Grotzinger, Jordan
Press and Siever
Understanding Earth
Fifth Edition
Chapter 13:
Earthquakes
Copyright © 2007 by W. H. Freeman & Company
Concepts
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Earthquake
Stress
Strain
Elastic Rebound Theory
Epicenter
Foreshocks, aftershocks
P, S and Surface waves (Love and Rayleigh)
Moment Magnitude
Earthquake Intensity
Tsunami
First Motion
What is an Earthquake??
An earthquake is caused by a sudden
rupture (break) of the earth
An earthquake generates vibrations that
travel through the earth, a.k.a
earthquake waves
1906
Why do Earthquakes happen?
•Plate Motion
How do earthquakes occur?
Elastic Rebound Theory is one
explanation of how earthquakes
take place.
Elastic Rebound Theory
•Pressure or stress, builds up in the
rock, much like a bending rubber
eraser in your hands.
•You can break a rubber eraser if the
pressure or stress you apply to it
exceeds the natural strength of the
rubber.
Elastic Rebound Theory
•There are two important concepts you should be
aware of. Stress in earth sciences refers to the
pressure the rock experiences and strain is the
actual damage or resultant deformation that we
can measure.
•While the pressure builds up in the rock the rock
DOES deform or strain. However, it does so
elastically. That is if you release the rock from
under stress it will BOUNCE back without any
signs of damage. Normal rocks are usually only
able to take a small (1-2%) of elastic strain.
Stretching or thinning beyond 1-2% can cause
permanent damage.
Elastic Rebound Theory
•When the rock breaks or ruptures some of the
energy goes into breaking the rock and a lot of it
goes into moving the surround rocks.
•The rupture itself may remain buried, invisible
underground (a blind fault). A rupture may or may
not reach the surface.
•After the stress or pressure is release the rock
seals ahd again starts to accumulate stress.
Elastic Rebound Theory
•Ruptures or breaks in the rock are planar cracks
that grow in all directions
•The rupture itself may remain buried, invisible
underground (a blind fault). A rupture may or may
not reach the surface.
Foreshocks and Aftershocks are smaller
earthquakes that precede and postdate the
main shock.
What is a foreshcok or aftershock depends on
when the main shock comes. But, this is really
only known AFTER the big earthquake occurs!
The “EARTHQUAKE GAME” -- 1988
A seismograph is a tool that measure
ground motion.
During the 1988 Auburn-LSU football
game LSU scored a late and winning
touchdown. The crowd cheered so
loudly that the “earthquake game”
made local history because the
ground motion appeared on the LSU
seismograph.
Seismograph
A seismograph consists of a real or
virtual mass that is so large that it
moves totally out of “sync” with the
frequency of the ground motion, that
is as IF IT WERE STILL. The mass
is not actually still but its large
inertia allows the faster motions of
the earthquake groundshake to be
registered accurately.
3 Types of Earthquake waves
• (P and S) When an earthquake
occurs it starts at depth, in the
BODY of the earth. At depth there
are only two types of particle motion
possible. (P and S waves) P and S
waves are known as BODY waves
because they always originate inside
the earth. These are not the most
damaging but they are the first to
be detected because they travel at
very fast speeds ( e.g. >6000 m/s.)
3 Types of Earthquake waves
• P waves are the fastest, and first
waves to arrive. S waves are the
second fastest (~50%) and Surface
waves (45%) are the slowest of the
three major types of earthquake
waves.
Particle Motion
--Image 1
Particle Motion
--Image 2
Particle Motion
--Image 3
Particle Motion
--Image 4
Particle Motion
--Image 5
3 Types of Earthquake waves
• (Surface) The most damaging
earthquake vibrations occur when
the P and S waves reach the surface.
At the surface they convert into
horizontal S waves and a combination
of vertical S and P waves called
Rayleigh waves.
The focus of an earthquake is the origin at
depth of that earthquake. The closest
place on the surface of the earth to the
focus is called the epicenter.
An epicenter is located by triangulating
between 3 or more seismograph stations,
using the arrival times of two or more of
the 3 types of earthquake waves.
Size of an Earthquake
An earthquake size can be
determined qualitatively or
quantitatively and there is an
equivalence between the two types
of scales.
Size of an Earthquake
(1) The Modified Mercalli Intensity
scale describes the perceived
intensity and damage, in 12
degrees, of the shaking and is
useful when there are no
seismographs, as in historical
earthquakes (e.g. New Madrid 7.5,
1811, Missouri). Damage depends
on whether there are people to
witness the earthquake effect and
on local conditions.
Size of an Earthquake
(2) The quantitative measure of
the strength of an earthquake is
the moment magnitude. Moment
magnitude is proportional to the
logarithm of the area faulted and
seismic energy released during the
rupture. It can be measured
directly from the seismograms.
ACTIVE FAULTS GENERATE
EARTHQUAKES (1)
Earthquakes generate different first motions
in different directions depending on the type
of fault that generates the earthquake.
•A first motion for a normal fault in a
horizontal direction is OUT from the fault
during the rupture. These tend to occur along
divergent plate margin boundaries.
ACTIVE FAULTS GENERATE
EARTHQUAKES (2)
Earthquakes generate different first motions
in different directions depending on the type
of fault that generates the earthquake.
•A first motion for a thrust fault in a
horizontal direction is IN to the fault during
the rupture. These tend to occur along
convergent plate margin boundaries.
ACTIVE FAULTS GENERATE
EARTHQUAKES (3)
•A first motion for a strike-slip fault in a
horizontal direction is IN to the fault in
certain orientations and OUT from the fault in
other orientations. The first motion changes
every 90 degrees around the epicenter.