Geography quiz 30 multiple questions

CH. 4 - TSUNAMIS

Energy and Natural Hazards

No subduction zones!

No faults that could

create even a large

tsunami, let alone a

“megatsunami”

Learning Objectives

• Explain the process of tsunami formation and development.

• Locate on a map the geographic regions that are risk tsunamis.

• Synthesize the effects of tsunamis and the hazards they pose to coastal regions.

• Summarize the linkages between tsunamis and other natural hazards.

• Tsunamis are not caused by or affected by human activities, but damages are compounded as coastal populations increase.

• Discuss what nations, communities, and individuals can do to minimize the tsunami hazard.

Introduction to Tsunamis

• Tsunami is Japanese for “harbour wave” • Caused by a sudden vertical displacement of ocean water

• Triggered by: • Large earthquakes that cause uplift or subsidence of sea floor

• Underwater landslides

• Volcano Flank Collapse

• Submarine volcanic explosion

• Asteroids

• Can produce Mega-tsunami

• “Tidal Wave” – misnomer! • Tsunamis are not related to tides

Some Historic Tsunamis

General Wave Attributes

Properties of Seismic Waves:

• Amplitude: height of wave

• Wavelength: distance between successive wave peaks

• Period [s]: time between wave peaks (= 1/frequency)

• Frequency [Hz]: number of wave peaks in one second

Tsunami Waves

• Series of waves with long wavelengths (20 km to over 800 km)

and long periods (10 minutes to 1 hour)

• The restoring force is gravity (compare with seismic waves:

elastic waves where restoring force is springiness of rocks)

• Velocity depends on water depth:

g is gravitational acceleration (9.8 m/s2)

d is water depth

v = gd

Tsunami vs. Wind-caused waves

• Wind-caused waves

- water rotates in circles

- short wavelength

- short period

• Tsunami

- flow as massive sheets of

water

- long period

- long wavelength (the

longer the wavelength, the

slower the wave loses

kinetic energy)

How do Earthquakes Cause a Tsunami :

Point source

• Volcano- and landslide-caused tsunami

• Trigger is a point source

- Energy flows away radially, high attenuation, local damage

Tsunami waves

propagate radially

when it is a point

source

How do Earthquakes Cause a Tsunami –

Fault Source Linear source:

fault (on seafloor)

that ruptures

Tsunami waves

propagate mainly in

direction

perpendicular to fault

Low attenuation:

potential for damage

far from source

How do Earthquakes Cause a Tsunami?

• Two mechanisms:

- Seafloor movement (more common)

- Triggering a landslide

• Takes an earthquake of M 7.5 or greater

- Creates enough displacement of the seafloor

- Upward or downward movement displaces the entire mass of water

- Starts a four-stage process

How do Earthquakes cause a Tsunami?

1. Earthquake uplifts or downshifts the seafloor

• Rupture uplifts the seafloor

• A dome forms on the surface of the water above the fault

• Dome collapses and generates the tsunami wave

• Waves radiate outward (like a pebble in a pond)

2. Tsunami moves rapidly in deep ocean

• Can travel 720 km per hour

• Spacing (frequency) of crests is large and small amplitude

• Boats in open ocean don’t notice the tsunami waves

Characteristics of Ocean Waves

• Characteristics common to all waves propagating in the

open ocean:

- Water moves in forward-rotating circles

- Diameter of circles decreases with depth

- Negligible for depth > L/2

In open ocean:

H < 1m

Shoaling

• Occurs when waves interact with the seafloor near

the shoreline

• Interaction starts when depth < L/2

• Friction slows wave down

• Wavelength decreases

- Energy is concentrated in a shorter length

• Amplitude increases

- Wave breaks

Near shore:

H ~ 6-15 m

Characteristics of Ocean Waves

Longer

wavelength in

deep water

Shorter

wavelength in

shallow water

How do Earthquakes cause a Tsunami?

3. Tsunami nears land, loses speed, gains height - Depth of ocean decreases, slowing tsunami waves

45 km per hour

- More water piles up increasing amplitudes and frequency

4. Tsunami moves inland destroying everything in its path - Can be meters to tens of meters high

- Often arrives as a quick increase in sea level

- Trough may arrive first, exposing seafloor

- Runup, furthest horizontal and vertical distance of the largest wave

- Water returns to ocean in a strong, turbulent flow

- Edge waves may be generated parallel to the shore

- Second and third waves may be amplified

How do Earthquakes cause a Tsunami?

