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© 2014 Pearson Education, Inc.

What Is an Earthquake?

GEOL 110: GEOLOGY

Chapter 11:

EARTHQUAKES AND

EARTHQUAKE

HAZARDS

This presentation is not for public distribution and only intended for educational and reference use within the associated course.



What is an Earthquake?

Occurs by the sudden and rapid movement of

one block of rock sliding past another, causing

the ground to shake.

Why does it occur?

• Build up of stress causes

sudden slippage

• Most earthquakes occur

along preexisting faults

Lisa James/Register



Points of an Earthquake

• Focus/Hypocenter: location of slippage (generally along a

fault)

• Epicenter: location at surface

• Seismic Waves: stored-up energy originating from

hypocenter.

• Waves radiate outward in all directions

• Seismic energy dissipates with distance



What causes an

Earthquake?

• Stress builds up in rocks on both

sides of a fault until they break/slip

• Large landslides, meteorites, and

volcanic eruptions produce weak

earthquakes

How does stress build up?

• Movement of plates bend rocks

• Friction keeps rock in place until too much stress

• Rocks have been moved to a new location


Aftershocks and Foreshocks

• Foreshocks precede the main

shock; smaller

– Can precede several day, weeks,

months

• Aftershocks follow the main shock;

smaller

– Due to fault surface adjusting to the

displacement caused by the main shock

– Fewer and fewer as time passes

– Aftershocks can cause damage to

already weakened structures


earthquakes.usgs.gov


Faults and Earthquakes

Plate Boundaries, Faults and Earthquakes

• Plate boundaries are the source of most large

earthquakes

• Earthquakes can occur on new and pre-existing

faults

3 types of Boundaries?

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Types of Faults that cause

earthquakes

• Normal fault: associated with divergent

boundaries; doesn’t cause many earthquakes

• Reverse and Thrust fault: associated with

convergent boundaries;

– Megathrust fault: boundary between

subducting and overlying plate; causes

very large earthquakes.

– Displace water and cause tsunami

• Strike-slip fault: associated with transform

boundaries

Divergent: Footwall

moves up

geology.about.com

Convergent:

Footwall moves

down


Major Fault Types

A. Normal Fault =

divergent

B. Reverse Fault =

convergent

B. Megathrust Fault =

convergent

C. Strike-slip Fault =

transform



San Andreas Fault

San Andreas is the most studied fault system in the world

• Slow, gradual displacement known as fault creep

• Other segments regularly slip, producing small earthquakes

• Other segments remain stuck and store elastic energy for a few

hundred years before they break loose, resulting in a major earthquake

USGS/2015

Creep along Calaveras Fault



Oct. 17,1989

Loma Prieta, M7.1

April 18, 1906 Great SF

Earthquake, M7.8;

Epicenter off shore ~2miles

Epicenter in Santa Cruz Mtns

2 Large Earthquakes on

San Andreas Fault



Fault Rupture and Travel• Most faults are locked except for brief,

abrupt movements

• Faults do not slip all at once

• Initial slip begins at focus and

travels along the fault surface

• Slippage adds strain to adjacent

sections triggering more slippage

• Slippage mainly travels in one

direction (Napa EQ traveled North)

• Fault slip is the amount of

displacement on the fault surface


Displacement of Structures Along a Fault

Fault Slip: amount of displacement

At least 46 cm of right-lateral displacement

C.Roughley 2014

1906 earthquake

displacement 8.5 ft.

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Seismology: The Study of Earthquake Waves

What is seismology?

India

Mexico

Japan

Mongolia, China



What is seismology?

• The study of earthquake

waves

• Earliest studies of

earthquake waves date

back almost 2000 years to

the Chinese

• The Chinese Seismoscope

Invented 132 AD by

inventor, Imperial Historian,

and Royal Astronomer

Zhang Heng



Instruments that Record Earthquakes

• Seismographs / seismometers record the

movement of Earth in relation to a stationary mass on

a rotating drum or magnetic tape

• Seismograms are the records produced by

seismographs


Seismic Waves

What are Seismic Waves?

• Seismic Waves: waves of energy that travel through earth’s

layers

• Wave: energy moves from one place to another and matter

doesn’t move from place to another (basic).

• Eg: ocean waves carry energy to the shore, but water doesn’t

build up at shoreline


Seismic Waves

Different types of energy travel in waves:

• Soundwaves carry noise through the air to

our ears

• Light, heat, radar carry energy waves as

electricity and magnetism.

