Ch11 Earthquakes

7/24/2019 Ch11 Earthquakes

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Chapter 11

Earthquakes


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What is an earthquake?

An earthquakeis the vibration of Earth

produced by the rapid release of energy

Energy released radiates in all directionsfrom its source, the focus

Energy is in the form of waves

Sensitive instruments around the worldrecord the event


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Earthquake focusand epicenter

Figure 11.2


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Earthquakes and faults

Movements that produce earthquakes are

usually associated with large fractures inEarths crust called faults

Most of the motion along faults can be

explained by the plate tectonics theory


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Elastic rebound

Mechanism for earthquakes was first

explained by !"! #eid #ocks on both sides of an existing fault are

deformed by tectonic forces

#ocks bend and store elastic energy

"rictional resistance holding the rocks together

is overcome


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Elastic rebound

Earthquake mechanism

Slippage at the weakest point $the focus% occurs &ibrations $earthquakes% occur as the deformed

rock 'springs back( to its original shape $elastic

rebound%

Earthquakes most often occur along existingfaults whenever the frictional forces on the

fault surfaces are overcome


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Figure 11.5AB


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Figure 11.5CD


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Offset produced by the 1906San Francisco earthquake

Figure 11.4


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"oreshocks and aftershocks

Ad)ustments that follow a ma)or earthquake

often generate smaller earthquakes calledaftershocks

Small earthquakes, called foreshocks, often

precede a ma)or earthquake by days or, in

some cases, by as much as several years


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San Andreas An acti!eearthquake "one

San Andreas is the most studied fault

system in the world

*isplacement occurs along discretesegments + to - kilometers long

Some portions exhibit slow, gradual

displacement known as fault creep .ther segments regularly slip producing

small earthquakes


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San Andreas An acti!eearthquake "one

*isplacements along the San Andreas fault

Still other segments store elastic energy for

hundreds of years before rupturing in greatearthquakes

/rocess described as stick0slip motion

1reat earthquakes should occur about every 2

to - years along these sections


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Seis#o$o%y

Seismographs, instruments that recordseismic waves

#ecords the movement of Earth in relationto a stationary mass on a rotating drum ormagnetic tape

More than one type of seismograph is

needed to record both vertical andhori3ontal ground motion

#ecords obtained are called seismograms


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A !ertica$ %round#otion seis#o%raph

Figure 11.8


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Figure 11.7


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&ypes of Seis#ic Wa!es

Surface waves

4ravel along outer part of Earth

5omplex motion

5ause greatest destruction

Exhibit greatest amplitude and slowestvelocity

6aves have the greatest periods $timeinterval between crests%

.ften referred to as long waves, or 7 waves

Fi 11 9C


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Figure 11.9C


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8ody waves 4ravel through Earths interior

4wo types based on mode of travel/rimary $/% waves

/ush0pull $compress and expand% motion,changing the volume of the interveningmaterial

4ravel through solids, liquids, and gases

1enerally, in any solid material, / waves

travel about +!9 times faster than S waves

Fi 11 9A


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Figure 11.9A


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Secondary $S% waves 'Shake( motion at right angles to their

direction of travel 4ravel only through solids

Slower velocity than / waves

Slightly greater amplitude than / waves

Fi 11 9D


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Figure 11.9D


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'ocatin% the sourceof earthquakes

4erms

"ocus0 the place within Earth where

earthquake waves originate Epicenter: location on the surface directly

above the focus

Epicenter is located using the difference invelocities of / and S waves


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7ocating the epicenter of an earthquake

4hree station recordings are needed to

locate an epicenter Each station determines the time interval

between the arrival of the first / wave and

the first S wave at their location

A travel0time graph is used to determine

each stations distance to the epicenter


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Seis#o%ra# sho(in% )* S*and surface (a!es

Figure 11.10

A $ i h


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A tra!e$+ti#e %raph

Figure 11.11


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7ocating the epicenter of an earthquake

A circle with a radius equal to the distance

to the epicenter is drawn around eachstation

4he point where all three circles intersect is

the earthquake epicenter

Fi di th k


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Findin% an earthquakeepicenter

Figure 11.12


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Earthquake belts

About ;2 percent of the energy released by

earthquakes originates in a few relativelynarrow 3ones that wind around the globe

