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Earthquakes
Chapter 5
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You will learn
• What earthquakes are
• Where earthquakes occur
• How fault ruptures
generate earthquakes
• How we measure and
study earthquakes
• What earthquake hazards
are
• How scientists attempt to
predict earthquakes
• How people mitigate
earthquake hazards
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Earthquakes Defined
• Trembling, shaking, or
vibration of Earth.
• Tectonic movements within
lithosphere causes
earthquakes and most occur
along preexisting faults.
• Movements generate seismic
waves that travel outward in
all directions from fault.
• Seismic waves originate at
focus and migrate outward;
epicenter marks projection of
focus directly to surface.
Figure 5-2 Movement on Faults
Causes Earthquakes
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Where Earthquakes Occur—
Transform Boundaries
• Motions along transform plate
boundaries cause many large
earthquakes globally
• Transform plate boundaries
are complex—San Andreas
fault zone is a system of many
connected faults that allow
plates to move past each other
• Main San Andreas fault cuts to
basal crust and extends from a
divergent plate boundary in the
Gulf of California northward to
near Cape Mendocino, where
it merges with an offshore
transform fault.
Figure 5-3 The San Andreas Fault Zone
Large fault system = transform plate boundary.
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Where Earthquakes Occur—
Convergent Boundaries
• Largest earthquakes on record
occurred along subduction zones,
where one plate slips beneath
another
• Reverse/thrust faults are common
Figure 5-4a The Largest Earthquakes
Figure 5-4c Tsunami generated from May
Occur at Convergent Plate Boundaries
1960 Chilean quake of 9.5 magnitude was
also devastating to Japan.
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Where Earthquakes Occur—
Divergent Boundaries
• Pulling apart or extension of
the lithosphere and normal
faulting along these oceanic
divergent plate boundaries
causes many shallow, smallto-moderate-size earthquakes
along MORs
• Pull-apart basins or rifts are
also common on land with
some quakes larger than
Figure 5-5a
those at the MORs
Earthquakes at Divergent Plate
Boundaries Earthquakes are common in the
East African rift zone, where normal faulting
marks new divergent plate boundaries.
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Basin and Range—
A Divergent History
Figure 5-6 Basin and Range Province—Crust
Extended
(a) The Basin and Range is centered on Nevada.
(b) Normal faults occur between long mountain
ranges and intervening valleys, or basins.
(c) Displacement on normal faults generates many
small-to-moderate size earthquakes and creates
surface scarps.
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Where Earthquakes Occur—Not all along
Boundaries: Intraplate Locations
• Central Mississippi
Valley experiences
many small-tomoderate-size intraplate
earthquakes.
• Map shows epicenters
of earthquakes recorded
since 1974.
Figure 5-7 Earthquakes in the
Mississippi Valley
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Elastic Rebound Theory
Figure 5-8 The Elastic
Rebound Theory Explains
Fault Rupture and
Earthquake Generation
Sudden release of elastic
strain that built up along
a fault releases seismic
energy, causing
earthquakes (right).
Theory developed from
observations of
displacements on San
Andreas fault in 1906 such
as the offset fence line on
left.
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Creepy Faults
Figure 5-9 The Hayward Fault Creeps
Part of the San Andreas Fault system, the Hayward Fault goes from goalpost to
goalpost across the UC Berkeley football stadium (left).
The creeping movement on the fault is very slowly tearing the stadium apart
(right).
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Earthquakes and Faults—
Body Waves
• Body waves travel through the interior of Earth
(i.e., through rock).
