonics The grand recycling of the upper few hundred kilometers of Earth is called the tectonic cycle. The Greek word tekton comes from architecture and means “to build”; it has been adapted by geologists as the term tectonics, which describes the building of topography and the deformation and move- ment within Earth’s outer layers.

Adding the horizontal components of movements on Earth allows us to understand the tectonic cycle. Ignoring

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shell into pieces that slip around as we try to pluck them off. This hand-held model of brittle pieces being moved atop a softer layer below is a small-scale analogue to the interac- tions between Earth’s lithosphere and asthenosphere.

DEVELOPMENT OF THE PLATE TECTONICS CONCEPT Our planet is so large and so old that the combined efforts of many geologists and philosophers over the past few hundred years have been required to amass enough observations to begin understanding how and why Earth changes as it does. The first glimpse of our modern understanding began after the European explorers of the late 1400s and 1500s made maps of the shapes and locations of the known continents and oceans. These early world maps raised intriguing possibilities. For example, in 1620, Francis Bacon of England noted the paral- lelism of the Atlantic coastlines of South America and Africa and suggested that these continents had once been joined. During the late 1800s, the Austrian geologist Eduard Suess presented abundant evidence in support of Gondwanaland, an ancient southern supercontinent composed of a united South America, Africa, Antarctica, Australia, India, and New Zealand, which later split apart. This process of the continents moving, splitting, and recombining is known as continental drift. The most famous and outspoken of the early proponents of continental drift was the German meteorologist Alfred Wegener. In his 1915 book, The Origin of Continents and Oceans, he collected all available evidence, such as similar rocks, fossils, and geologic structures, on opposite sides of the

complexities for the moment, the tectonic cycle can be sim- plified as follows ( figure 2.13 ). First, melted asthenosphere flows upward as magma and cools to form new ocean floor/ lithosphere. Second, the new lithosphere slowly moves later- ally away from the zones of oceanic crust formation on top of the underlying asthenosphere; this phenomenon is known as seafloor spreading. Third, when the leading edge of a moving slab of oceanic lithosphere collides with another slab, the older, colder, denser slab turns downward and is pulled by gravity back into the asthenosphere, a process called subduction, while the less-dense, more buoyant slab overrides it. Last, the slab pulled into the mantle is reab- sorbed. The time needed to complete this cycle is long, com- monly in excess of 250 million years.

If we adopt the perspective of a geologist-astronaut in space and look down upon the tectonic cycle, we see that the lithosphere of Earth is broken into pieces called plates ( figure 2.14 ). The study of the movements and interactions of the plates is known as plate tectonics. The gigantic pieces of lithosphere (plates) pull apart during seafloor spreading at divergence zones, slide past at transform faults, or collide at convergence zones. These plate-edge interactions are directly responsible for most of the earthquakes, volcanic eruptions, and mountains on Earth.

Another way that plate tectonics can be visualized is by using a hard-boiled egg as a metaphor for Earth. Consider the hard-boiled egg with its brittle shell as the lithosphere, the slippery inner lining of the shell as the asthenosphere, the egg white (albumen) as the rest of the mantle, and the yolk as the core. Before eating a hard-boiled egg, we break its brittle

Asthenosphere

Mesosphere

Lithosphere

Continent

M an

tl e

C ru

st

Seafloor

Spreading center at volcanic ridge

Subduction zone at deep-ocean trench

M a g

m a

Figure 2.13 Schematic cross-section of the tectonic cycle. Magma rises from the asthenosphere to the surface at the oceanic volcanic ridges where it solidifies and adds to the plate edges. As the igneous rock cools, the plate subsides and gravity pulls the plates from their topographic highs. The plate continues to cool, grows thicker at its base, becomes denser, collides with a less-dense plate, and turns down into the mantle, where it is ultimately reassimilated. Adapted from A. Cox and R. B. Hart, Plate Tectonics: How It Works .

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38 Chapter 2 Internal Energy and Plate Tectonics

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Earth’s Magnetic Field Anyone who has ever held a compass and watched the free-turning needle point toward the north has experienced the magnetic field that surrounds Earth. The Chinese invented and were the first to use magnetic compasses. They in turn taught 14th-century European travelers, who brought this knowledge back to Europe, where it was developed into the navigational tool that helped late 15th- century explorers make their voyages of discovery.

Earth’s magnetic field operates as if a gigantic bar magnet were located in the core of Earth inclined 11° from vertical ( figure 2.15 ). The magnetic pole and geographic North Pole do not coincide, but the magnetic pole axis has apparently always been near the rotational pole axis. Notice in figure 2.15 that the inclination of the magnetic lines of force with respect to Earth’s surface varies with latitude. At the magnetic equator, the magnetic lines of force are parallel to Earth’s surface (inclination of 0°). Toward the poles, either northward or southward, the angle of inclination continuously increases until it is perpendicular to the surface at both the north and south magnetic poles (inclinations of 90°). Notice also that the lines of force are inclined downward and into Earth’s surface near the North Pole and upward and out of Earth’s surface near the South Pole.

In reality, the interior of Earth is much too hot for a bar magnet to exist. Magnetism in rocks is destroyed by temperatures above 550°C (1,020°F), and temperatures in Earth’s core are estimated to reach 5,800°C (10,470°F). The origin of Earth’s magnetic field involves movements of the iron-rich fluid in the outer core, which generate electric currents that in turn create the magnetic field. Fluid iron is an excellent conductor of electricity. The molten iron flowing around the solid inner core is a self-perpetuating dynamo deriving its energy both from the rotation of Earth and from the convection of heat released by the crystallization of minerals at the boundary of the inner and outer cores.

A closer look at Earth’s magnetic field yields several problems awaiting resolution. The simplified magnetic field portrayed in figure 2.15 does not show the complexities that occur over years and centuries as the magnetic field's strength waxes and wanes. More than 400 years of measurements document variations in the strength and stability of the magnetic field. At present, the strength is 10% weaker than in the year 1845, but the field strength is still about twice as strong as the long-term average. The flow of fluid iron in the outer core has regions of turbulence, including motions as complex as whirlpools. Change is normal, and in turn, the magnetic field fluctuates.

In addition, the magnetic pole moves about the geographic North Pole region in an irregular pattern. The rapid rotation of Earth holds the magnetic pole near the pole of rotation, but the magnetic pole wanders enough that it crosses 5° to 10° of latitude each century. In recent decades, the magnetic pole has moved at rates of 10 to 40 km (6 to 25 mi) per year.

Every several thousand to tens of millions of years, a highly dramatic change occurs in the magnetic field: the magnetic polarity

In Greater Depth

Geographic North Pole

Magnetic North Pole

Figure 2.15 Schematic diagram of Earth’s magnetic field. The bar magnet pictured does not exist, but it would create the same magnetic field achieved by the electrical currents in Earth’s liquid, iron-rich outer core. Notice that (1) the magnetic pole and the rotational pole do not coincide, (2) the magnetic lines of force are parallel to Earth’s surface at the magnetic equator and perpendicular at the magnetic poles, and (3) the lines of force go into Earth at the North Pole and out at the South Pole. From P. J. Wyllie, The Way the Earth Works . Copyright © 1976 John Wiley & Sons, Inc., New York. Reprinted with permission of John Wiley & Sons, Inc.

reverses. In a reversal, the orientation of the magnetic field flip- flops from a north (normal) polarity to a south (reverse) polarity or vice versa. It has been 780,000 years since the last long-term rever- sal. Models run on supercomputers indicate that reversals take less than a thousand years to complete. During a reversal, it appears that the magnetic field does not disappear; it just gets more com- plex. The magnetic lines of force become twisted and tangled, but a magnetic field still exists to protect life on Earth from much of the incoming solar and space radiation.

The change in orientation of the magnetic field leaves its imprint in rocks, where geologists (paleomagnetists) can read it. The paleomagnetic history contained in the rocks has provided the most important evidence of seafloor spreading; it also has allowed charting of the paths of continents as they have moved through different latitudes. In addition, the record of magnetic reversals provides the data for a magnetic timescale, a third geologic time- scale. (The first timescale is based on the irreversible sequence of fossils occurring in sedimentary rocks, and the second timescale is founded on the decay of radioactive isotopes.)

Atlantic Ocean. Wegener suggested that all the continents had once been united in a supercontinent called Pangaea ( pan meaning “all” and gaea meaning “earth”).

Much is made of the fact that during his lifetime, Wegener’s hypothesis of continental drift garnered more

ridicule than acceptance. But why were his ideas not widely accepted? Wegener presented an intriguing hypothesis well supported with observations and logic, but his mechanism was deemed impossible. Geologists and geophysicists could not visualize how a continent could break loose from the

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underlying rocks and plow a path over them. The break- through in understanding came when the ocean floors were studied and the data were best explained by the formation of new seafloor that spread apart and later was consumed by subduction. When scientists realized that the lithosphere decouples from the asthenosphere and moves laterally, they understood how the relatively small, low-density continents, set within the oceanic crust, could be carried along as inci- dental passengers (see figure 2.13 ).

In the mid-1960s, evidence abounded, mechanisms seemed plausible, and the plate tectonic theory was devel- oped and widely accepted. Wegener was restored to an ele- vated status. Scientific understanding grew with the addition of new data, old hypotheses were modified, and new theories were created. Science is never static; it is a growing, evolving body of knowledge that creates ever-better understanding of how Earth works.

It is rare in science to find widespread agreement on a large-scale hypothesis such as plate tectonics. But when data from Earth’s magnetic field locked inside seafloor rocks were widely understood, skeptics around the world became convinced that seafloor spreading occurs and that the con- cept of plate tectonics is valid.

Figure 2.16 A stratified pile of former lava flows of the Columbia River Basalt exposed in the east wall of Grand Coulee, Washington. The oldest flow is on the bottom and is overlain by progressively younger flows. Photo by John S. Shelton.

MAGNETIZATION OF VOLCANIC ROCKS Lava is magma that erupts from a volcano, flows outward as a sheetlike mass, slows down, and stops. Then, as the lava cools, minerals begin to grow as crystals. Some of the earliest formed crystals incorporate iron into their structures. After the lava cools below the Curie point, about 550°C (1,020°F), atoms in iron-bearing minerals become magnetized in the direction of Earth’s magnetic field at that time and place. The lined-up atoms in the iron-rich crystals behave like compass needles pointing toward the magnetic pole of their time (mea- sured as declination or “compass bearing”); they also become inclined at the same angle as the lines of force of the magnetic field (measured as inclination or dip). Ancient magnetic fields have been measured in rocks as old as 3.5 billion years.

Lava flows pile up as sequences of stratified (layered) rock, and the magnetic polarity of each rock layer can be measured ( figure 2.16 ). Many of the volcanic rocks also contain minerals with radioactive isotopes that allow us to determine the age of the volcanic rock—that is, how long ago the lava flow solidified. When this information is plot- ted together in a vertical column, a timescale of magnetic

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Volcanic rocks: former lava flows

Reversed polarity

Normal polarity

Radiometric timescale (in millions of years) 0 (today)

Magnetic polarity reversal timescale

1

2

3

4

Figure 2.17 A portion of the magnetic polarity timescale. Magnetic polarity measurements in volcanic rocks combined with radiometric ages determined from the same rocks allow formation of a timescale based on magnetic polarity reversals. Notice the unique and nonrepetitive pattern of the polarity reversals. From P. J. Wyllie, The Way the Earth Works . Copyright © 1976 John Wiley & Sons, Inc., New York. Reprinted with permission of John Wiley & Sons, Inc.

Paleomagnetic timescale

with radiometric ages (in millions of years)

0

2

4

6

8

10

62° N

60°

30° 25° W

Ri dg

e ax

is

Figure 2.18 Map of the magnetically striped Atlantic Ocean floor southwest of Iceland. Black areas are magnetized pointing to a north pole and white areas to a south pole. Notice the near mirror images of the patterns on each side of the volcanic ridge (spreading center).

polarities emerges ( figure 2.17 ). It is interesting to note that the timing of polarity reversals appears to be random. There is no discernible pattern to the lengths of time the magnetic field was oriented either to the north or to the south. The processes that reverse the polarity of the magnetic field are likely related to changes in the flow of the iron-rich liquid in the outer core. The reversal-causing mechanism does not occur at any mathematically definable time interval.

Magnetization Patterns on the Seafloors Since the late 1940s, oceanographic research vessels criss- crossing the Atlantic Ocean have towed magnetometers to measure the magnetization of the seafloor. As the number of voyages grew and more data were obtained, a striking pattern began to emerge ( figure 2.18 ). The floor of the Atlantic Ocean is striped by parallel bands of magnetized rock that show alternating polarities. The pattern is symmetrical and parallel with the midocean volcanic ridge (spreading cen- ter). That is, each striped piece of seafloor has its twin on the other side of the oceanic mountain range.

A remarkable relationship exists between the time of reversals of magnetic polarity, as dated radiometrically from a sequence of solidified lava flows (see figure 2.17 ), and the widths of alternately polarized seafloor ( figure 2.18 )—they are comparable. How stunning it is that the widths of mag- netized seafloor strips have the same ratios as the lengths of time between successive reversals of Earth’s magnetic field. This means that distance in kilometers is proportional to time in millions of years. Now, if Earth’s magnetic field is

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42 Chapter 2 Internal Energy and Plate Tectonics

reversing polarity in a known timescale and if that timescale reappears in distances, then the relationship must take the form of a velocity. That is, magma is injected into the oce- anic ridges where it is imprinted by Earth’s magnetic field as it cools to form new rock. Then the seafloor/ocean crust/ lithosphere is physically pulled away from the oceanic ridges as if they were parts of two large conveyor belts going in opposite directions ( figure 2.19 ).

The evidence provided by the paleomagnetic time-scale and the magnetically striped seafloors is compelling. These

phenomena are convincing evidence that seafloor spreading occurs and that plate tectonics is valid.

Other let’s now consider evidence supporting plate tectonics.

Earthquake Evidence The map of earthquake epicenters ( figure 2.20 ) can

be viewed as a connect-the-dots puzzle. Each epicenter represents a place where one major section of rock has moved past another section. Take your pen or pencil, con-

6 m.y. 5 4 3 2 1 10 2 3 4 5 6 m.y.

Today

Magnetic polarity

Normal Reverse

North South

Lithosphere

Asthenosphere

Lithosphere

Figure 2.19 Cross-section of magnetically striped seafloor. Numbers above the seafloor are radiometrically determined ages in millions of years (m.y.). The near mirror-image magnetic pattern is like a tape recorder that documents “conveyor belt” movements away from volcanic ridges.