Ma)or earthquake 3ones include the

5ircum0/acific belt, Mediterranean Sea

region to the imalayan complex, and the

oceanic ridge system

Figure 11 13


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Figure 11.13


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Earthquake depths

Earthquakes originate at depths ranging

from 2 to nearly 9 kilometers

Earthquake foci arbitrarily classified as

Shallow $surface to 9 kilometers%


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Earthquake depths

*efinite patterns exist

Shallow focus occur along the oceanic ridgesystem

Almost all deep0focus earthquakes occur in the

circum0/acific belt, particularly in regions

situated landward of deep0ocean trenches


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Earthquake Monitoring

http://earthquake.usgs.gov/eqcenter/recenteqs

ww/Quakes/quakes_all.php http://www.emsc-csem.org/ine!.php"page#ho

mehttp://www.iris.eu/seismon/
http://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/quakes_all.phphttp://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/quakes_all.phphttp://www.emsc-csem.org/index.php?page=homehttp://www.emsc-csem.org/index.php?page=homehttp://www.iris.edu/seismon/http://www.iris.edu/seismon/http://www.emsc-csem.org/index.php?page=homehttp://www.emsc-csem.org/index.php?page=homehttp://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/quakes_all.phphttp://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/quakes_all.php


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,easurin% the si"eof earthquakes

4wo measurements that describe the si3e of

an earthquake are


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Figure 11.14


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Magnitude scales

#ichter magnitude0 concept introduced by

5harles #ichter in +;=2 #ichter scale

8ased on the amplitude of the largest seismic

wave recorded

Accounts for the decrease in wave amplitude

with increased distance

Figure 11.15


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Figure 11.15


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Magnitude scales

#ichter scale

7argest magnitude recorded on a 6ood0Anderson seismograph was >!;

Magnitudes less than -! are not felt by humans

Each unit of #ichter magnitude increase

corresponds to a tenfold increase in waveamplitude and a =-0fold energy increase


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Magnitude scales

.ther magnitude scales

Several '#ichter0like( magnitude scales havebeen developed

Moment magnitudewas developed because none

of the '#ichter0like( magnitude scales

adequately estimates very large earthquakes *erived from the amount of displacement that

occurs along a fault


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Earthquake destruction

Amount of structural damage attributable

to earthquake vibrations depends on


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*estruction from seismic vibrations

1round shaking

#egions within - to 2 kilometers of the

epicenter will experience about the same

intensity of ground shaking

owever, destruction varies considerably

mainly due to the nature of the ground on

which the structures are built


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-a#a%e caused by the 196.Anchora%e* A$aska quake

Figure 11.17

Figure 11.C


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*estruction from seismic vibrations

7iquefaction of the ground

@nconsolidated materials saturated with waterturn into a mobile fluid

Seiches

4he rhythmic sloshing of water in lakes,

reservoirs, and enclosed basins

6aves can weaken reservoir walls and cause

destruction


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&suna#is

*estructive waves that are ofteninappropriately called 'tidal waves(

#esult from vertical displacement along afault located on the ocean floor or a largeundersea landslide triggered by anearthquake


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For#ation of a tsuna#i

Figure 11.20

Figure 11.21


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Figure 11.22AB


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Can earthquakesbe predicted?

Short0range predictions 1oal is to provide a warning of the location

and magnitude of a large earthquake withina narrow time frame

#esearch has concentrated on monitoringpossible precursors : phenomena that

precede a forthcoming earthquake such asmeasuring uplift, subsidence, and strain inthe rocks


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Short0range predictions 5urrently, no reliable method exists for

making short0range earthquake predictions 7ong0range forecasts

1ive the probability of a certain magnitude

earthquake occurring on a time scale of =to + years, or more


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7ong0range forecasts

8ased on the premise that earthquakes are

repetitive or cyclical @sing historical records or paleoseismology

Are important because they provide

information used to

*evelop the @niform 8uilding 5ode

Assist in land0use planning

Figure 11.25


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Figure 11.26


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Earthquakes E!idencefor p$ate tectonics

A good fit exists between the plate tectonics

model and the global distribution of

earthquakes 4he connection of deep0focus earthquakes

and oceanic trenches is further evidence

.nly shallow0focus earthquakes occur along

divergent and transform fault boundaries

Figure 11.27


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End of Chapter 11

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Ch11 Earthquakes