• Body waves include Primary (P) (a) and
Secondary (S) (b) waves
• P waves are known also known as
compressional (think P for Pressure) waves
• S waves are known also as Shear waves
• P waves can be transmitted through both
liquid and solids
• S waves can be transmitted only through
solids (shear cannot be transmitted through
liquid)
Figure 5-10a and b
How Seismic Waves Pass through Rock
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Earthquakes and Faults—
Surface Waves
• When body waves approach the
Earth’s surface, a part of their energy
is converted into surface waves
• Surface waves travel along Earth’s
outer edges rather than through its
interior, as in the case of body waves
• Love waves (c) and Rayleigh waves (d)
are surface waves
Figure 5-10c and d
How Seismic Waves Pass through Rock
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Investigating Earthquakes
• Satellites in space are used to measure
changes in land surface in seismic
zones such as upwarps or downwarps
• Seismometers are used to measure
magnitudes of earthquakes and locate
fault ruptures associated with quakes
• Seismometers measure vertical and
horizontal earth vibrations
• A seismogram is a recording of ground
motions caused by seismic waves
Figure 5-11 Earthquake Vibrations Are
Recorded by a Seismometer
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How Big Was That Quake?—Magnitude
Figure 5-13 Amount of Energy Released by Earthquakes of Different
Magnitudes Comparing amount of energy released by earthquakes to energy
associated with other natural and human-produced phenomena helps us understand
how incredibly powerful they can be. The figure also gives the energy equivalents of
moment magnitudes.
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Quantifying the Magnitude
• Moment magnitude, Mw (the “w” stands for “work”) = numerical
scale of the amount of energy released by an earthquake.
• Mw calculated on the basis of:
1) the total area of the fault rupture,
2) how far the rocks move along the fault during quake, and
3) the strength of the rock that ruptures.
• These 3 aspects of an earthquake are related to long-period
seismic waves recorded on seismograms. a good estimate of
the seismic energy released by earthquakes, especially very
large ones.
• On the Mw scale a change of one represents a change of 31.6
times the power released.
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Intensity—the Degree of
Shaking
Mercalli Scale can be used to make a comparison between the
shaking and damage
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Modified Mercalli Intensity Maps
Figure 5-14 Mercalli
Intensity Maps of
Two Famous Bay
Area Earthquakes
• Maps show that the 1906 earthquake (far left) was much more
extensive and destructive than was the 1989 Loma Prieta
quake in California.
• Red areas show highest intensities between IX and X.
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Locating Earthquakes
Figure 5-15 Locating the Epicenter of an
Earthquake
P and S arrival times and triangulation with 3 or
more stations is used to locate the epicenter.
• Analyzing the difference in
arrival times of P and S
waves at several different
seismometers allows the
epicenter to be identified.
• Since travel time of P and S
waves is different the
distance away from a given
recording station can be
determined.
• Depths of quakes can also
be determined using a 3-D
analysis similar somewhat to
method for finding the
epicenter.
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Building Design and Earthquakes
Figure 5-16 Earthquakes Don’t Kill People—Collapsing Buildings Do
(a) Bam, Iran, before earthquake of December 26, 2003, (top) and afterward (below),
reduced to piles of mud bricks. Magnitude 6.6 earthquake caused 41,000+ deaths.
(b) Northridge, California, earthquake of 1994, though equally strong,
claimed only 57 lives.
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Earthquake Hazards—
Ground Shaking
Factors to consider:
• Magnitude—quake size
• Distance from focus—shallow or deep?
• Site geology—consolidated or not?
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Earthquake Hazards—Ground
Displacement and Failure
Factors to Consider:
• Liquifaction
• Slope Failure
• Surface Ruptures
• Crustal Deformation
Figure 5-17 Kobe, Japan
Liquefaction of soft fill after a 6.9.
Figure 5-18 Earthquakes May Trigger
Landslides
Peru devastated by a large mud and rock flow
caused by a 1970 earthquake in the Andes.
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Surface Ruptures
Figure 5-19 Surface Rupture
Surface rupture can permanently scar the landscape and cause great damage.
Views captured after the great Anchorage, Alaska earthquake of 1964.
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Earthquake Hazards—Tsunami
• On December 26, 2004,
a subduction zone
earthquake of Mw 9.1—
3rd largest ever
recorded—occurred off
coast of Sumatra,
Indonesia
• 225,000 people in 11
countries perished in this
great tsunami
Figure 5-20 Earthquakes May Cause
Tsunami (a) Tsunami form when water is
displaced above a sudden offset on the
seafloor; (b) As they enter shallow water,
tsunami slow down and gain height.
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Tsunami Destruction—
December 26, 2004
Figure 5-21 Tsunami Devastation—before (on left) and after (on right).
Note nearly complete wiping out of landscape.
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Earthquake Hazards—Fire
• Greatest hazards to urban areas stricken by
earthquakes
• Great 1906 San Francisco earthquake—fires burned
3 days, devastating 12 square kilometers (4.6 mi2)
• Great 1923 Tokyo earthquake, tens of thousands of
people perished in fires.