USGS National Earthquake Information Center

60°

30°

–30°

–60°

–30° 0° 30° 60° 90° 120° 150° 180° –150° –120° –90° –60° –30°

–800 km (496 mi)

–501 km (311 mi)

–301 km (187 mi) D E P T H

–151 km (94 mi)

–71 km (44 mi)

–33 km (20 mi)

0

0°

Figure 2.20 Map of earthquake epicenters, 1975–1995. Notice that epicenters are concentrated in linear belts. Color-coding of epicenters indicates depths of earthquakes; notice that depths increase toward continents. After USGS National Earthquake Information Center.

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nect the dots (epicenters), and you will outline and define the edges of the tectonic plates, the separately moving pieces of lithosphere. Remember that these plates are about 100 km (60� mi) thick and can be thousands of kilometers across.

Earthquakes at depth commonly occur along inclined planes ( figure 2.21 ) adjacent to deep-ocean trenches. These deep earthquakes define the subducting plates being pulled forcefully back into the mantle. In figure 2.20 , the earth- quake epicenters are color-coded according to depth. The

increasing depths toward and beneath continents define the subducting oceanic plates.

Ages from the Ocean Basins One of the most stunning facts discovered during the recent exploration of the oceans is the youthfulness of the ocean basins. The oldest rocks on the ocean floors are about 200 million years in age; this is less than 5% of the age of Earth ( figure 2.22 ). Remember that some continental rocks are more than 4,000 million years old. Meteorites are more

Eurasian Continent Sea of Japan

Japan Deep-ocean

trench Pacific Ocean

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Figure 2.21 Cross-section showing earthquake (fault movement) locations at depth; notice the inclined plane defined by the earthquake sites (black dots). The earthquake locations define the subducting plate beneath Japan. At shallow depths, earthquakes are generated in brittle rocks in both subducting and overriding plates. At greater depths, only the interior of the subducting Pacific plate is cold enough to maintain the rigidity necessary to produce earthquakes. Striped areas are hot rocks defined by relatively lower-velocity seismic waves.

South America

North America

Asia

Australia

Africa

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Greenland

0 10 20 33 40 48 56 69 83 118 127132 142 150157 180

Figure 2.22 Age of the ocean floor in millions of years. Red is youngest; blue is oldest. Photo from NOAA/NGDC.

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44 Chapter 2 Internal Energy and Plate Tectonics

than 4,500 million years old. Some Moon rocks are more than 4,500 million years old, and none are younger than 3,100 million years. But the ocean basins (not the water in them) and their contained volcanic mountains, sediments, and fossils are all much, much younger. Why? Because the ocean basins are young features that are continuously being formed and destroyed.

Along the oceanic ridges, volcanism is active, and new seafloor/oceanic crust is forming (see figure 2.19 ). Moving away from the ridges, the seafloor volcanic rocks and islands become progressively older. The oldest seafloor rocks are found at the edges of the ocean basins.

At certain locations, deep-seated hot spots produce more heat, causing hotter rocks with lower density. These masses of buoyant hot rock rise upward as plumes through the mantle, begin to melt near the top of the overlying asthe- nosphere, and pass up through the lithosphere as magma. Hot spots have active volcanoes above them on Earth’s surface. The volcanoes rest on moving plates that carry them away from their hot-spot source. This process forms lines of extinct volcanoes on the ocean floor, from youngest to oldest, point- ing in the direction of plate movement ( figure 2.23 ). The hot-spot–fed plume moves also and probably is responsible for the prominent bend in the island/seamount chain.

The blanket of sediment on the seafloor ranges from very thin to nonexistent at the volcanic ridges and thickens toward the ocean margins ( figure 2.24 ). The older the sea- floor, the more time it has had to accumulate a thick cover of sand, silt, clay, and fossils.

Oceanic Mountain Ranges and Deep Trenches The greatest mountain ranges on Earth lie on the ocean bot- toms and extend more than 65,000 km (40,000 mi). These long and continuous volcanic mountains are seen to form at spreading centers where plates pull apart and magma rises to fill the gaps.

The ocean bottom has an average depth of 3.7 km (2.3 mi), yet depths greater than 11 km (nearly 7 mi) exist in elongate, narrow trenches (see top of figure 2.23 a ). The long and deep trenches were known since the Challenger oceanographic expedition in the 1870s, but they were not understood until the 1960s, when geologists recognized that they are the tops of the subducting plates turning downward to reenter the mantle.

Systematic Increases in Seafloor Depth Above the oceanic ridges, the ocean water depths are rela- tively shallow. However, moving progressively away from the ridges, the ocean water depths increase systematically with seafloor age ( figure 2.24 ). This is due to the cooling and contraction of the oceanic crust with a resultant increase in density. Also, some isostatic downwarping occurs due to the weight of sediments deposited on the seafloor. The progres- sive deepening of the seafloor with increasing age also testi- fies to the existence of seafloor spreading.

28

Hawaii

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P lu

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Figure 2.23 A hot spot and its path. (a) Map shows the Hawaiian Islands–Emperor seamount chain of hot-spot–fed volcanoes with plots of their radiometric ages in millions of years. The map pattern of volcano ages testifies to movement of the Pacific plate through time. (Ma = mega-annum, 1 million years.) (b) Cross-section shows a hot spot at a depth where hot mantle rock rises up through the asthenosphere and passes through the lithosphere as a plume of magma supplying a volcano. Because the lithospheric plate keeps moving, new volcanoes are formed. (a) Reprinted by permission from World Ocean Floor by Bruce C. Heezen and Marie Tharp, copyright 1977.

The Fit of the Continents If the continents have really drifted apart, then we should be able to take a map, cut out the continents to make puzzle pieces, and then reassemble them in their former configura- tion. In fact, this can be done if we know where to cut the map. On two-dimensional world maps, the landmasses occupy

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Plate Tectonics 45

Sediments and fossils Youngest at surface

Ages of fossils

Oldest in bottom layer of sediment

18016014012010080604020020406080100120140160180

Asthenosphere

Thickest

Older

Sedimen ts

Thick er

Litho spher

e Yo

ung er

Thin

Ocean-ridge volcanism

Seafloor age (millions of years)

Figure 2.24 Schematic cross-section through oceanic lithosphere perpendicular to a volcanic ridge. Moving away from the ridge: (1) radiometric ages of oceanic lithosphere increase, (2) thicknesses of accumulated sediments increase, and (3) ages of fossils in the sediments increase. The systematic increases in water depth are due to cooling, shrinking, and increase in density of the aging seafloor rocks.

about 29% of Earth’s surface and the oceans the other nearly 71%. If we cut the puzzle pieces at the land-sea shoreline and then attempt to reassemble them, the fit will not be good. The problem here is that the significant boundary is not between land and water but instead at the real edge of the continent— the change from low-density continental rocks to higher- density oceanic rocks. This change occurs at about a 1,800 m (6,000 ft) water depth. If we remove the oceans, we find that the continental masses cover 40% of Earth’s surface and the ocean basins the other 60%. If we cut the puzzle pieces at the 1,800-meter water-depth line, then the continental puzzle

pieces fit together quite well. There are some overlaps and gaps, but these are reasonably explained by changes during the last 220 million years, since the last major split of the conti- nents. Examples of the changes include deformation during the process of rifting; growth of river deltas, volcanoes, and coral reef masses; erosion of the continents; and land movements.

Changing Positions of the Continents Undoing the seafloor spreading of the last 220 million years restores the continents of today into the supercontinent Pangaea, which covered 40% of Earth ( figure 2.25 ). Although

Tethys Sea

Panthalassa

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Figure 2.25 Pangaea, the supercontinent, 220 million years before present. The modern continents are drawn to be recognizable in this restoration. The superocean of the time (Panthalassa) exists today in shrunken form as the Pacific Ocean. After R. S. Dietz and J. C. Holden, “Reconstruction of Pangaea: Breakup and Dispersion of Continents, Permian to Present” in Journal of Geophysical Research 75:4, 939–56, 1970. Copyright © 1970 American Geophysical Union.

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46 Chapter 2 Internal Energy and Plate Tectonics

L a u r a s i a

Tethys Sea

EurasiaNorth America

South America

Africa

India

Australia

Antar ctica

80°

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G o n d w a n a

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Figure 2.26 Changing positions of the continents. (a) 180 million years ago. (b) 135 million years ago. (c) 65 million years ago. (d) Today.

the present continents had yet to form, this figure shows their relative positions within Pangaea before its breakup. The remaining 60% of the Earth’s surface was a massive ocean called Panthalassa (meaning “all oceans”).

Figure 2.26 a shows the breakup of Pangaea at 180 million years before present. An equatorial spreading center separated the northern supercontinent Laurasia from the southern supercontinent Gondwanaland. Much of the sediment deposited in the Tethys Sea at that time has since been uplifted to form mountain ranges, from the Himalayas to the Alps. Another spreading center began opening the Indian Ocean and separating Africa-South America from Antarctica-Australia.

At 135 million years ago, seafloor spreading had begun opening the North Atlantic Ocean, India was moving toward Asia, and the South Atlantic Ocean was a narrow sea similar to the Red Sea today ( figure 2.26 b ).

By 65 million years ago, seafloor spreading had opened the South Atlantic Ocean and connected it with the North Atlantic, and Africa came into contact with Europe, cutting off the western end of the Tethys Sea to begin the Mediterranean Sea ( figure 2.26 c ). Although the modern world had become recognizable, note that North America and Eurasia were still connected and that Australia had not yet left Antarctica.

Nearly half of the present ocean floor was created during the last 65 million years ( figure 2.26 d ). India has rammed

into Asia, continued opening of the North Atlantic has split Eurasia from North America, and Australia has moved a long way from Antarctica.

The Grand Unifying Theory Figure 2.27 shows how Earth’s outer layers are operating today in plate-tectonic action. The following model explains how it happens. Rising hot rock in the mantle reaches the asthenosphere and begins to melt; the liquid rises to Earth’s surface. The buildup of magma and heat causes expansion and topographic elevation of the overlying oceanic litho- sphere or continent, which then fractures because of the uplift and begins to be pulled apart laterally by gravity. A continent can be split, forming a rift zone, a young divergent plate boundary such as the East African Rift Valley. In an ocean basin, the pulling apart of oceanic lithosphere reduces pressure on the superheated asthenosphere rock, which liq- uefies and rises upward to fill the fractures and create new oceanic lithosphere via seafloor spreading. The continuing elevation of the volcanic mountain chain (ridge) forms a set- ting for gravity to keep pulling material downward and out- ward (spreading). The lateral spreading may be aided by

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The Grand Unifying Theory 47

movements are now so well understood due to the magnetic record of seafloor rocks that the plates are not only outlined, but their rates of movement are defined as well (see figure 2.14 ).

Note also on figure 2.27 that oceanic spreading ridges are offset by faulting at transform plate boundaries. Deep below the realm of plate tectonics, hot spots send up plumes of hot, buoyant rock that turns into magma near the surface, building shield volcanoes on the seafloor, such as in Hawaii, or explo- sive mega-volcanoes on continents, such as in Yellowstone National Park, Wyoming.

Plate tectonics is a great scientific concept. It provides us with new perspectives about Earth that are quite different from those encountered in our life or historical experiences. Because Earth is so much older and so much larger than a human being, we must set aside our personal time and size scales. Our lives are measured in decades, and our personal measuring rods are our 5- to 6-feet-tall bodies; with these as reference guides, we can be only mystified by Earth. However, if we change our time perspective to millions and billions of years and our size scales to continents and plates, then—and only then—can we begin to understand Earth. An active plate may move 1 cm (0.4 in) in a year; this is only 75 cm (30 in) in a human lifetime. The rates of plate movement are comparable to those of human fingernail growth.

How can we explain the building of mountains or the formation of ocean basins? We must consider Earth over its own time span of 4,570 million years. Then there is plenty of

convection cells of mantle heat, which rise and move later- ally beneath the lithosphere before descending.

As the lithosphere spreads, cools, and becomes denser, it is pulled ever more strongly by gravity. When oceanic litho- sphere collides with another plate, the denser (older, colder) plate goes beneath the less-dense (younger, warmer) plate in the process of subduction. If an oceanic plate goes beneath another oceanic plate at a convergent plate boundary, an island arc of volcanoes next to a trench can form, such as the Aleutian Islands of Alaska. If the subducting oceanic plate is pulled beneath a continent-carrying plate, the top of the down-bending oceanic plate forms a trench, and a line of active volcanoes builds on the continent edge, such as the Cascade Ranges of northern California, Oregon, Washington, and British Columbia.

As the leading edge of the negatively buoyant subducting plate turns downward, gravity exerts an even stronger pull on it, which helps tear the trailing edge of the plate away from the spreading center. The combination of gravity pulling on ele- vated spreading-center mountains and especially on denser, down-going plates at subduction zones (slab pull) keeps the lithospheric plates moving. Thus, an ongoing tectonic cycle operates whereby each moving part stimulates and maintains motions of the others in a large-scale, long-term recycling operation. Subducted plates are reassimilated into the mantle as physical slabs that remain solid enough to be recognized by their effects on the travel velocities of seismic waves. Plate

Convergent plate boundary

Convergent plate boundary

Continental rift zone (young divergent plate boundary)

Continental crust

Transform plate boundary

Divergent plate boundary

Strato- volcano

Shield volcano

Lithosphere Lithosphere

Asthenosphere

Hot spot

Oceanic spreading ridge

Trench

Oceanic subducting plate Su

bd uct

ing pla

te

Island arc

Trench

Figure 2.27 Three-dimensional schematic view of tectonic plates with divergent, convergent, and transform boundaries plus volcanoes above subducting plates and a hot spot. From Kious and Tilling, US Geological Survey.

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48 Chapter 2 Internal Energy and Plate Tectonics

time is long, and everyday changes on Earth add up to major results.

UNIFORMITARIANISM Hutton’s thought pattern, called uniformitarianism, has revolutionized our understanding of Earth. Uniformitarian- ism implies that natural laws are uniform through time and space. Physical and biological laws produce certain effects today, as they have in the past, and will in the future. If we can understand how Earth works today, we can use this knowledge to read the rock and fossil record to understand Earth’s history. The present is the key to the past.