• Gamma Rays produced by radioactivity.

• Radio waves carry radio and tv signals


Seismic Waves: waves of energy that travel

through earth’s layers

Types of seismic waves:

• Body waves: travel through earth’s interior

• Surface waves: travel in the rock layers just

below Earth’s surface


www.studyblue.com

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Body Waves

• P-Waves (primary waves):

compressional waves; push/pull;

stretch rocks parallel to wave

direction; travel through solid,

liquid, gas

• S-Waves (secondary

waves/shear waves): move

particles perpendicular to wave

direction; changes shape of

material; travel through solids

only! (change shape; liquids not

rigid therefore cannot shear or

change shape)

http://www.geo.mtu.edu/UPSeis/images/P-wave_animation.gif

http://www.geo.mtu.edu/UPSeis/images/S-wave_animation.gif



Body Waves


S-wave changes shape of

material

P-wave


Surface Waves

Travel through the crust

• Raleigh waves: rolls along the

ground like an ocean wave;

ground moves up and down

• Love waves: moves the

ground side to side

• Largest amplitude on

seismogram; most damage

http://www.geo.mtu.edu/UPSeis/images/Rayleigh_anim

ation.gif

http://www.geo.mtu.edu/UPSeis/images/Love_animatio

n.gif



Surface Waves



Body Waves vs. Surface Waves

S-waves do not

travel through

outer core

(liquid)

P-waves are faster than S-waves and arrive first.

Surface waves are slowest ; last to arrive

at a recording station; cause most property

damage

P-wave speed:

Water: 1450 m/s

Granite: 5000 m/s

S-wave speed:

~60% of P-wave

Surface wave speed: ~90% of S-wave© 2014 Pearson Education, Inc.

Typical Seismogram

Waves on a seismogram

http://www.geo.mtu.edu/UPSeis/images/P-wave_animation.gif

http://www.geo.mtu.edu/UPSeis/images/S-wave_animation.gif

http://www.geo.mtu.edu/UPSeis/images/Rayleigh_animation.gif

http://www.geo.mtu.edu/UPSeis/images/Love_animation.gif

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Typical Seismogram

How to find the epicenter:

1. Amount of time between the P and S wave on a seismogram can

tell how far away the earthquake was from that location.

2. Draw a circle on a map around the seismograph station; radius is

the distance to the earthquake; earthquake lies somewhere on the

circle.

3. Point of intersection is location of epicenter.


Determining the Size of Earthquakes

How do we determine the size of an

earthquake?

2 ways to describe:

• Intensity: a measure of the degree of

earthquake shaking at a given locale based

on the amount of damage

• Magnitude: an estimate of the amount of

energy released at the source of the

earthquake



Intensity scales (shaking)

• 1902, Guiseppe Mercalli developed intensity scale based

on CA buildings

• Modified Mercalli Intensity scale based on damage to

structures and where the earthquake is felt

• Drawback: destruction may not be a true measure of the

earthquake’s actual severity


Seismic Intensity Map

DYFI? Intensity Scale:

South Napa

Earthquake, Aug 24

Mw 6.0

7.0 miles deep

www.studyblue.com

http://earthquake.usgs.gov/earthquakes/map/


Magnitude scales (energy released)


Richter magnitude ML

• Introduced by Charles Richter in 1935;

seismologist and geophysicist; worked at

CalTech with Beno Gutenberg

• The Richter scale is ratio of amplitude of

the largest seismic wave to a minor

amplitude

• 1-10 relate to energy earthquake releases;

each level indicates ~32-fold increase in

energy released.

• Not accurate for large earthquakes


Magnitude scales (energy released)


Moment magnitude Mw

• Developed in the 1970s to replace

Richter magnitude scale. Today’s

standard.

• Calculated by determining the average

amount of slip on the fault, area of the

fault, and strength of the fault rock.

• Each unit increases ~32-fold increase

in energy released

• Used to update Richter magnitude

earthquakes


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Earthquake Richter Scale Moment Magnitude

New Madrid, MO, 1812 8.7 8.1

San Francisco, CA 1906 8.3 7.7

Prince William, AK 1964 8.4 9.2

Northridge, CA,1994 6.4 6.7

Comparison between the Richter and Moment

Magnitude Scales


Annual Occurrence of Earthquakes with

Various Magnitudes


Earthquake size example:

What is the intensity? What is the magnitude?


http://earthquake.usgs.gov/earthquakes/map/© 2014 Pearson Education, Inc.