• Offshore earthquakes can cause tsunamis to go toward land and out to sea • Uplifted dome of water splits in

two waves

• Distant tsunami • Travels out to sea and travels

long distances with little loss of energy

• Local tsunami • Travels quickly towards land

• People have little time to react

How do landslides cause a tsunami?

• Submarine landslides occur

when landslides occur

underneath the water

- Displaces water vertically causing

tsunamis

• On land, rock avalanches from

mountains can cause tsunami

- Example: Lituya Bay, Alaska

- 30.5 million cubic meters of rock

fell into ocean

- Bay water surged to 524 m (1790

ft.) above normal

Geographic Regions at Risk from Tsunamis

• All oceans and some lake shorelines have some risk

- Greater risk is for coasts near sources of tsunamis

- Which are??

• Greatest risk is to areas near or across from

subduction zones

- Example: Cascadia zone, Chilean Trench, off Coast of Japan

Where do the most tsunamis occur?

a) Indian and Atlantic oceans

b) Atlantic and Pacific oceans

c) Pacific and Indian oceans

d) Indian and Arctic oceans

e) Arctic and Atlantic oceans

Ring of Fire = Subduction zones

Where do the most tsunamis occur?

a) Indian and Atlantic

oceans

b) Atlantic and Pacific

oceans

c) Pacific and Indian

oceans

d) Indian and Arctic

oceans

e) Arctic and Atlantic

oceans

Why is the tsunami hazard in the Atlantic so low?

Tsunamis arrive as the leading edge of an elevated mass of water

- Similar to a very rapidly rising tide

- NOT breaker shape

Runs up and over the beach, floods inland for many minutes

Near shore topography (bays,

inlets) can focus the energy and

locally create enormous waves

Why does water often recede ahead of

a tsunami wave?

Water recedes

Direction of

propagation of

tsunami wave

e q u il ib

ri u m

Trough hits

shore first

Why does water often recede ahead of

a tsunami wave?

Water runs up

Direction of

propagation of

tsunami wave

e q u il ib

ri u m

Peak follows a

few minutes later

1960 Chile M 9.5 earthquake

• Most powerful earthquake ever measured

Earthquake-generated tsunami: 1960 Valdiva M 9.5

Case Study : Sumatra

• 26 December, 2004

• M9.2 megathrust earthquake

• 3-4 min. of ground shaking, 250 km offshore

• Death and destruction in 13 countries: - 198,000 deaths Indonesia

- 30,000 deaths Sri Lanka

- 11,000 deaths India

- 6000 deaths Thailand

• 1200 km long fault

• Seafloor offsets of up ~10 m

• Travelled 1.5 km inland

Case Study – Sumatra

• Megathrust event - Most lethal tsunami in recorded

history

- No warning system in Indian Ocean

- Few people knew what tsunami meant prior to event

• Education (or lack of) was a major reason for so many deaths - Many did not know how to

recognize a tsunami

- Many went to beach to watch

- Few knew what to do

- Tourists and first-generation residents

2004 Sumatra Tsunami killed people

on both sides of Indian Ocean

2004 Sumatra Earthquake

2004 Sumatra Earthquake, cont. • Those that were educated

• Scientists on beach in Sri Lanka • Noticed the sea level drop

• Sounded warning for those that went to beach to watch

• Animal behavior • Elephants started trumpeting about time of earthquake

• Ignored handlers and headed up hill

• Education of tsunami could have saved thousands more, especially with the distant tsunamis

June 23, 2004 December 28, 2004

Effects of Tsunamis and Linkages with Other

Natural Hazards

• Primary effects

- Inundation of water and resulting flooding and erosion

- Shorten the coastline

- Debris erodes both landscape and human structures

• Secondary effects

- Fires

- From ruptured gas lines or other sources

- Contaminated water supplies

- Floodwaters, wastewater treatment plants, rotting animal carcasses and

plants

- Disease

- Come in contact with polluted water or soil

• Coastline erosion