The term uniformitarianism has come under attack by some who assume it says that earth processes have always acted at a uniform and slow rate, but we all know that rates can vary. For example, seafloor spreading has operated at slower rates in the past, and it has also run at faster rates, but the laws governing how seafloor spreading operates do not change just because the rates vary. Some suggest using the term actualism instead of uniformitarianism, but the concept is basically the same. Actualism tells us to understand physical, chemical, and biological processes actually operating on, in, and outside Earth today, and to use these known and testable processes to interpret the past; it advises us not to invent undemonstrated and untestable supernatural causes to explain away problems.

How do we go about understanding Earth? We study the present to understand the past and then make probabilistic forecasts about the future.

time for small events to add up to big results. The plate mov- ing 1 cm/yr travels 10 km (6� mi) in just 1 million years. The 1 cm/yr process is fast enough to uplift a mountain in a small amount of geologic time.

How We Understand Earth Thousands of years ago, human thought had already made great advances in topics such as philosophy, government, religion, drama, and engineering. But our understanding of Earth was insignificant until Earth’s great age was realized. This recognition came late in human history; it started with James Hutton in the 1780s. Hutton carefully observed his Scottish landscape and thought deeply about it. For example, he saw rock walls built by the Romans that had stood for 15 centuries with only slight change. If 1,500 years was not long enough to break down a wall, Hutton wondered, how much time had been required to break down some of the hard rock masses of Scotland into the abundant pebbles and sand grains he saw? And how much more time had been necessary to lift the pebbly and sandy sedimentary rocks to form hills? All the active processes Hutton observed worked slowly, so his answer to the questions was that great lengths of time were required. In 1788, Hutton described the history of Earth as follows: “The result, therefore, of our present inquiry is that we find no vestige of a beginning, no prospect of an end.” And this was Hutton’s great gift to human thought:

Summary Massive amounts of internal heat within the early Earth caused widespread melting. Gravity has pulled Earth into lay- ers of differing density, beginning with a heavy metallic core and proceeding outward through layers of decreasing density: from the mantle to the continents, the ocean, and finally, the atmosphere. These layers exist in a state of flotational equilib- rium known as isostasy. Up-and-down movements of the land due to isostatic adjustments are readily measurable.

The radioactive isotopes that help heat Earth’s interior by their decay do so at measurable rates quantified by half- lives. Elements that radioactively decay act as clocks that can be used to date the events of Earth history. Earth is 4.57 bil- lion years old.

The outer layers of Earth are involved in a grand recy- cling known as the tectonic cycle. Hot buoyant rock and magma rise up from the mantle, through the lithosphere, to build world-encircling mountain ranges of volcanoes (oce- anic ridges). The injection of magma elevates ridges that are pulled apart by gravity (slab pull) in gigantic slabs (plates) to form ocean basins in the process known as seafloor spreading. When these moving lithospheric plates collide, if one plate is

composed of denser rock, it will turn back down into the mantle, a process called subduction, to become melted and reabsorbed. The entire lithosphere is fractured into plates that pull apart (diverge), slide past, and collide (converge) with each other. The plate collisions cause mountains to rise, sea- floors to bend down forming trenches that are elongate and deep, volcanoes to erupt, and earthquakes to be generated; this cyclic process is the topic of plate tectonics. Continents are composed of lower-density rock that rides on top of the denser rock of the moving plates.

The evidence for plate tectonics is overwhelming. Ancient magnetic fields locked into iron-bearing minerals in rocks point toward former south or north magnetic poles in patterns, indicating that seafloor spreading and continental drift occur. The ages of rocks, sediments, and fossils, as well as the depth of water, all increase away from the oceanic ridges, indicating that oceanic crust/lithosphere is continu- ously forming and spreading apart. The oldest rocks and fossils in the ocean basins are less than 5% of Earth’s age, indicating that oceanic material is destroyed by recycling into the mantle.

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Questions for Further Thought 49

5. How does nuclear fusion differ from nuclear fission? 6. What is the age of Earth? How is this determined? 7. After freeing zircon crystals from an igneous rock, how could

you determine when the rock formed (solidified)? 8. Where are the oldest known Earth rocks found? How old

are they? 9. What are the differences between brittle, ductile, and elastic

behavior? 10. Draw and label a cross-section that explains the tectonic cycle. 11. Explain some other evidence indicating that seafloors spread. 12. What is a tectonic plate? 13. What are the ages of the oldest (a) rocks on the continents,

and (b) rocks making up the ocean floor? 14. Explain several lines of evidence indicating that the

continents move about Earth. 15. Describe a deep-ocean trench. How does one form? 16. Why do deep earthquakes tend to occur within inclined bands? 17. Draw a cross-section that shows a hot spot and plume. How

do they help determine the directions of plate motions? 18. How is Earth’s magnetic field formed? Describe the reversals

of magnetic polarity from north to south. 19. Explain the concept of uniformitarianism. 20. What is the Curie temperature? How is this related to

magnetism? 21. Explain the paleomagnetic evidence for seafloor spreading. 22. How can the magnetic record inside a volcanic rock be used

to determine the latitude at which the lava cooled?

Questions for Further Thought 1. Earth is commonly called terra firma . Does this make good

geologic sense? 2. Are new natural nuclear reactors likely to spring into action

on Earth? 3. Your lifetime will be what percentage of geologic time? 4. How much does the ground sink under a load or rise after

removal of a load during isostatic adjustments? 5. Why does the polarity of Earth’s magnetic field switch from

north to south and back again? 6. How can the rate of motion of a plate be calculated?

Terms to Remember acre foot 32 actualism 48 asthenosphere 29 brittle 31 buoyancy 32 centigrade 27 continental drift 37 convection 47 convergence zone 37 core 28 crust 28 curie 36 Curie point 40 divergence zone 37 ductile 31 elastic 31 element 31 epicenter 42 Fahrenheit 27 fluids 30 fossil 44 Gondwanaland 37 half-life 34 hot spot 34 igneous rock 35 isostasy 32 latitude 39 Laurasia 46 lava 40 liquid 29 lithosphere 29 magma 45

magnetic field 39 magnetic pole 39 magnetism 39 mantle 28 mesosphere 29 nuclear fission 36 nuclear fusion 27 Pangaea 39 Panthalassa 46 plastic 31 plate 42 plate tectonics 37 plume 44 radioactive isotope 34 reef 45 ridge 41 seafloor spreading 37 sediment 36 solar radiation 27 spreading center 44 strain 31 stress 37 subduction 37 tectonic cycle 36 tectonics 36 tidal friction 35 topography 36 transform fault 37 trench 44 uniformitarianism 48 viscous 29 yield stress 30

Questions for Review 1. Describe how Earth became segregated into layers of

differing density. 2. How did Earth’s continents, oceans, and atmosphere form? 3. Describe some examples of isostasy. 4. What energy sources caused the interior of the early Earth

to heat up?

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Earthquake Geology and Seismology 3

CHAPTER

Houses built on vertical posts in Bosques de las Lomas, Mexico City, have precious little shear strength to respond to seismic waves.

LEARNING OUTCOMES Earthquakes are shaking most commonly caused by earth movements along faults. Energy from movements is carried long distances by seismic waves. After studying this chapter, you should:

• be able to describe the types of faults.

• know the types of seismic waves.

• understand the different ways of calculating earthquake magnitude.

• be familiar with the variables that determine earthquake intensity, as in the Mercalli intensity scale.

• comprehend the relationships between periods and frequencies of seismic waves, buildings, and geologic foundations.

• recognize the types of buildings and building materials that fail during earthquakes.

• understand how to construct buildings that do not fail during earthquakes.

OUTLINE • Understanding Earthquakes

• Types of Faults

• Development of Seismology

• Seismic Waves

• Locating the Source of an Earthquake

• Magnitude of Earthquakes

• Ground Motion During Earthquakes

• Earthquake Intensity—What We Feel During an Earthquake

• A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971

• Building in Earthquake Country

Photo by Pat Abbott. Diseased nature oftentimes breaks forth In strange eruptions: oft the teeming earth Is with a kind of colic pinch’d and vex’d By the imprisoning of unruly wind Within her womb; which, for enlargement striving, Shakes the old beldam earth, and topples down Steeples, and moss-grown towers.

—William Shakespeare, 1598, King Henry IV

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Understanding Earthquakes 51

T he earth beneath our feet moves, releasing energy that shifts the ground and sometimes topples cities. Some earthquakes are so immense that their energy is equivalent to thousands of atomic bombs exploded simultaneously. The power of earthquakes to destroy human works, to kill vast numbers of people, and to alter the very shape of our land has left an indelible mark on many civilizations .

Earthquake unpredictability instills an uneasy respect and fear in humankind that, through the millennia, have helped shape thought about life and our place in it. Ancient accounts of earthquakes tend to be quite incomplete. Instead of providing rigorous descriptions of Earth behavior, they emphasize interpretations. For more than 2,000 years, based on Aristotle’s ideas, many explanations of earth- quakes were based on winds rushing beneath Earth’s surface. Even Leonardo da Vinci wrote in his Notebooks, about 1500 ce, that:

A Classic Disaster The Lisbon Earthquake of 1755 Portugal in the 18th century, and especially its capital city of Lisbon, was rich with the wealth its explorers brought from the New World. Portugal’s decline probably began with a set of earthquakes. On the morning of 1 November 1755—All Saints Day—Lisbon rocked under the force of closely spaced earthquakes originating offshore under the Atlantic Ocean. On this day of religious observance, the churches were full of worshippers. About 9:40 a.m., a thunderous underground sound began, followed by violent ground shaking. The severe ground movement lasted two to three minutes, causing widespread damage to the buildings in this city of more than 250,000 people. Most of Lisbon’s churches were built of masonry; they collapsed into the narrow streets, killing thousands of trapped and fleeing people. Tapestries fell onto candles and lamps—all lit on this holy day—and started fires that burned unchecked for six days.

Before an hour had passed, crippled Lisbon was rocked by a second earthquake, more violent but shorter-lived than the first. In the panic, many of the frightened survivors of the first earthquake had rushed to the shore for safety, only to be swept away by quake-caused sea waves up to 10 m (33 ft) high. These walls of water spilled onto the land, car- rying boats and cargo more than 0.5 km inland. As the seawater with- drew, it dragged people and debris from the earthquake-shattered structures back to the ocean.

The two earthquakes killed almost 70,000 people and destroyed or seriously damaged about 90% of the buildings in Lisbon ( figure 3.1 ). At the time, the city was rich in bullion, jewels, and merchandise, and it had great commercial and cultural importance. The destruction of this famous city by earthquakes and their resulting sea waves and fires was a shock to Western civilization. Not only were the losses of lives and buildings staggering, but the fires also incinerated irre- placeable libraries, maps and charts of the Portuguese voyages of

Figure 3.1 The Lisbon earthquake.

discovery, and paintings by such masters as Titian, Correggio, and Rubens. The Lisbon earthquakes did more than devastate a city; they changed the prevailing philosophies of the era. All was not well in the world after all.

When mountains fall headlong over hollow places they shut in the air within their caverns, and this air, in order to escape, breaks through the Earth, and so pro- duces earthquakes.

Despite the profound effects that earthquakes have had on civilizations for so many centuries, scientific observations did not begin until the early 19th century, when good descrip- tions were made of earthquake effects on the land. Today, less than two centuries later, our knowledge of earthquakes has increased enormously. We have a fairly comprehensive understanding of what earthquakes are, why and where they happen, and how big and how often they occur at a given site. Our scientific data and theories allow us to understand phe- nomena that even the greatest minds of the past could not have glimpsed. Such are the rewards from the pyramidal building of knowledge we call science.

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52 Chapter 3 Earthquake Geology and Seismology

To visualize this fault movement, snap your fingers. As you prepare your finger snap, you push your thumb and fin- ger together and sideways, but friction resists their moving past each other. When stress builds high enough, your thumb and finger slip rapidly, releasing energy as sound waves. Both a fault rupture in the earth and your finger snap feature the same sudden slips that release energy in waves.

FAULTS AND GEOLOGIC MAPPING The 19th-century recognition that fault movements cause earthquakes was a fundamental advance that triggered a whole new wave of understanding. With this relationship in mind, geologists go into the field to map active faults, which in turn identifies earthquake-hazard belts. Because a fault moves formerly continuous rock layers apart, the careful mapping of different rock masses can define sharp lines that separate offset segments of single rock masses. Fault sur- faces can be vertical, horizontal, or at any angle to Earth’s surface. Some faults rupture the ground, some do not.

The principles that help us understand faults begin with some of the earliest recognized relationships about rocks, which are still useful today. In 1669, the Danish physician Niels Steensen, working in Italy and known by his Latinized name of Steno, set forth several laws that are fundamental in interpreting geologic history. His law of original horizontality explains that sediments (sands, gravels, and muds) are originally depos- ited or settled out of water in horizontal layers. This is important because some older sedimentary rock layers are found at angles ranging from horizontal to vertical. But since we know they started out as horizontal layers ( figure 3.3 ), their postdeposi- tional history of deformation can be unraveled by mentally returning their orientations back to horizontal ( figure 3.4 ).

Understanding Earthquakes The word earthquake is effectively a self-defining term—the Earth quakes, the Earth shakes, and we feel the vibrations. Earth- quakes, or seisms, may be created by volcanic activity, meteorite impacts, undersea landslides, explosions of nuclear bombs, and more; but most commonly, they are caused by sudden earth movements along faults. A fault is a fracture surface in the Earth across which the two sides move past each other ( figure 3.2 ). Stresses build up in rocks, but friction along fault surfaces holds the rocks together. When stress builds high enough, the rocks along the fault snap and move suddenly, releasing energy in waves we feel as the shaking of an earthquake.

Figure 3.2 Offset of tilled farmland by 1979 movement of the Imperial fault, southernmost California. View is to the east; the west side of the fault (closest to you) has moved northward (to your left). Photo courtesy of Pat Abbott.

Figure 3.3 North wall of the upper Grand Canyon. At the canyon bottom, the once horizontal sedimentary rock layers have been tilted to the east. Their uptilted ends have been eroded and buried by horizontal younger rock layers. Photo by John S. Shelton.

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Understanding Earthquakes 53

In the law of superposition, Steno stated that in an unde- formed sequence of sedimentary rock layers, each successive layer is deposited on top of a previously formed, and hence older, layer. Thus, each sedimentary rock layer is younger than the bed beneath it but older than the bed above it (figures 3.3 and 3.4).

