Geography quiz 30 multiple questions

CH. 3 - EARTHQUAKES

https://www.usgs.gov/news/updat

e-magnitude-71-earthquake-

southern-california

Earthquake

Alert!

M6.4 and M7.1

earthquakes occurred

in Southern California

within 36 hours of

each other, 11 km

apart

Learning Objectives

• Compare and contrast the different types of faulting.

• Explain the formation of seismic waves.

• Summarize the processes that lead to an earthquake and the release of seismic waves.

• Differentiate between the magnitude scales used to measure earthquakes.

• Identify global regions at most risk for earthquakes, and describe the effects of earthquakes.

• Describe how earthquakes are linked to other natural hazards.

• Explain how human beings interact with and affect earthquake hazards.

• Propose ways to minimize seismic risk and suggest adjustments we can make to protect ourselves.

Energy and Natural Hazards

2011 Tohoku Earthquake

• Japan located just 200 km (~124 mi) west of Japan Trench • Pacific plate is subducting beneath Eurasian plate (9 cm/yr)

• Experiences frequent large earthquakes

• March 11, 2011 • Strongest recorded earthquake to hit Japan

• Significantly greater than considered possible • Released about 600 million times more energy than bomb on Hiroshima

• Well engineered buildings helped reduce the loss of lives due to structural collapse

• Greatest loss of life was due to tsunami

Shaking and Damage During the Tohoku Earthquake

Introduction to Earthquakes

• What is an earthquake? • The sudden slip on a fault (release of elastic energy), and the

resulting ground shaking and radiated seismic energy caused by the

slip {USGS, 2002}

• People feel approximately 1 million earthquakes a year • Few are noticed very far from the source

• Even fewer are major earthquakes

• Most earthquakes occur along plate boundaries

Earthquake Distribution

Faults and Faulting

• Earthquakes occur along faults • Plane of weakness in Earth’s crust

• Semi-planar fracture or fracture system where rocks are broken and displaced

• Fracture (crack) in the earth, where the two sides of the earth move past each other

• Centuries-old mining terminology used • Footwall

• Block below the fault plane

• Miner would stand here

• Hanging wall

• Block above the fault plane

• Hang a lantern here

Basic Fault

Features

Footwall • Block below the fault plane • Miner would stand here

Hanging wall • Block above the fault plane • Hang a lantern here

Faults and Faulting, cont.

• Faulting – process of fault rupture • Similar to sliding one rough board past another

• Slow motion due to friction

• Stresses the rocks along the fault

• Rocks rupture and displaced when stress exceeds strength of rocks

• Stress • Force that results from plate tectonic movements

• Tensional

• Compressional

• Shearing

• Strain • Change in shape or location of the rocks due to the stress

Faults ≠ Plate boundaries

• However, most faults occur along plate boundaries

• Fault types

- Distinguished by direction of rock displacement

• Three basic types:

1. Dip-slip

a) Normal

b) Reverse

2. Strike-slip

a) right-lateral

b) left-lateral

3. Oblique slip

Normal dip-slip • Vertical motion

• Hanging wall moves down

relative to footwall

Reverse dip-slip • Vertical motion

• Hanging wall moves up relative to footwall

Strike-slip • Crust moves in horizontal direction

Faults and Faulting, cont.

• Blind faults do not extend to the surface

Types of Plate Boundaries and Stress

• Divergent = Extensional Stress >> Normal Faulting

• Convergent = Compressional Stress >> Thrust or Reverse Faulting

• Transform = Shear Stress >> Strike-Slip Faulting

Block diagram of fault surface

Faults are not simple planar

surfaces!

Faults are complex zones of

breakage where rough and

interlocking rock is held

together over an irregular

surface.

Stress builds up over many

years before enough energy

is stored to allow rupture on

the fault.

Elastic Rebound Theory Gradual build up of stress along a fault until the strength of the rock is

exceeded, resulting in a release of energy in the form of an earthquake

The Earthquake Cycle

• Change in strain • Accumulation before an earthquake

• Drop after an event

• Three or four stages 1. Long period of inactivity

2. Accumulated elastic strain produces small earthquakes

3. Foreshocks • Hours or days before large earthquake

• May not occur

4. Mainshock • Major earthquake

• Includes aftershocks: few minutes to a year after

Elastic Rebound Rocks deform elastically until a

critical point is reached and the

fault slips, releasing the stored

elastic energy

Time 1

Time 2

Time 3

Time 4

The Earthquake Cycle, cont.