Earthquake Destruction

Damage from Earthquakes

Dependent on:

• Magnitude of the earthquake

• Proximity to the epicenter



Damage from Seismic VibrationsThe amount of damage to human-made structures depends on:

• Intensity

• Duration (Loma Prieta 15 seconds; 1963 Alaska 3-4minutes)

• Geology

• Building materials and construction practices/codes of the region

2010 Haiti Earthquake

Mw 7.0

Depth: 8.1miles

Intensity: X (extreme)

52 aftershocks Mw 4.5 or greater

Photo Marco Dormino/ The United Nations United Nations Development Programme - originally posted to Flickr as Haiti Earthquake



Damage from Seismic Vibrations

• Amplification of seismic waves

• Soft sediments amplify seismic waves more than solid bedrock

• Liquefaction• https://www.youtube.com/watch?v=TzlodnjPAuc


https://www.youtube.com/watch?v=TzlodnjPAuc

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• Landslides and Ground

Subsidence

• Ground shaking causes

loose sediments on a slope

to slump

• Fire

• Can start when gas and

electrical lines are

destroyed by an

earthquake

• Broken water lines make

fire control problematic



Tsunami

Series of large ocean waves• Most are generated by displacement from a megathrust fault

• In open water, the wave amplitude is less than 1 meter and the

wavelength can be larger than 700 meters

• Close to shore, the water “piles up” and some tsunamis can

exceed 30 meters in height


Tsunami Generated by Displacement of the

Ocean Floor

Tsunami generated from the 2011 Tohoku earthquake (9.0Mw)

•40 meters high and traveled up to 6 mi inland

•Affected not only Japan but also the west coast of North America

K.Haichi 2010 K.Haichi 2011

Sign reads “Tsunami of 1933 came to here”. After 2011 Tsunami


Japan Tsunami

K.Haichi 2011

Tsunami is expected to come to here.

The city of Taro is famous because the

city was most enthusiastic to protect

the city from the Tsunami in Japan.

They built the seawall whose height is

10m, probably highest in Japan.

The Tsunami of 2011 was higher than

this seawall and destroyed most of

downtown area.



Tsunami warning system

• Observations in the Pacific

Ocean.

• A series of deep-water

buoys in the Pacific Ocean

detect energy released by

earthquakes

• Tidal gauges measure sea

level rise and fall


Earthquake Belts and Plate Tectonics

Circum-Pacific Belt

• 25,000 miles of subduction boundaries along the Circum-

Pacific Belt

• Most earthquakes occur along megathrust faults of

convergent plate boundaries

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Earthquake Belts and Plate Tectonics

Alpine-Himalayan Belt

• Tectonic activity is attributed to the collision of the

African and Indian Plates with the Eurasian Plate

• Divergent plate boundaries are associated with frequent,

but weak seismic activity


Can Earthquakes Be Predicted?

Can earthquakes be predicted?Short-Range Predictions

• The goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame

• Research has concentrated on monitoring possible precursors of major earthquakes:

• Monitor changes in ground elevation

• Measure strain in the rocks

• Measure changes in groundwater level

• Frequency of foreshocks

Currently, no reliable methods exist for making short-range earthquake predictions



Early Warning Systems

• Berkeley early warning system

• Japan early warning system– Over 4200 seismometers

– 2 record P-wave and give warning

– Few seconds to a minute

Seismo.berkeley.edu

http://blogs.agu.org/landslideblog/2015/10/28/af

ghanistan-earthquake-1/


Long-Range Forecasts

• Give the probability of earthquakes of a certain magnitude occurring on a time scale of 30 to 100 years (or more)

• Useful guide for building codes. Eg: Building the Trans-Alaskan Pipeline over the Denali Fault




Long-Range Forecasts• Seismic gaps: tectonically

quiet zones along a fault where strain is currently building up

• The stored strain will be released in a future earthquake

• Paleoseismology is the study of prehistoric earthquakes

• Trench across a fault zone, look for evidence of ancient faulting (mud volcanoes and offset sedimentary strata)



Earthquakes and Faults

exercise

• Afghanistan earthquake

•> 360 deaths (Pakistan) and

~2,000 injured.•http://blogs.agu.org/landslideblog/2015/10/28/afgha

nistan-earthquake-1/

This presentation is not for public distribution and only intended for educational and reference use within the associated course.


http://blogs.agu.org/landslideblog/2015/10/28/afghanistan-earthquake-1/

http://blogs.agu.org/landslideblog/2015/10/28/afghanistan-earthquake-1/


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