Figure 3.5 Aerial photo of part of South Island, New Zealand (see figure 3.6 for location). The Alpine fault cuts a prominent slash from near the lower left (southwest) corner of the photo to the top center (northeast). Arrowheads line up with the fault. Photo courtesy of Pat Abbott.

Steno’s law of original continuity states that sediment layers are continuous, ending only by butting up against a topographic high, such as a hill or a cliff, by pinching out due to lack of sediment, or by gradational change from one sedi- ment type to another. This relationship allows us to appreci- ate the incongruity of a sedimentary rock layer that abruptly terminates. Something must have happened to terminate it. For example, a stream may have eroded through it, or a fault may have truncated it. Geologists spend a lot of time locating and identifying offsets of formerly continuous rock layers. In this way, we can determine the lengths of faults and estimate the magnitude of earthquakes they produce. Longer lengths of fault rupture create bigger earthquakes.

On a much broader scale, we can find large offsets on long-acting, major faults. figure 3.5 shows a pronounced line cutting across the land in a northeast-southwest trend; this is the Alpine fault on the South Island of New Zealand. The west (left) side has been moved 480 km (300 mi) toward the north. In Otago province in the southern part of the South Island, gold was discovered in 1861 in stream gravels. This set off a gold rush that brought in prospectors and miners from all over the world. The gold fever that had attracted so many fortune seek- ers to California in 1849 now moved to New Zealand. Prospec- tors panned the streams and worked their way upstream into bedrock hills to find the source of the gold. Yet much of the

Figure 3.4 These sedimentary rocks were deposited in horizontal layers, but have since been compressed into contorted layers by movements of the San Andreas fault. Photo by John Shelton.

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Thus, many mines consist of adits (passages) dug along old, inactive faults. Ores are common along faults because when one block of rocks moves past another in a fault zone, the tremendous friction tends to shatter and pulverize the rocks in the fault zone. The broken rock creates an avenue of permeability through which water can flow. If the under- ground water carries a concentration of dissolved metals, they may precipitate as valuable elements or minerals within the fault zone. Early miners working in excavated fault zones called the floor beneath their feet the footwall and the rocks above their heads the hangingwall ( figure 3.8 ). This terminology is used to define the types of faults dominated by vertical movements, called dip-slip faults. Faults with the major amounts of their offset in the dip or vertical direction are caused by either a pulling ( tension) or a pushing ( compression) force.

wealth lay 480 km to the northeast in Nelson province, where the same gold-bearing rock masses had been offset along the Alpine fault by more than 23 million years of fault movements ( figure 3.6 ). As this example shows, fault studies also can have tremendous implications for locating mineral wealth.

Types of Faults As tectonic plates move, mountains are elevated and basins are warped downward. The brittle rocks of the lithosphere respond by fracturing (also called jointing or cracking). When regional forces create a large enough stress differential in rocks on either side of a fracture, then movement occurs and the frac- ture becomes a fault. Accumulated movements of rocks along faults range from millimeters to hundreds of kilometers. These movements can cause originally horizontal sedimentary rock layers to be tilted and folded into a wide variety of orientations ( figure 3.7 a). To describe the location in three-dimensional (3-D) space of a deformed rock layer, a fault surface, or any other planar feature, geologists make measurements known as dip and strike. Dip is seen in the two-dimensional (2-D) verti- cal view ( cross-section ) as the angle of inclination from the horizontal of the tilted rock layer ( figure 3.7 b). It is also impor- tant to note the compass direction of the dip in the horizontal plane—for example, toward the northeast. Strike is viewed in the 2-D horizontal view ( map ) as the compass bearing of the rock layer where it pierces a horizontal plane.

DIPSLIP FAULTS The classification of faults uses some terminology of early miners. Many ore veins were formed in ancient fault zones.

Figure 3.6 Generalized geologic map of South Island, New Zealand. Each map color records a different type of rock. Locate the Alpine fault, and then match up the rock patterns across the fault. The gold-bearing rocks near Queenstown have been offset 480 km (300 mi) to near Nelson.

Queenstown

Nelson block

Nelson

N

Fiordland block

Otago schists

0 100 200 300 km

Area covered by photo in figure 3.5

Alpin e

fault

Alp ine

faul t

Figure 3.7 (a) A 75-million-year-old sandstone layer at La Jolla Bay, California, exposed at a moderately high tide. The sea surface forms a horizontal plane against the inclined sandstone bed. (b) The strike of a rock layer is the compass bearing of the “shoreline.” The dip angle is the number of degrees below horizontal that the rock layer is inclined. Photo by Pat Abbott.

Strike DipHorizontal line of

intersection

Water surface

(b)

Dip angle

Dip dire

ctio n

(a)

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Types of Faults 55

repeated, when viewed in cross-section ( figure 3.10 ). With compressional forces, the hangingwall moves upward rela- tive to the footwall; this type of fault is referred to as a reverse fault. The compressional motions of reverse faults are commonly found at areas of plate convergence where subduction or continental collision occurs.

The extensional versus compressional origins of move- ment can have enormous economic implications. Look again at figures 3.9 and 3.10. Visualize the emphasized (dotted) rock layer in each figure as being an oil reservoir. Now imag- ine yourself to be the landowner above either the zone of omission or the zone of repetition. In one case, it could mean poverty; in the other, great wealth.

There are two major types of dip-slip faults: normal faults and reverse faults. A normal fault occurs when the hangingwall moves down relative to the footwall. The domi- nant force is extensional, as recognized by the separation of the pulled-apart rock layers in a zone of omission ( figure 3.9 ). The word normal as a name for this type of fault is unfortu- nate because it carries a connotation of normalcy, as if this were the standard or regular mode of fault movement; such is not the case. Extensional or normal-style faults are typical of the faults at seafloor spreading centers and in regions of con- tinents being pulled apart.

If the dominant force that creates a fault movement is compressional, then the rock layers are pushed together, or

Figure 3.8 Schematic cross- section of miners excavating ore that precipitated in broken rock within an old fault zone. Notice that the rock layers in the footwall and hangingwall are no longer continuous; this gives evidence of the movements that occurred along the fault in the past.

Mine Ground surface

Hangingwall

Footwall

Figure 3.9 Schematic cross- section of a normal fault; that is, the hangingwall has moved downward (in a relative sense). Extensional forces are documented by the zone of omission, where the originally continuous rock layers are missing. The small arrows indicate movement; the larger arrows show force.

Zone of

omission

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that this convention for naming the fault works no matter which way you are straddling the fault; try it facing both directions with figure 3.11 . Simi- larly, if features on the left-hand side of the fault have moved closer to you, then it is a left-lateral, or sinistral, fault.

We have looked at a large strike- slip fault in New Zealand, the Alpine fault, but the most famous strike-slip fault in the world is the San Andreas in California. This right-lateral fault is more than 1,300 km (800 mi) long. On 18 April 1906, a 430 km (265 mi) long segment of the San Andreas fault ruptured and moved horizon- tally as much as 6.5 m (20 ft) in 60 seconds. The great burst of energy generated by the fault move- ment was actually the release of

elastic energy that had built up and been stored in the rocks for many decades.

Faults are not simple planar surfaces that glide readily when subjected to stress. Instead, faults are complex zones of breakage where rough and interlocking rocks are held together over an irregular surface that extends many miles below the ground. Stress must build up over many years before enough potential energy is stored to allow a rupture on a fault. The initial break occurs at a weak point on the fault and then propagates rapidly along the fault surface. Much of the energy stored in the rocks is released as radiating seismic waves that humans call an earthquake. The point where the fault first ruptures is known as the hypocenter, or focus. The point on Earth’s surface directly above the hypocenter is called the epicenter ( figure 3.12 ).

A fault rupture is not a simple, one-time movement that produces “the earthquake.” In fact, we never have just one earthquake. The stresses that build up in the rocks in an area are released by a series of movements along the fault, or several faults, that continue for weeks to months to years. Each fault movement generates an earthquake.

Steps in Strike-Slip Faults Strike-slip faults do not simply split the surface of Earth along perfectly straight lines. The rupturing fault tears apart the rocks along its path in numerous subparallel breaks that stop and start, bend left, and bend right. For analogy, visualize a sheet cake or pan of moist mud. Put your right hand on the upper right corner and your left hand on the lower left corner. Now pull toward you with your right hand and push away with your left. Do you visualize the cake ripping along one straight line? Or along several breaks that stop and start, bend left and bend right? So it is with Earth when it ruptures during an earthquake- generating fault movement. Normal and reverse faults also have bends; we just don’t see them as easily on the surface.

STRIKESLIP FAULTS When stress produces shear and causes most of the movement along a fault to be horizontal (parallel to the strike direction), the fault is referred to as a strike-slip fault. These fault offsets are seen in map view as though from a balloon or airplane looking down on Earth’s surface. Strike-slip faults are further classified on the basis of the relative movement directions of the fault blocks. If you straddle a fault and the block on your right-hand side has moved relatively toward you, then it is called a right-lateral, or dextral, fault ( figure 3.11 ). Notice

Figure 3.10 Schematic cross-section of a reverse fault; that is, the hangingwall has moved upward (in a relative sense). Compressional forces are documented by the zone of repetition, where the originally continuous rock layers have been split, shoved together, and stacked above each other.

Zone of

repetition

Figure 3.11 Map of a right-lateral, strike-slip fault. As the man straddles the fault, the right-hand side of the fault has moved relatively closer to him. If he turns around, will the right-hand side of the fault still have moved closer to him?

Right-lateral fault

Straddle the fault; right-hand side moves toward you.

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The bends along a fault have profound implications for the creation of topography. figure 3.13 a is a sketch of a right-lateral fault with a bend (step) in it—a left-stepping bend. Stand to either side of the fault and look at the region of the bend. Note that the fault segment left of the bend is closest to you; hence, this is a left-stepping, right-lateral fault. Notice what occurs at the bend in the fault when the two sides slide past each other—compression, pushing together, collision, constraint. The photo in figure 3.13 b shows a left step in the right-lateral Superstition Hills fault west of Brawley, Califor- nia, which was created on 16 November 1987. Notice how the compression at the bend produced a little hill. What size could this hill attain if movements at this left step were to occur for millions of years? It could grow into a mountain.

Similarly, figure 3.14 a depicts a right step along a right- lateral fault. Visualize what happens at the bend in the fault. In this case, the two sides pull apart from each other, extend, diverge, release. The photo in figure 3.14 b is from the same earthquake, along a different length of the same fault, as in figure 3.13 b. At this right step, the two sides pulled apart and created a down-dropped area—a wide crack or a little basin.

TRANSFORM FAULTS Transform faults are a special type of horizontal-movement fault first recognized by the Canadian geologist J. Tuzo Wilson in 1965. Figure 3.15 depicts how a transform fault

Figure 3.12 Block diagram of a fault surface. The hypocenter (focus) is the point on the fault surface where the rupture began; the epicenter is the point on Earth’s surface directly above the hypocenter. Notice that because the fault surface is inclined (it dips), the epicenter does not plot on the trace of the fault at the surface. Source: J. Ziony, ed., “Earthquakes in the Los Angeles Region.” US Geological Survey .

Rupture area

Slip

Surface rupture

Hypocenter

Epicenter

Fault line on surface

Fault surface

Figure 3.13 (a) Left step in right-lateral fault. Notice that the land is pushed together at the fault bend whenever the fault moves. Movements will create a hill, which could grow to a mountain if the fault remains active for a long enough time. (b) Land offset along the Superstition Hills right-lateral fault during its 16 November 1987 earthquake. See the left step and the uplift at the bend. (Black Arrows indicate directions of land movement.) Photo by Pat Abbott.

Left-stepping, right-lateral fault

Stand to the side, look at bend in fault; left-hand side steps toward you.

Push

Together

(a)

(b)

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Figure 3.15 Plate-tectonic model of a transform fault. Notice that the transform fault connects the two separated spreading centers; the seafloor moves in opposite directions here. Beyond the spreading centers, the two plates move in the same direction and are separated by a fracture zone; there is no transform fault here.

Fracture zone Transform fault

Asthenosphere

Lithosphere

Magma

Spreading center

Deep-ocean trench

Sp re

ad in

g ce

nt er

S ub

du ct

io n

C on

tin en

t

zo ne

Figure 3.14 (a) Right step in right-lateral fault. Notice that the land is pulled apart at the fault bend whenever the fault moves. Movements will create a hole, which could become a basin if the fault stays active for a geologically long time. (b) Land offset along the Superstition Hills right- lateral fault during its 1987 rupture. See the right step and the pull apart at the bend. (Black Arrows indicate directions of land movement.) Photo by Pat Abbott.

Right-stepping, right-lateral fault

Stand to the side, look at bend in fault; right-hand side steps toward you.

Pu ll

Ap art

(a)

(b)

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device known was invented in China in 132 ce by Chang Heng. The modern era of seismologic instrumentation began about 1880. Instrumenta- tion continues to evolve through many different styles, but a basic need is to record the 3-D move- ment of earthquake waves. This is achieved by having instruments detect Earth motions ( seis- mometers ) and record them ( seismographs ) as north-south horizontal movements, east-west horizontal movements, and vertical movements. To accurately record the passage of seismic waves, a seismometer must have a part that remains as stationary as possible while the whole Earth beneath it vibrates. One way to accomplish this is by building a frame that suspends a heavy mass ( figure 3.16 ). The support frame rests on Earth and moves as Earth does, but the mass sus- pended by a wire must have its inertia overcome before it moves. The principle of inertia explains that a stationary object—for example, the sus- pended mass—tends to remain stationary. The differences between motions of the frame and the hanging mass are recorded on paper by pen and ink or, increasingly, as digital data. Visualize the process this way: hold an ink pen steady in your hand and then vibrate the entire Earth beneath your pen to make an inked line.

Other important pieces of information to record include the arrival times and the durations of the various seismic waves. This is accom-

plished by having time embedded in the seismographic record either as inked tick marks on the paper graph or within the digital data. Time is standardized in the United States by the national clock in Boulder, Colorado.

First-order analysis of the seismic records allows seismologists to identify the different kinds of seismic waves generated by the fault movement, to estimate the amount of energy released (magnitude), and to locate the epicenter/ hypocenter (where the rock hit the water, so to speak).