• Epicenter • Given by news reports

• Location on surface above the rupture

• Focus (hypocenter) • Point of initial breaking

or rupturing

• Displacement of rocks starts here • Propagates up, down,

and laterally along the fault plane

• Produces shock waves, called seismic waves (cause ground shaking)

Seismic Waves

• Caused by a release of energy from rupture of a fault

• Body waves: travel through the body of the Earth

• P waves, primary or compressional waves

- Move fast with a push/pull motion

- Can move through solid, liquid, and gas

• S waves, secondary or shear waves

- Move slower with an up/down motion

- Can travel only through solids

P waves, primary or compressional waves

- Body waves, travel through the body of the Earth - Move fast with a push/pull motion - Can move through solid, liquid, and gas

P waves, primary or compressional waves

- Velocity depends on density

and compressibility of the

materials through which they

pass

- Greater resistance to

compression, greater the

velocity

- Seismic waves pass

through packed atomic

structures

- Velocity through igneous rocks

(eg. granite) ~5.0 km/s

- Velo. in sed. rocks (eg.,

sandstone) ~3.0 km/s

S waves, secondary or shear waves

- Body waves, travel through the body of the Earth - Move slower with an up/down motion - Can travel only through solids

S waves, secondary or shear waves

- Transverse waves that

propagate by shearing

particles at right angles to the

direction of propagation in the

vertical and horizontal plane

- Velocity depends on density

and resistance to shearing of

materials

- Velo. in igneous rocks ~ 3.0

km/s

- Velo. in sedimentary rocks ~1.7

km/s

Seismic Waves, cont.

Surface waves: move along

Earth’s surface • P and S waves that reach the

surface

• Travel more slowly than body

waves

• Complex horizontal and vertical

ground movement

Rayleigh Waves • Rolling motion

• Responsible for most of the

damage near epicenter

• Shaking produces both

vertical and horizontal

movement

Seismic Waves, cont.

Surface waves: move along Earth’s surface

• P and S waves that reach the surface

• Travel more slowly than body waves

• Complex horizontal and vertical ground movement

Love Waves • Horizontal ground shaking

• Faster than Rayleigh waves

• Do not move through water or air

• Very hazardous!

Wave direction

Seismic Waves and 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

Seismic Waves and Wave Attributes

Properties of Seismic Waves:

• Attenuation: amplitude of seismic waves decreases with

increasing distance from the hypocenter

- More pronounced for high-frequency waves

- Less pronounced for low-frequency waves

How do we detect and record seismic waves?

Horizontal component Vertical component

Before computers…

Modern 3-component seismograph station

3 orthogonally aligned seismometers:

- Veritcal

- North-south

- East-west

Seismogram (a recording of the ground motion)

P

S

Analysis of seismic

records allows

seismologists to

identify the different

kinds of seismic

waves generated by

fault movement

Distance to Epicenter

Use difference between first P and S wave arrival times:

- P waves will appear first

- Seismographs across globe record arrivals of waves to station sites

- Distance to epicenter can be found by comparing travel times of the waves

Distance to

Earthquake

Epicenter

Note:

P-wave first

S-wave second

Surface waves last

Time lag between P and S-wave

arrival is called Δt, or the S-P time.

Ex. 1994 M 6.7

Northridge earthquake

Calculating Epicentral Distance

P wave has velocity VP ; S wave have velocity VS

VS < VP

Both originate at the same place – the hypocenter – and travel the same distance, but

the S wave takes longer to arrive than the P wave.

Time for S wave to travel a distance D:

Time for P wave to travel a distance D:

The time difference between them is:

Now solve for the distance D:

Time = Distance

Velocity

T S

= D

V S

T P

= D

V P

(T S -T

P ) = D

V S

- D

V P

= D 1

V S

- 1

V P

æ

è ç

ö

ø ÷ = D

V P

-V S

V P V S

æ

è ç

ö

ø ÷

𝐷 = 𝑉𝑃𝑉𝑆 𝑉𝑃 − 𝑉𝑆

𝑇𝑠 − 𝑇𝑝

Locating an Earthquake

• Location of epicenter • At least three stations

are needed to find exact epicenter

• Distances from epicenter to each station are used to draw circles representing possible locations

• The place where all three circles intersect is the epicenter

• Process is called triangulation

Tectonic Creep and Slow Earthquakes

• Tectonic creep: gradual movement such that

earthquakes are not felt - Can produce slow earthquakes

- Also called fault creep

• Can slowly damage roads, sidewalks, and building

foundations

• Can last from days to months

https://seismo.berkeley.edu/blog/2008/10/14/the-hayward-fault.html

Earthquake Shaking

• Shaking experience depends on:

1. Earthquake magnitude

2. Location in relation to epicenter and direction of rupture

3. Local soil and rock conditions

• Strong shaking from a moderate magnitude or higher