Figure 5-22 Fires
Commonly Are Major
Secondary Effects
of Earthquakes
1906 San Francisco
fires (left), 1995 Kobe,
Japan, fires (right)
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Earthquake Predictions
• Ability to accurately predict the size, location,
and time of earthquakes remains elusive even
with today’s technology and information.
• Limited success of prediction based on a variety
of possible warning signs or precursors such as
animal behavior.
• Prediction has been superseded by a more
achievable goal of making long-term, lessspecific earthquake forecasts.
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Seismic Gaps
Figure 5-23 Earthquake history: Strike-Slip
Fault in Turkey Quakes migrated E to W over
60 yrs; lack of activity in W identified a seismic
gap—marking vicinity of a Mw 7.6 quake in
1999.
Figure 5-24 Damage in
Izmit, Turkey—1999
17,000 deaths
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Recurrence Intervals and Predictions
• Records of past earthquakes,
including the size of the
rupture, allow us to identify
where future earthquakes are
likely to occur.
• The big question is “When?”
• Long-term records of quake
size and characteristics
establish the average time, or
recurrence interval.
FIGURE 5-25
Waiting for the Big
One Parkfield, California, residents
were ready for the predicted
earthquake, but it arrived 15 yrs late.
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Making Earthquake Forecasts
Figure 5-26 Looking to the Future
Earthquake forecasts give probability that a quake
will occur on a fault per specific time period. Globally
few areas have been sufficiently studied to justify a
forecast. Two areas of exception however are: San
Francisco Bay Region (a) and Los Angeles area (b).
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Mitigating Earthquake Hazards
through Mapping
Figure 5-27 The USGS National Shaking
Hazard Map
Colors on map show levels of horizontal
shaking that have a 10% chance of being
exceeded in a 50-yr period. Shaking = %
acceleration of falling object due to gravity (g).
Highest hazards are orange to red zones with
16–32+% s.
• USGS maintains national
maps showing peak horizontal
ground motion accelerations
expected from earthquakes
• Maps are part of the National
Earthquake Hazard Reduction
(NEHRP) program
• Mapping reveals major
seismic hazard zones such as
western states and New
Madrid region
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Engineering for Earthquakes
• Geologists mapped fault location and size of expected
earthquakes on it.
• Engineers designed
pipeline for crossing the
fault. The design worked.
• Pipeline withstood 5.5 m.
• (18 ft) of horiz. and 1 m
(3 ft) of vert. surface
displacement caused by
the November 3, 2002,
Mw 7.9 earthquake on
Figure 5-28 Trans-Alaska Pipeline
fault.
System (TAPS) Crosses the Active StrikeSlip Denali Fault
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Being Prepared for Earthquakes
Being prepared includes:
• Have a planned
emergency response
• Having an early
warning system
• Providing public
education and training
for preparedness
Figure 5-29 Map of Shaking Intensity
Created Immediately after the
Northridge Earthquake in California
Emergency services to tens of thousands
of people in the Northridge area were
efficiently provided and allocated for
based on the produced map.
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Can We Improve the Tsunami
Warning System?
• Communication is critical
throughout the world in the event
of a large earthquake occurring
in the ocean.
• Despite a network of warning
systems, the December 26,
2004, tsunami severely affected
many countries around the Indian
Ocean and killed over 225,000
people.
• This was more than any other
tsunami in recorded history.
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SUMMARY
• Movements in the dynamic geosphere cause
earthquakes—most along plate boundaries
but significant intraplate quakes also occur
• Earthquakes present many hazards, in some
cases even far away from where they occur
such as via development of tsunami
• Elastic rebound theory explains the
relationship between earthquakes, tectonic
stresses, and faulting.
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SUMMARY (cont.)
• Seismic energy moves as body waves or
surface waves
• Seismometers record ground motions from
seismic waves
• Scales: Richter, and more accurate Moment
Magnitude (Mw), ranks earthquakes based on
∑seismic energy released, and Modified Mercalli
Intensity
• Major hazards include liquefaction, permanent
surface displacements, landslides, tsunamis,
and fires
© 2011 Pearson Education, Inc.