WAVES Throw a rock into a pond, play a musical instrument, or experience a fault movement, and the water, the air, or the Earth will transmit waves of energy that travel away from the initial disturbance. All these waves have the following similarities: amplitude, the height of the wave above the starting point ( figure 3.17 ); wavelength, the distance between successive waves; period, the time between waves measured in seconds; and frequency, the number of waves passing a given point during 1 second. Frequencies are measured in hertz (Hz) , where 1 Hz equals one cycle per

forms. Seafloor crust forms at oceanic volcanic ridges and is pulled apart by gravity and slab pull of subducting plates. When plates collide, the denser plate subducts. But what happens along the sides of the plates? They slide past each other at trans- form faults. Visualize this process in three dimensions. The spreading plates are rigid slabs of oceanic rock, tens of kilome- ters thick, that are being wrapped around a near-spherical Earth. How does a rigid plate move about a curved surface? The plates must fracture, and these fractures are transform faults. In fact, transform faults must link spreading centers or connect spreading centers with subduction zones.

In figure 3.15 , notice that in the region between the two spreading centers, the relative motions of the two plates are in opposite directions in typical strike-slip fault fashion. However, passing both to the right and left of the spreading centers, notice that the two slabs are moving in the same direction and there they are called fracture zones there is no active offset across a fracture zone.

Development of Seismology The study of earthquakes is known as seismology (after seism, meaning “earthquake”). The earliest earthquake-indicating

Figure 3.16 A basic seismograph. Earth moves, the seismograph framework moves, and the hanging wire vibrates, but the suspended heavy mass and pen beneath it remain relatively steady. Ideally, the pen holds still while Earth moves beneath the pen to produce an inked line. Three seismometers sensing vibrations in orthogonal directions of ground shaking are required to record the full 3-D shaking at a point.

Pen

Seismograph

Heavy weight does not move

Wire vibrates

Framework

Concrete base moves

Earth moves

moves

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Secondary Waves The secondary (S) wave is the second wave to reach a recording station. S waves are transverse waves that propa- gate by shearing or shaking particles in their path at right angles to the direction of advance. This motion is probably most easily visualized by considering how a jump rope moves when you shake one end up and down ( figure 3.18 b). S waves travel only through solids. S waves do not propagate through fluids. On reaching fluid or gas, the S wave energy is reflected back into rock or is converted to another form. The velocity of an S wave depends on the density and resis- tance to shearing of materials. Fluids and gases do not have shear strength and thus cannot transmit S waves. Representa- tive velocities for S waves in dense rocks (e.g., granite) are about 3 km/sec (about 6,700 mph). With their up-and-down and side-to-side motions, S waves shake the ground surface and can do severe damage to buildings.

SEISMIC WAVES AND EARTH’S INTERIOR Large earthquakes generate body waves energetic enough to be recorded on seismographs all around the world. These P waves and S waves do not follow simple paths as they pass through Earth; they speed up, slow down, and change direc- tion, and S waves even disappear. Analysis of the travel paths of the seismic waves gives us our models of Earth’s interior ( figure 3.19 ). Earth is not homogeneous. Following the paths of P and S waves from Earth’s surface inward, there is an initial increase in velocity, but then a marked slowing occurs at about 100 km (62 mi) depth; this is the top of the astheno- sphere. Passing farther down through the mantle, the veloci- ties vary but generally increase until about 2,900 km (1,800 mi) depth; there, the P waves slow markedly and the S waves disappear. This is the mantle-core boundary zone. The disappearance of S waves at the mantle-core boundary, due to their reflection or conversion to P waves, indicates that the outer core is mostly liquid. Moving into the core, P wave velocities gradually increase until a jump is reached at about 5,150 km (3,200 mi) depth, suggesting that the inner core is solid.

SURFACE WAVES Surface waves are created by body waves disturbing the sur- face. They are of two main types—Love waves and Rayleigh waves. Both Love and Rayleigh waves are referred to as L waves (long waves) because they take longer periods of time to complete one cycle of motion and are the slowest moving. The frequencies of surface waves are low—less than one cycle per second. The low-frequency, long-period waves carry significant amounts of energy for much greater dis- tances away from the epicenter.

Love Waves Love waves were recognized and first explained by the British mathematician A. E. H. Love. Their motion is

second. Note that period and frequency are inversely related:

1 Period �

frequency (in hertz)

For example, if five waves passed a given point in 1 second, then the frequency is 5 Hz and the period of time between each wave is 0.2 second.

Seismic Waves When a fault slips, or an explosion occurs, it releases energy in seismic waves that pass through the whole body of the planet ( body waves ) and others that move near the surface only ( surface waves ).

BODY WAVES Body waves are the fastest and are referred to as either pri- mary or secondary waves. Body waves ranging from about 0.02 Hz to tens of Hz produce measurable ground shaking. These high-frequency, short-period waves are most energetic for short distances close to the hypocenter/epicenter.

Primary Waves The primary (P) wave is the fastest and thus the first to reach a recording station. P waves move in a push-pull fashion, alternating pulses of compression (push) and extension (pull); this motion is probably best visualized using a Slinky toy ( figure 3.18 a). P waves radiate outward from their source in an ever-expanding sphere, like a rapidly inflating balloon. They travel through any material, be it solid, liquid, or gas. Their speed depends on the density and compressibility of the materials through which they pass. The greater the resistance to compression, the greater the speed of the seismic waves passing through packed atomic lattices. Representative veloc- ities for P waves in hard rocks (e.g., granite ) are about 5.1 to 5.5 km/sec (about 11,400 to 12,300 mph). P waves in water slow to 1.4 km/sec (about 3,100 mph). Because P waves and sound waves are both compressional waves, they can travel through air. P waves may emerge from the ground, and if you are near the epicenter, you may be able to hear those P waves pulsing at around 15 cycles per second as low, thunderous noises. The arrival of P waves at your home or office is simi- lar to a sonic boom, including the rattling of windows.

Figure 3.17 Wave motion. Amplitude is the height of the wave above the starting point. Wavelength is the distance between wave crests B and A. Period is the amount of time in seconds for wave crest B to travel to site A.

B A Wavelength

Travel direction

Amplitude

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Seismic Waves 61

Rayleigh Waves Rayleigh waves were predicted to exist by Lord Rayleigh 20 years before they were actually recognized. They advance in a backward-rotat- ing, elliptical motion ( figure 3.18 c) similar to the orbiting paths of water molecules in wind-blown waves of water, except that waves in water are forward-rotating ( figure 3.18 d). The shaking produced by Rayleigh waves causes both vertical and horizontal movement. The shallower the hypo- center, the more P and S wave energy will hit the surface, thus putting more energy into Rayleigh waves. The rolling waves pass through both ground and water. The often-heard report that an earthquake feels like being rocked in a boat at sea well describes the passage of Rayleigh waves. These waves have long periods, and once started, they go a long way.

SOUND WAVES AND SEISMIC WAVES Waves are fundamental to both music and seismology. Musicians use instruments to produce the sound waves we hear as music. For exam- ple, a trombone player controls the amount of sound with his breath, and changes the frequencies of the sound waves by extending and retracting the slide on the trombone. Earth- quakes generate body and surface waves; seismologists record and analyze the seismic wave patterns to determine where the earthquake occurred and how much energy was released during the event.

Music is a common part of our lives and we are familiar with hear- ing sound waves. Sound waves and seismic waves can be presented in the same visual form. Waveforms for a trombone and a moderate-size

earthquake are shown in figure 3.20 . Both a trom- bone and an earthquake have more higher-frequency waves if a shorter path is traveled—that is, the trom- bone is retracted and has a short length, and the fault- rupture length is short. As the travel paths become longer for both trombone (extended) and earthquake (longer fault rupture), the number of low-frequency

similar to that of S waves, except it is from side-to-side in a horizontal plane roughly parallel to Earth’s surface. As with S waves, their shearing motion is at right angles to the direction of advance; to understand this, visualize the jump rope in figure 3.18 b lying on the ground. Love waves gener- ally travel faster than Rayleigh waves. Like S waves, they do not move through water or air.

Figure 3.18 Types of seismic waves. (a) P waves exhibit the push-pull motion of a Slinky toy. (b) S waves move up and down perpendicular to the direction of advance, like a shaken jump rope. (c) Rayleigh waves advance in a backward-rotating motion, as opposed to (d) wind-blown ocean waves, which cause water to move in forward-rotating circles.

(d)

Ocean surface Ocean waves

beach

(b)

(c)

P wave

S wave

Direction of Rayleigh wave motion

(a)

Wind

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Figure 3.19 Varying velocities of P waves and S waves help define the internal structure of Earth.

Asthenosphere (“soft plastic”)

Lithosphere (solid)

Hydrosphere (liquid)Atmosphere

Mantle

Outer core

Inner core

(solid)

Wave velocity (km/sec)

S wave

P wave

P wave

4 6 8 10 12 14

0

1,000

2,000

3,000

4,000

5,000

6,000

D ep

th (

km )

Figure 3.20 Comparison of wave patterns for a trombone and an earthquake for short and long-distance travel paths. Source: A. Michael, S. Ross, and D. Schaff, “The Music of Earthquakes; Waveforms of Sound and Seismology” in American Scientist (2002).

One Note or Source on the Trombone But Varying the Path

Slide retracted — short path

Slide extended — long path

Time 0.01

second

A ir

pr es

su re

Magnitude 5.1 Earthquake at Two Distances

Station 10 km from earthquake

Station 120 km from earthquake

Time

Relative magnification 36x

2 seconds

G ro

un d

ve lo

ci ty

waves increases. Musically, as the path through the trombone lengthens, the vibrations per second decrease, the frequencies are lower, and the tone is lower. Seismically, a rupturing fault sends off high-frequency seismic waves, and as the fault rup- ture grows longer, more low-frequency seismic waves are generated. The ranges of some common frequencies are listed in table 3.1 .

Locating the Source of an Earthquake Using the lengths of time the various seismic waves take to reach a seismograph, the locations of the epicenter and hypo- center can be determined. P waves travel about 1.7 times faster than S waves. Thus, the farther away from the

earthquake origin, the greater is the difference in arrival times between P and S waves ( figure 3.21 ). When a seismograph records an earthquake, the difference in arrival times of P and S waves is determined by subtracting the P arrival time from the S time (S–P). Inspection of the seismogram in figure 3.22 shows that S waves arrived 11 minutes after P waves. figure 3.21 indicates that an S–P arrival time difference of 11 minutes cor- responds to an earthquake about 8,800 km (5,400 mi) away. But in what direction?

Epicenters can be located using seismograms from three recording stations. As an example, S–P wave arrival time differences yield distances to the epicenter of 164 km (102 mi) from Memphis State University in Tennessee, 236 km (146 mi) from St. Louis University in Missouri, and 664 km (412 mi) from Ohio State University in Columbus. If the distance from each station is plotted as the radius of a

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Magnitude of Earthquakes 63

Some Common Frequencies (in hertz)

Sound Waves 30,000 Hz—heard by dogs 15–20 Hz to 15,000–20,000 Hz—range of human hearing 15–20 Hz—P waves in air heard by humans near epicenter

Seismic Waves 0.02–30 Hz—body waves

0.002–0.1 Hz—surface waves

TABLE 3.1

Figure 3.21 Plot of travel time versus distance from earthquake for seismic waves. Note that the arrival time difference for P and S waves of 11 minutes in figure 3.22 corresponds to a distance of about 8,800 km (5,400 mi).

circle, the three circles will intersect at one unique point—an epicenter at New Madrid, Missouri ( figure 3.23 ). Computers usually make the calculations to determine epicenter locations; however, a better mental picture of the process is gained via the hand-drawn circles.

The difference in arrival times of P and S waves (S–P) actually measures the distance from the recording station to the hypocenter (or focus) of the earthquake, the site of initial fault movement (see figure 3.12 ). If the hypocenter is on Earth’s surface, then the hypocenter and epicenter are the same. How- ever, if the hypocenter is deep below the surface, it will affect the arrival time of surface (L) waves because L waves do not begin until P waves strike the Earth’s surface. The depth to a

hypocenter is best determined where an array of seismometers is nearby, thus allowing careful analysis of P wave arrival times.

Magnitude of Earthquakes Magnitude is an estimate of the relative size or energy release of an earthquake. It is commonly measured from the seismic wave traces on a seismogram.

RICHTER SCALE In 1935, Charles Richter of the California Institute of Technology devised a quantitative scheme to describe the magnitude of California earthquakes, specifically events with shallow hypocenters located near (less than 300 mi from) the seismometers. Richter based his scale on the idea that the bigger the earthquake, the greater the shaking of Earth and thus the greater the amplitude (swing) of the lines made on the seismogram. To standardize this relationship, he defined magnitude as:

the logarithm to the base ten of the maximum seismic wave amplitude (in thousandths of a millimeter) re-corded on a standard seismograph at a distance of 100 kilometers from the earthquake center.

M in

ut es

a fte

r st

ar t o

f e ar

th qu

ak e

Distance from earthquake in kilometers

25

20

15

L

S

P

10

5

0 2,000 4,000 6,000 8,000 10,000

Su rfa

ce w

av es

S w ave

P wa ve

11 min

P S

11 minutes

Figure 3.22 Seismogram recorded in Finland of the Sumatran earthquake on 26 December 2004. Notice that the difference in arrival times of P and S waves is 11 minutes. See figure 3.21 to read the distance traveled by the seismic waves.

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64 Chapter 3 Earthquake Geology and Seismology

step up in magnitude); it certainly does not increase as much as the values in table 3.2 might lead us to think. In effect, the bigger earthquake means that more people in a larger area and for a longer time will experience the intense shaking. A longer duration of shaking can greatly increase the amount of damage to buildings.

Computing a Richter magnitude for an earthquake is quickly done, and this is one of the reasons for its great popu- larity with the deadline-conscious print and electronic media. Upon learning of an earthquake, usually by phone calls from reporters, one can rapidly measure (1) the amplitude of the seismic waves and (2) the difference in arrival times of P and S waves. Figure 3.24 has reduced Richter’s equation to a nomo- graph, which allows easy determination of magnitude. Take a couple of minutes to figure out the magnitude of the earth- quake whose seismogram is printed above the nomograph.

Each year, Earth is shaken by millions of quakes that are recorded on seismometers. Most are too small to be felt by humans. Notice the distinctive “pyramidal” distribution

Because not all seismometers will be sitting 100 km from the epicenter, corrections are made for distance. Richter assigned simple, whole numbers to describe magnitudes; for every 10-fold increase in the amplitude of the recorded seismic wave, the Richter magnitude increases one number—for example, from 4 to 5. The energy released by earthquakes increases even more rapidly than the 10-fold increase in amplitude of the seismic wave trace. For example, if the amplitude of the seismic waves increased 10,000 times (10 � 10 � 10 � 10), the Richter magnitude would move up from a 4 to an 8. However, the energy release from 4 to 8 increases by 2,800,000 times ( table 3.2 ).

What does this increase mean in everyday terms? If you feel a magnitude 4 earthquake while sitting at your din- ner table, and then a magnitude 8 comes along while you are still at the table, would you really be shaken 2,800,000 times as hard? No. The greater energy of the magnitude 8 earthquake would be spread out over a much larger area, and over a time interval about 20 times longer (e.g., 60 seconds as opposed to 3 seconds). At any one location, the felt shaking in earthquakes above magnitude 6 does not increase very much more (maybe three times more for each

Figure 3.23 Location of an earthquake epicenter. S–P arrival time difference calculations gave a radius of 164 km from Memphis, 236 km from St. Louis, and 664 km from Columbus. The circles plotted with these values intersect uniquely at New Madrid, Missouri—the epicenter.

Columbus

New Madrid

St.Louis

Memphis Earthquakes in the World Each Year

Magnitude

Number of Quakes per Year Description

8.5 and up 0.3

8–8.4 1 Great

7.5–7.9 3

7–7.4 15 Major

6.6–6.9 56

6–6.5 210 Strong (destructive)

5–5.9 800 Moderate (damaging)

4–4.9 6,200 Light

3–3.9 49,000 Minor

2–2.9 350,000 Very minor

0–1.9 3,000,000

TABLE 3.3

Energy of Richter Scale Earthquakes

TABLE 3.2

Richter Magnitude

Energy Increase

Energy Compared to Magnitude 4

4 1

5 = 48 Mag 4 EQs 48

6 = 43 Mag 5 EQs 2,050

7 = 39 Mag 6 EQs 80,500

8 = 35 Mag 7 EQs 2,800,000

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Magnitude of Earthquakes 65

OTHER MEASURES OF EARTHQUAKE SIZE An earthquake is a complex event, and more than one number is needed to assess its magni- tude. Although the Richter scale is useful for assessing moderate-size earthquakes that occur nearby, the 0.1- to 2-second-period waves it uses do not work well for distant or truly large earthquakes. The short-period waves do not become more intense as an earth- quake becomes larger. For example, the Rich- ter scale assesses both the 1906 San Francisco earthquake and the 1964 Alaska earthquake as magnitude 8.3. However, using other scales, the San Francisco earthquake is a magnitude 7.8 and the Alaska seism is a 9.2. The Alaska earthquake was at least 100 times bigger in terms of energy.

The Richter scale is now restricted to mea- suring only local earthquakes with moderate magnitudes (noted as M L ). Because earthquakes generate both body waves that travel through Earth and surface waves that follow Earth’s uppermost layers, two other magnitude scales have long been used: m b and M s . The body-wave (m b ) scale uses amplitudes of P waves with 1- to 10-second periods, whereas the surface- wave scale (M s ) uses Rayleigh waves with 18- to 22-second periods. Early on, all magni- tude scales were considered equivalent, but now we know that earthquakes generate differ- ent proportions of energy at different periods. For example, larger earthquakes with their larger fault-rupture surfaces radiate more of their energy in longer-period seismic waves. Thus, for great and major earthquakes, body- wave magnitudes (m b ) will significantly under- estimate the actual size of the earthquake. Even a composite of these three methods of deter- mining earthquake magnitude (M L , m b , and M s ) does not necessarily yield the true size of an earthquake.

Moment Magnitude Scale Seismologists have moved on to other measures to more accurately determine earthquake size. The seismic moment (M o ) relies on the amount of movement along the fault that generated the earthquake; that is, M o equals the shear strength of the rocks times the rupture area of the fault times the average displacement (slip) on the fault. Moment is the most reliable measure of earthquake size; it measures the amount of strain energy released by the movement along the whole rupture surface. Seismic moment has been incorporated into a new earthquake

of earthquakes by size—the smaller the earthquake magni- tude, the greater their numbers ( table 3.3 ). Yet the fewer than 20 major and great earthquakes (magnitudes of 7 and higher) each year account for more than 90% of the energy released by earthquakes. At the upper end of the magnitude scale, the energy increases are so great that more energy is released going from magnitude 8.9 to 9 than from magni- tude 1 to 8. These facts underscore the logarithmic nature of the Richter scale; each step up the scale has major significance.

500

6

100

50

20

10

5

2

1

0.5

0.2

0.1

5

4

3

2

1

0

400

300

200

100

60

50

40

30

20

10 8 6

4

2

40

20

0–5

Distance (km)

S–P (sec)

Magnitude Amplitude (mm)

0 10 20

30

20

10 m ill

im et

er s

Amplitude (peak height)

seconds S–P

(arrival time difference)

P S

Figure 3.24 Nomograph of the Richter scale allowing earthquake magnitudes to be estimated. On the seismogram, read the difference in arrival times of P and S waves in seconds and plot the value on the left column of the nomograph. Next read the amplitude of the peak height of the S wave and plot this value on the far right column. Draw a line between the two marked values, and it will pass through the earthquake magnitude on the center column. Check your answer in Questions for Review at the end of the chapter.

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66 Chapter 3 Earthquake Geology and Seismology

magnitude scale by Thomas Hanks and Hiroo Kanamori, the moment magnitude scale (M w ), where:

Mw � 2/3 log10(Mo) − 10.7

The moment magnitude scale is used for big earth- quakes. It is more accurate because it is tied directly to physical parameters such as fault-rupture area, fault slip, and energy release. For great earthquakes, it commonly takes weeks or months to determine M w because time is required for the aftershocks to define the area of the rupture zone.

The three largest moment magnitudes calculated to date are the 1960 Chile earthquake (M s of 8.5; M w of 9.5), the 1964 Alaska earthquake (M s of 8.3; M w of 9.2), and the 2004

Sumatra event (M w of 9.1). These gigantic earthquakes occurred at subduction zones. A variety of energetic events are placed on a logarithmic scale for comparison in figure 3.25 . Each step or increment up the scale is a 10-fold increase in magnitude.

FORESHOCKS, MAINSHOCK, AND AFTERSHOCKS Large earthquakes do not occur alone; they are part of a series of movements on a fault that can go on for years. The biggest earthquake in a series is the mainshock. Smaller earthquakes that precede the mainshock are foreshocks, and those that follow are aftershocks. Realistically, there are no differences between these earthquakes other than size; they are all part of the same series of stress release on the fault.

A large-scale fault movement increases the stress on adjacent sections of a fault, helping trigger the additional fault movements that we feel as aftershocks. The danger of large aftershocks is especially acute in the three days follow- ing the mainshock. Sometimes a big earthquake is followed by an even bigger earthquake, and then the first earthquake is reclassified as a foreshock.

MAGNITUDE, FAULTRUPTURE LENGTH, AND SEISMICWAVE FREQUENCIES Fault-rupture length greatly influences earthquake magni- tude. As approximations, these fault-rupture lengths yield the following earthquake magnitudes:

• 100 m (328 ft) rupture ≈ magnitude 4 • 1 km (0.62 mi) rupture ≈ magnitude 5 • 10 km (6.2 mi) rupture ≈ magnitude 6 • 40 km (25 mi) rupture ≈ magnitude 7 • 400 km (250 mi) rupture ≈ magnitude 8 • 1,000 km (620 mi) rupture ≈ magnitude 9

A rupture along a fault during an earthquake typically moves 2 to 4 km/sec. A lengthier rupture gives a lengthier duration of movement ( table 3.4 ).

Fault-rupture lengths and durations in seconds also affect the frequencies of seismic waves produced during earthquakes. Faults that move for short distances and short amounts of time generate mostly high-frequency seismic waves. Faults that rupture for longer distances and longer times produce increasingly greater amounts of low-frequency seismic waves.

Seismic waves die off with distance traveled. High- frequency seismic waves die out first—at shorter distances from the hypocenter. Low-frequency seismic waves carry significant amounts of energy farther—through longer dis- tances. High-frequency seismic waves cause much damage at short distances from the epicenter. But at longer distances, it is the low-frequency seismic waves that do most of the damage.

Meteorite impact (10 km diameter, 20 km/sec velocity)

Earth's daily receipt of solar energy

Earth's annual internal heat flow

U.S. annual energy consumption

Chile 1960 earthquake (M9.5)

Average annual seismic energy release on Earth

Hurricane (kinetic energy)

Average annual seismicity in

continent interiors

Average hurricane (10-day lifetime)

Alaska 1964 and Sumatra 2004 earthquakes (M9.2)

World's largest nuclear explosion

Mount St. Helens eruption New Madrid 1812 earthquake (M7.5)

1 megaton nuclear explosion

Hiroshima 1945 atomic bombE ne

rg y

(E rg

s) (

lo ga

rit hm

ic s

ca le

)

Electrical energy of typical thunderstorm

Average tornado (kinetic energy)

Lightning bolt

Equivalent moment magnitude (M) (unitless numbers)

2.0 4.0 6.0 8.0 10.0 12.0

1016

1018

1020

1022

1024

1026

1028

1030

Figure 3.25 Equivalent moment magnitude of a variety of seismic (green dots), human-made (yellow squares), and other phenomena (red triangles). Source: A. C. Johnston, “An earthquake strength scale for the media and the public” in Earthquakes and Volcanoes 22 (no. 5): 214–16. US Geological Survey .

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Ground Motion During Earthquakes 67

Figure 3.26 This inadequately braced house failed due to horizontal acceleration during the 1971 San Fernando earthquake. Photo by Al Boost.

Rupture Length and Duration

Magnitude

Rupture Length (km)

Duration (seconds)

1964 Alaska 9.2 1,000 420

1906 San Francisco, CA 7.8 400 110

1992 Landers, CA 7.3 70 24

1983 Borah Peak, ID 7.0 34 9

2001 Nisqually, WA 6.8 20 6

1933 Long Beach, CA 6.4 15 5

2001 Yountville, CA 5.2 4 2

TABLE 3.4

fast-moving train or on a small boat in high seas. A problem for building designers is that earthquake accelerations have locally been in excess of 1 g. For example, in the hills above Tarzana, California, the 1994 Northridge earthquake gener- ated phenomenal accelerations—1.2 g vertically and 1.8 g horizontally.

PERIODS OF BUILDINGS AND RESPONSES OF FOUNDATIONS The concepts of period and frequency also apply to build- ings. Visualize the shaking or vibration of a 1-story house and a 30-story office building. Do they take the same amount of time to complete one cycle of movement, to shake back and forth one time? No. Typical periods of swaying for build- ings are about 0.1 second per story of height. The 1-story house shakes back and forth quickly at about 0.1 second per cycle. The 30-story building sways much slower, with a period of about 3 seconds per cycle.

The periods of buildings are also affected by their con- struction materials. A building of a given height and design will have a longer period if it is made of flexible materials such as wood or steel; its period will be shorter if it is built with stiff materials such as brick or concrete.

The velocity of a seismic wave depends on the type of rock the wave is traveling through. Seismic waves move faster through hard rocks and slower through softer rocks and loose sediments. Seismic waves are modified by the rocks they pass through; they become distorted. When seismic waves pass from harder rocks into softer rocks, they slow down and thus must increase their amplitude to carry the same amount of energy. Shaking tends to be stronger at sites with softer sediments because seismic waves move more slowly but with greater amplitude.

When seismic waves of a certain period carry a lot of energy and their period matches the period of a building, the

Ground Motion During Earthquakes Seismic waves radiate outward from a fault movement. The interactions among the various seismic waves move the ground both vertically and horizontally. Buildings usually are designed to handle the large vertical forces caused by the weight of the building and its contents. They are designed with such large factors of safety that the additional vertical forces imparted by earthquakes are typically not a problem. Usually, the biggest concern in designing buildings to withstand large earthquakes is the sideways push from the horizontal components of movement ( figure 3.26 ).

ACCELERATION Building design in earthquake areas must account for accel- eration. As seismic waves move the ground and buildings up and down, and back and forth, the rate of change of velocity is measured as acceleration. As an analogy, when your car is moving at a velocity of 25 mph on a smooth road, you feel no force on your body. But if you stomp on the car’s accel- erator and rapidly speed up to 55 mph, you feel a force push- ing you back against the car’s seat. Following the same thought, if you hit the brakes and decelerate rapidly, you feel yourself being thrown forward. This same type of accelera- tive force is imparted to buildings when the ground beneath them moves during an earthquake.

The usual measure of acceleration is that of a free- falling body pulled by gravity; it is the same for all objects, regardless of their weight. The acceleration due to gravity is 9.8 m/sec2 (32 ft/sec2), which is referred to as 1.0 g and is used as a comparative unit of measure. Weak buildings begin to suffer damage at horizontal accelerations of about 0.1 g. At accelerations between 0.1 to 0.2 g, people have trouble keeping their footing, similar to being in the corridor of a

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68 Chapter 3 Earthquake Geology and Seismology

shaking is amplified and resonance can occur. The reso- nance created by shared periods for seismic waves and build- ings is a common cause of the catastrophic failure of buildings during earthquakes.

Understanding the concept of shared periods and reso- nance may be advanced by visualizing a tall flagpole with a heavy metal eagle on top. First, if you shake this pole, you will quickly learn that the pole has a strong tendency to move back and forth only at a certain rate or period. If the flagpole swings a complete cycle in 2 seconds, it has a period of 2 seconds. Second, if seismic waves of a 2-second period begin to shake the ground, the amount of movement of the flagpole starts to increase. The pole is now resonating, the forces it must withstand have increased, and the greater forces created by the combined periods may cause destruction.

Earthquake Intensity— What We Feel During an Earthquake During the tens of seconds that a large earthquake lasts, we feel ourselves rocked up and down and shaken from side to side. It is an emotional experience, and the drama of our personal accounts varies according to our location during the shaking and our personalities. But for personal narratives to have meaning that can be passed on to succeeding genera- tions, common threads are needed to bind the accounts together. In the late 1800s, descriptive schemes appeared that were based on the intensity of effects experienced by people and buildings. The most widely used scale came from the Italian professor Giuseppi Mercalli in 1902; it was modified by Charles Richter in 1956. The Mercalli Intensity Scale has 12 divisions of increasing intensity labeled by Roman numer- als ( table 3.5 ).

Earthquake magnitude scales are used to assess the energy released during an earthquake; earthquake intensity scales assess the effects on people and buildings ( table 3.6 ). The difference between magnitude and intensity can be

illuminated by comparison to a lightbulb. The wattage of a lightbulb is analogous to the magnitude of an earthquake. Wattage is a measure of the power of a lightbulb, and magni- tude is a measure of the energy released during an earthquake.

A lightbulb shining in the corner of a room provides high-intensity light nearby, but the intensity of light decreases toward the far side of the room. The intensity of shaking caused by a fault movement is great near the epicenter, but in general, it decreases with distance from the epicenter. (This generalization is offset to varying degrees by variations in geologic foundations and building styles.)

Mercalli intensities also are crucial for assessing magni- tudes of historical events before there were instrumented records, thus allowing us to assess recurrence intervals between major earthquakes.

DID YOU FEEL IT? Mercalli intensity maps can be generated immediately fol- lowing an earthquake using your input processed by com- puter. After you feel an earthquake, google Did You Feel It? Click on the webpage, enter your ZIP code, and then answer questions such as “Did the earthquake wake you up?” and “Did objects fall off shelves?” Within minutes, a computer- generated intensity map will begin to take shape, providing information so quickly that it helps emergency-response per- sonnel know where to respond to crises. The computer- produced map will show Mercalli intensities for each ZIP code in the region.

MERCALLI SCALE VARIABLES The Mercalli intensity value at a given location for an earth- quake depends on several variables: (1) earthquake magni- tude; (2) distance from the hypocenter/epicenter; (3) type of rock or sediment making up the ground surface; (4) building style—design, kind of building materials, height; and (5) duration of the shaking. These factors must be considered in assessing the earthquake threat to any region and even to each specific building.

F � ma Newton’s second law of motion explains that force (F) is equal to mass (m) times acceleration (a). When a force is applied to a mass, it produces a proportional acceleration. Mass is measured in kilo- grams (kg). Acceleration is in meters per second per second (m/s/s). Force is measured in newtons (1 newton = 1 kg m/s 2 ). A force of 1 newton will give a 1-kg mass an acceleration of 1 meter per sec- ond per second. The sudden slip on a fault is a localized source of energy that creates forces on rock, which result in accelerations that shake us.

In Greater Depth For an analogy, hold your arm upright in front of you and wave

it back and forth. You create rapid acceleration and high velocity, but no damage is done because the mass of your arm is small and the inertial forces are low. However, if a large mass, such as a build- ing weighing thousands of tons, is subjected to the same accelera- tion, the motion produces large inertial forces that are difficult for the building to withstand. If these forces last long enough, the building may fail.

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Earthquake Intensity—What We Feel During an Earthquake 69

TABLE 3.5 Modified Mercalli Scale of Earthquake Intensity

Magnitude Mercalli Intensity Acceleration (% g)

2 and less I–II Usually not felt by people Less than 0.1–0.19

3 III Felt indoors by some people 0.2–0.49

4 IV–V Felt by most people 0.5–1.9

5 VI–VII Felt by all; building damage 2–9.9

6 VII–VIII People scared; moderate damage 10–19.9

7 IX–X Major damage 20–99.9

8 and up XI–XII Damage nearly total More than 100 � more than 1 g

TABLE 3.6 Comparison of Magnitude, Intensity, and Acceleration

I. Not felt except by a very few people under especially favor- able circumstances.

II. Felt by only a few people at rest, especially those on upper floors of buildings or those with a very sensitive nature. Delicately suspended objects may swing.

III. Felt quite noticeably indoors, especially on upper floors, but many people do not recognize it as an earthquake. Vibra- tions are like those from the passing of light trucks. Standing automobiles may rock slightly. Duration of shaking may be estimated.

IV. Felt indoors during the day by many people, outdoors by few. Light sleepers may be awakened. Vibrations are like those from a passing heavy truck or a heavy object striking a building. Standing automobiles rock. Windows, dishes, and doors rattle; glassware and crockery clink and clash. In the upper range of IV, wooden walls and frames creak.

V. Felt indoors by nearly everyone, outdoors by many or most. Awakens many. Frightens many; some run outdoors. Some broken dishes, glassware, and windows. Minor cracking of plaster. Moves small objects, spills liquids, rings small bells, and sways tall objects. Pendulum clocks misbehave.

VI. Felt by all; many frightened and run outdoors. Excitement is general. Dishes, glassware, and windows break in considerable quantities. Knickknacks, books, and pictures fall. Furniture moves or overturns. Weak plaster walls and some brick walls crack. Damage is slight.

VII. Frightens all; difficult to stand. Noticed by drivers of automo- biles. Large bells ring. Damage negligible in buildings of good design and construction, slight to moderate in well-built ordi- nary buildings, considerable in badly designed or poorly built buildings, adobe houses, and old walls. Numerous windows

and some chimneys break. Small landslides and caving of sand and gravel banks occur. Waves appear on ponds; water becomes turbid.

VIII. Fright is general and alarm approaches panic. Disturbs drivers of automobiles. Heavy furniture overturns. Dam- age slight in specially designed structures; considerable in ordinary substantial buildings, including partial collapses. Frame houses move off foundations if not bolted down. Most walls, chimneys, towers, and monuments fall. Spring flow and well-water levels change. Cracks appear in wet ground and on slopes.

IX. General panic. Damage considerable in masonry struc- tures, even those built to withstand earthquakes. Well- built frame houses thrown out of plumb. Ground cracks conspicuously. Underground pipes break. In soft sediment areas, sand and mud are ejected from ground in fountains and leave craters.

X. Most masonry structures are destroyed. Some well-built wooden structures and bridges fail. Ground cracks badly with serious damage to dams and embankments. Large landslides occur on river banks and steep slopes. Railroad tracks bend slightly.

XI. Few, if any, masonry structures remain standing. Great damage to dams and embankments, commonly over great distances. Supporting piers of large bridges fail. Broad fissures, earth slumps, and slips occur in soft and wet ground. Underground pipelines completely out of service. Railroad tracks bend greatly.

XII. Damage nearly total. Ground surfaces seen to move in waves. Lines of sight and level distort. Objects thrown up in air.

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70 Chapter 3 Earthquake Geology and Seismology

In Greater Depth

What to Do Before and During an Earthquake Before We have seen that earthquakes don’t kill us—it is our own build- ings and belongings that fall during the shaking and harm us. What should you do to be prepared for an earthquake? First, walk into each room of your house, assume that strong shaking has begun, and carefully visualize (virtual reality) what might fall—for example, ceiling fan, chandelier, mirror, china cabinet, gas water heater. Now reduce the risk. Nail them. Brace them. Tie them. Velcro them. Lower them. Remove them.

Second, walk outside, assume strong shaking, and visualize what might fall—for example, trees, power lines, brick chimney. Now reduce the risk. Trim them. Chop them. Replace them.

Third, repeat the visits inside and outside your home. This time, locate safe spots where protection exists—for example, under a heavy table, beneath a strong desk, under a bed. Remember these safe spots so you can occupy them quickly when shaking begins. Drop, cover, and hold on.

During After preparing your home, program yourself to stay composed during the shaking. Remember that the severe shaking probably will last only 5 to 60 seconds. So, be calm and protect yourself for 1 minute. In most places, if you are inside, you should stay inside; if you are outside, stay outside. This advice was underscored in the San Simeon earthquake in California on 22 December 2003. Both fatalities occurred to women running out the door of a 19th-century building; they were killed by debris falling off the building front. The people who stayed inside the building were unharmed.

1. Earthquake Magnitude: The relation between magni- tude and intensity is obvious—the bigger the earthquake (the more energy released), the higher the odds are for death and damage.

2. Distance from Hypocenter/Epicenter: The relation between distance and damage also seems obvious; the closer to the hypocenter/epicenter, the greater the dam- age. But this is not always the case, as will be seen in chapter 4 with the 1989 World Series (Loma Prieta) and 1985 Mexico City earthquakes.

3. Foundation Materials: The types of rock or sediment foundation are important. For example, hard rock foun- dations can vibrate at high frequencies and be excited by energetic P and S waves from a nearby epicenter; the shaking of soft or water-saturated sediments can be amplified by surface (L) waves from distant earth- quakes; and steep slopes often fail as landslides when severely shaken.

4. Building Style: Building style is of vital importance. What causes the deaths during earthquakes? Not the shaking of the earth, but the buildings, bridges, and other structures that collapse and fall on us. Earthquakes don’t kill, buildings do. Buildings have frequencies of

vibration in the same ranges as seismic waves. The vibra- tions of high-frequency P and S waves are amplified by (1) rigid construction materials, such as brick or stone, and (2) short buildings. If this type of building is near the epicenter, beware!

The movements of low-frequency surface waves are increased in tall buildings with low frequencies of vibration. If these tall buildings also lie on soft, water- saturated sand or mud and are distant from the epicenter, disaster may strike.

5. Duration of the Shaking: The duration of the shaking is underappreciated as a significant factor in damages suffered and lives lost. Consider the ranges of shaking times in table 3.7 . For example, if a magnitude 7 earthquake shakes vigorously for 50 seconds, rather than 20, the increase in damages and lives lost can be enormous.

A Case History of Mercalli Variables: the San Fernando Valley, California, Earthquake of 1971 The San Fernando Valley (Sylmar) earthquake of 9 February 1971 occurred within the northwestern part of the Los Angeles megalopolis at 6:01 a.m., causing 67 deaths (including nine heart attacks). One of the most critical factors in determining life loss from earthquakes is the time of day of the event. In California, the best time for an earthquake for most people is when they are at home; their typical one- and two-story wood- frame houses are usually the safest buildings to occupy.

1. Earthquake Magnitude: The magnitude was 6.6, with 35 aftershocks of magnitude 4.0 or higher occurring in

Magnitude versus Duration of Shaking Richter

Magnitude Duration of Strong Ground

Shaking in Seconds

8–8.9 30 to 180

7–7.9 20 to 130

6–6.9 10 to 30

5–5.9 2 to 15

4–4.9 0 to 5

TABLE 3.7

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A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971 71

the first 7 minutes after the main shock. This is a lot of energy to release within an urban area.

2. Distance from Epicenter: The distance from the epicen- ter was a fairly consistent variable in this event. A rather regular bull’s-eye pattern resulted from contouring the damages reported in Mercalli numerals ( figure 3.27 ).

3. Foundation Materials: The types of foundation materi- als were not a major factor in this event.

4. Building Style: Poorly designed buildings, bridges, and dams were the major problem. Three people died at the Olive View Hospital with the collapse of its “soft” first story featuring large plate-glass windows. “Soft” first- story buildings support the heavy weight of upper floors without adequate shear walls or braced frames to with- stand horizontal accelerations ( figure 3.28 a). Many of these buildings still exist, despite their known high odds of failure during earthquakes ( figure 3.28 b).

Another hospital failure was responsible for 47 deaths. Some of the pre-1933 buildings at the Veterans Administration Hospital used hollow, clay-tile bricks to build walls designed to carry only a vertical load. Many of the hollow-core clay bricks shattered under horizontal accelerations that measured up to 1.25 times gravity.

Freeway bridges collapsed and took three lives. A freeway bridge is a heavy horizontal mass (roadbed) sus- pended high atop vertical columns. Swaying of these top- heavy masses, which have poor connections between their horizontal and vertical elements, resulted in collapse as support columns moved out from under elevated road- beds. The lessons learned from these 1971 failures had not

Figure 3.27 Contour map of Mercalli intensities from the San Fernando earthquake of 9 February 1971 shows an overall decrease in intensity away from the epicenter.

Pacific Ocean

I–IV

I–IV

VI

VI

VII VIII–XI

V

V

Mexico

California

Nevada Limit of felt area

0 50 100 mi

0 80 160 km

Santa Catalina Island

Lo s A

ng ele

s

San Diego

Salton City

Big Bear City

Palm SpringsSanta Ana

Pasadena

San Fernando

Bakersfield

Las Vegas Fresno

Parkfield

Yosemite Nat'l Park

Sa nta

Ba rba

ra

Figure 3.28 Buildings with “soft” first stories. (a) Bracing is inadequate on the first floor, and there are no shear walls to transmit seismic loads to the ground. Thus, seis- mic stresses are concentrated at the join between the first and second floors. When the ground accelerates to the right, the building lags behind and the first story flattens. (b) This eight- story medical-office building atop a “soft” first story is located in a California city near active faults. Sources: (a) “Improving Seismic Safety of New Buildings,” 1986, Federal Emer- gency Management Agency; (b) Photo by Pat Abbott.

(a)

(b)

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72 Chapter 3 Earthquake Geology and Seismology

Figure 3.29 This freeway collapsed in Los Angeles during the 1994 Northridge earthquake. Vertical supports and horizontal roadbeds move at different periods. If not bound together securely, they separate and fall when shaken. Photo by M. Celebi, US Geological Survey .

been acted upon by 1989, when the Interstate 880 ele- vated roadway collapsed, killing 42 people in Oakland during the World Series earthquake. Failure happened again in Los Angeles in 1994 during the Northridge earthquake ( figure 3.29 ).

5. Duration of Shaking: The strong ground shaking lasted 12 seconds. Earthquakes in the magnitude 6 range typ- ically shake from 10 to 30 seconds (see table 3.7 ). The significance of the relatively short time of strong shak- ing in the San Fernando Valley earthquake is enormous. The Lower Van Norman Reservoir held 11,000 acre- feet of water at the time of the quake. Its dam was begun in 1912 as a hydraulic-fill structure where sediment and water were poured into a frame to create a large mass; this is not the way to build a strong dam. During the earthquake, the dam began failing by landsliding and had lost 30 ft of its height (800,000 cubic yards of its mass) and stood only 4 ft above the water level when the shaking stopped ( figure 3.30 ). If the strong shaking

Figure 3.30 Failure of the Lower Van Norman Dam. (a) A few more seconds of strong shaking would have unleashed the deadly force of 11,000 acre-feet of water on San Fernando Valley residents below the dam. (b) Landsliding lowered the dam by 30 feet. (a) Photo by Al Boost. (b) Data source: US Geological Survey Fact Sheet 096–95, “The Los Angeles Dam Story,” January 1995.

Bedrock

Water level

Water level

Alluvium

After 1971 earthquake

(b)

Before 1971 earthquake

Dam crest

Thin dirt wall

Earthen dam

(a)

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Building in Earthquake Country 73

movements are resisted by the shear walls, and the forces are transmitted back to the ground.

Even a “house of cards” is a shear-wall structure, although each “wall” does not have much strength. The walls must be at right angles and preferably in a simple pattern ( figure 3.31 ). The house of cards is made enormously stron- ger if horizontal and vertical elements are all securely fastened—for example, by taping them together.

A structure commonly built with insufficient shear walls is the multistory parking garage. Builders do not want the added expense of more walls, which eliminate parking spaces and block the view of traffic inside the structure. These buildings are common casualties during earthquakes ( figure 3.32 ).

Bracing is another way to impart seismic resistance to a structure. Bracing gives strength to a building and offers resistance to the up, down, and sideways movements of the ground ( figure 3.33 ). The bracing should be made of ductile materials that have the ability to deform without rupturing.

had lasted another 5 seconds, the dam would have failed and released the water onto a 12-square-mile area below the dam where 80,000 people were at home.

LEARNING FROM THE PAST The 1971 San Fernando Valley earthquake unequivocally demonstrated the hazard in this region. It has been eloquently stated that “past is prologue” and that “those who do not learn the lessons of history are doomed to repeat them.” How well were the lessons of 1971 learned? Another test was painfully administered on 17 January 1994, when the magni- tude 6.7 Northridge earthquake struck the immediately adja- cent area. This time, 57 people died and damages escalated to $30 billion. The same types of buildings again failed, and freeway bridges again fell down. The lessons from 1971 were poorly learned.

Building in Earthquake Country One of the problems in designing buildings for earthquake country is the need to eliminate the occurrence of resonance. This can be done in several ways: (1) Change the height of the building; (2) move most of the weight to the lower floors; (3) change the shape of the building; (4) change the type of building materials; and (5) change the degree of attachment of the building to its foundation. For example, if the earth foundation is hard rock that efficiently transmits short-period (high-frequency) vibrations, then build a flexible, taller building. Or if the earth foundation is a thick mass of soft sediment with long-period shaking (low frequency), then build a stiffer, shorter building. For building materials, wood is flexible and lightweight, has small mass, and is able to handle large accelerations. Concrete has great compressional strength but suffers brittle failure all too easily under ten- sional stress. Steel has ductility and great tensional strength, but steel columns fail under compressive stress.

Ground motion during an earthquake is horizontal, ver- tical, and diagonal—all at the same time. The building com- ponents that must handle ground motion are basic. In the horizontal plane are floors and roofs. In the vertical plane are walls and frames. An important component in building resis- tance is how securely the floors and roofs are tied or fastened to the walls so they do not separate and fail.

SHEAR WALLS AND BRACING Walls designed to take horizontal forces from floors and roofs and transmit them to the ground are called shear walls . In a building, shear walls must be strong themselves, as well as securely connected to each other and to roofs and floors. In a simple building, seismic energy moves the ground, pro- ducing inertial forces that move the roofs and floors. These

Figure 3.31 A “house of cards” is a structure with walls and floors but no strength. Earthquake resistance is greatly increased by tying the walls and floors together with tape. Source: “Improving Seismic Safety of New Buildings,” 1986, Federal Emergency Management Agency.

Stronger

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74 Chapter 3 Earthquake Geology and Seismology

RETROFIT BUILDINGS, BRIDGES, AND HOUSE CONSTRUCTION The process of reinforcing existing buildings to increase their resistance to seismic shaking is known as retrofitting. Figure 3.34 shows how some common designs in building retrofits give seismic strength to a building.

Highway bridges and elevated roadways commonly col- lapse during major earthquakes. Part of the problem comes from the different frequencies of movement of vertical supports and horizontal roadbeds, but part comes from the behaviors of different construction materials. Bridge builders combine steel (for its ductility) with concrete (for its strength). During the 1994 Northridge earthquake, support-column failures occurred as concrete cracked and steel deformed ( figure  3.35 a). The rebuilding process employs additional alternating layers of con- crete and steel to avoid future failures ( figure 3.35 b).

Modern one- and two-story woodframe houses perform well during seismic shaking. Houses must be able to move

Figure 3.33 A six-story building with a braced frame incorporated in its design. Photo by Pat Abbott.

up, down, and sideways without failing. The ability to with- stand earth movements is given by building shear walls and by using bracing and other elements that tie the walls, foun- dation, and roof together ( figure 3.36 ).

For retrofitting, older houses must have these same resisting elements added to the foundation walls that hold the house above the ground. Additionally, much of the damage, injury, and even death during an earthquake occurs inside homes as personal items are thrown about—items such as unsecured water heaters, ceiling fans, cabinets, bookshelves, and electronic equipment. Bolt down or secure with Velcro your personal items so they don’t become airborne missiles inside your home during an earthquake.

BASE ISOLATION When the earth shakes, the energy is transferred to buildings. How can buildings be saved from this destructive energy? One approach is to build structures so huge and strong that an earthquake cannot knock them down. But earthquakes can knock them down. For an example, see the failure of the mas- sive support column in the 6.9 M W even in Kobe, Japan ( figure 3.37 ). If buildings cannot stand up against the most powerful seismic waves, then we need to learn to roll with them. Modern designs employ base isolation whereby devices are placed on the ground or within the structure to absorb part of the earthquake energy. For example, visualize yourself standing on Rollerblades during an earthquake. Would you move as much as the earth? Base isolation uses wheels, ball bearings, shock absorbers, “rubber doughnuts,” rubber and steel sandwiches, and other creative designs to isolate a building from the worst of the ground shaking ( fig- ure 3.38 ). The goal is to make the building react to shaking much like your body adjusts to accelerations and decelera- tions when you are standing in a moving train or bus. This concept has recently been used in building San Francisco’s new airport terminal. The 115-million-pound building rests on 267 stainless steel sliders that rest in big concave dishes. When the earth shakes, the terminal will roll up to 20 inches in any direction.

Figure 3.34 How to strengthen buildings. (a) Add braces. (b) Infill walls. (c) Add frames to exterior or interior. (d) Add buttresses. (e) Isolate building from the ground. Source: After AIA/ACSA Council on Architectural Research.

(a) Brace it. (b) Infill it.

(e) Isolate it.

(c) Frame it.

(d) Buttress it.

Figure 3.32 This automobile parking structure at California State University–Northridge collapsed during the 17 January 1994 earthquake. The structure had 2,500 parking spaces and was built in 1991 for $11.5 million. Photo by Gregory A. Davis.

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Figure 3.35 Support columns on Freeway 118 in Simi Valley, California. (a) Problem: This column failed during the 1994 earthquake when brittle concrete cracked and ductile steel rebar buckled. (b) Solution: New columns have vertical steel rebar wrapped by circular rebar, and both are encased in concrete. In addition, columns are confined by bolted steel jackets that will be encased in concrete. Photos by Peter W. Wiegand.

(a) (b)

Double top

Blocking

Studs

Plywood panel shear wall

Metal brackets

Foundation Bolts

Diagonal brace Figure 3.36 How can a house be built to resist seismic waves? Bolt it. Bracket it. Brace it. Block it. Panel it.

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76 Chapter 3 Earthquake Geology and Seismology

Figure 3.37 Despite their huge size, the stiff and massive beams (note car for scale) supporting Kobe’s elevated expressway failed in the January 17, 1995, earthquake of 6.9 Mw.

Figure 3.38 (a) The Office of Disaster Preparedness in San Diego County is housed in a two-story, 7,000 ft2 building sitting on top of 20 lead-impregnated rubber supports (base isolators) that each weigh 1 ton. (b) An example of a base isolator. Cutaway view into a 1 m wide by 1 m tall sandwich shows alternating layers of rubber (each 15 mm thick) and steel (each 3 mm thick) with a central core of lead. During an earthquake, the rubber and steel flex and the lead absorbs energy. Photo by Pat Abbott.

Lead core

(b)

Rubber layers

Steel layers

(a)

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Terms to Remember 77

Earthquakes are shaking ground caused most often by sud- den movements along cracks in the Earth called faults. Some major faults acting for millions of years have offset rock layers by hundreds of kilometers. Sedimentary rock layers originally are continuous, horizontal, and in superpositional order (oldest on bottom, youngest on top); however, fault movements cut rocks into discontinuous masses, and in places, fault deformation has tilted rock layers and even overturned the super-positional sequence. Geologists mea- sure the 3-D orientation of rock layers via dip (angle and direction of inclination) and strike (compass bearing of rock cutting a horizontal plane).

Dip-slip fault types have dominantly vertical move- ments. Normal faults are due to extensional (pull-apart) forces. Reverse faults are due to compressional (push- together) forces. Strike-slip fault types have dominantly horizontal (shear) offsets. Straddling the fault, if the right-hand side moves toward you, it is a right-lateral fault; if the left-hand side moves toward you, it is a left-lateral fault. Bends (steps) in strike-slip faults cause the land to either uplift or downdrop. Another type of fault, called a transform fault, connects offset spreading-center segments.

Earthquakes, also called seisms, disperse their energy in seismic waves that radiate away from the hypocenter or point of fault rupture. The point on the surface above the fault rupture is the epicenter. Some seismic waves pass through the body of Earth; these are the P waves (primary waves with a push-pull motion) and the S waves (secondary waves with a shearing motion). Other seismic waves travel along the surface (Love and Rayleigh waves).

Summary

Earthquake energy is assessed by its magnitude. Differ- ent estimates of magnitude are derived from different meth- ods, based on local shaking (Richter scale), body waves (m b ), surface waves (M s ), or seismic moment (M w ). Earth has more than a million earthquakes each year, but more than 90% of the energy is released by the 12 to 18 largest events.

Seismic waves have different periods (time between cycles) and frequencies (number of cycles per second):

1 Period �

frequency

P waves commonly have from 1 to 20 cycles per second; surface waves commonly have 1 cycle every 1 to 20 seconds. Where the frequencies of seismic waves match the vibration frequencies of foundations and buildings, destruction may be great.

Earthquake effects on structures and people are assessed via the Mercalli Intensity Scale. Its variables are earthquake magnitude, distance from the hypocenter/epicenter, type of rock or sediment foundation, building style, and duration of shaking. Mercalli intensities are of more than just scientific interest because earthquakes don’t kill, buildings do.

Building components that must stand up to seismic shaking are horizontal (floors, roofs) and vertical (walls, frames). But horizontal and vertical components move at different frequencies. For buildings to stand up to earth- quakes, the horizontal and vertical components must be securely tied together using bolts, brackets, braces, and such. New designs of large buildings utilize energy-absorbing base isolation devices placed between the building and the ground.

Terms to Remember acceleration 67 aftershock 66 amplitude 59 base isolation 74 body waves 60 compression 54 cross-section 54 dip 54 dip-slip fault 54 fault 52 footwall 54 foreshock 66 fracture 52 frequency 59

friction 52 granite 60 hangingwall 54 hertz (Hz) 59 hypocenter 66 inertia 59 joint 54 law of original

continuity 53 law of original

horizontality 52 law of superposition 53 left-lateral fault 56 magnitude 63

mainshock 66 map 54 normal fault 55 period 59 permeability 54 primary (P) wave 60 resonance 68 retrofit 74 reverse fault 55 right-lateral fault 56 secondary (S) wave 60 seism 52 seismic moment 65 seismic wave 60

seismogram 62 seismograph 59 seismology 59 seismometer 59 shear 56 stress 52 strike 54 strike-slip fault 56 surface waves 60 tension 54 transform fault 57 wavelength 59

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78 Chapter 3 Earthquake Geology and Seismology

Questions for Review Ans. In figure 3.24 , the earthquake magnitude is close to 5.

1. Draw a cross-section of a sequence of sedimentary rock layers. Label and explain the laws of original horizontality, superposition, and original continuity.

2. Draw cross-sections of a normal fault and a reverse fault. What are the differing forces that determine which one forms? Which one involves tension? Compression?

3. Draw a map of a left-stepping, right-lateral fault. Explain what happens to the land at the step (bend) in the fault.

4. Draw a cross-section showing an inclined fault with a hypocenter at 15 km (9 mi) depth. Does the epicenter plot on the surface trace of the fault?

5. Sketch a map of a strike-slip and a transform fault. Explain their similarities and differences.

6. What do P and S seismic waves tell us about the nature of Earth’s interior?

7. How can arrival times of P and S waves be used to determine distance to the epicenter?

8. How are foreshocks distinguished from aftershocks? 9. What are typical P wave velocities in hard rock? Water? Air? 10. What are typical S wave velocities in hard rock? 11. How damaging to buildings are P waves? S waves? Rayleigh

waves? 12. What is the frequency of a seismic wave with a period of

1 second? ¼ second? 1/10 second? 13. What are typical frequencies for 1-story buildings? 10-story?

30-story? 14. Will a tall building be affected more by high- or low-

frequency seismic waves? Why? 15. Is resonance more likely for a 20-story building when shaken

by P waves or Rayleigh waves? 16. Building designers must account for acceleration. What does

this statement mean? 17. What are the differences between earthquake magnitude and

earthquake intensity? 18. List five main variables affecting Mercalli intensities.

19. How does the Richter magnitude scale for earthquakes differ from moment magnitude?

20. Explain how base isolation systems can reduce the shaking of buildings during an earthquake.

Questions for Further Thought 1. Immediately after the start of a big earthquake, how can the

greater velocity of P waves be utilized to provide some protection for hospitals, computer systems, and trains?

2. What is the quake potential of the Moon (moonquakes)? Does the Moon have similar numbers and magnitudes of quakes as Earth? Why?

3. If you are in an airplane over the epicenter of a great earthquake, what will you experience?

4. How earthquake safe is your home or office? What are the nearest faults? What kind of earth materials is your home or office built upon? How will your building size, shape, and materials react to shaking? What nearby features could affect your home? What hazards exist inside your home?

5. Make a list of the similarities between snapping your fingers and the movement of a fault.

Disaster Simulation Game Your challenge is to protect a city from earthquake disaster by constructing new buildings and retrofitting old ones. You are given a budget. Then you have real choices to make.

The city you must protect has a specified population of people. You are provided with a map of the town and charged with protecting as many people, buildings, and livelihoods as possible. You must build a hospital and two schools plus retrofit 10 old buildings. Are you ready for the challenge? Go to http://www. stopdisastersgame.org . Click on Play Game. On the next page, click on Play Game again. Select the Earthquake scenario, then choose your preferred difficulty level: Easy (small map); Medium (medium-size map); or Large (large map).

Good luck! Save as many people as you can.

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