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Chapter 10: Earthquakes and Earth’s
Interior
Introduction
 When the Earth quakes, the energy stored in
elastically strained rocks is suddenly released.
 The more energy released, the stronger the
quake.
 Massive bodies of rock slip along fault surfaces
deep underground.
 Earthquakes are key indicators of plate motion.
How Earthquakes Are Studied (1)
 Seismometers are used to record the shocks and
vibrations caused by earthquakes.
 All
seismometers make use of inertia, which is the
resistance of a stationary mass to sudden movement.

This is the principal used in inertial seismometers.
 The seismometer measures the electric current
needed to make the mass and ground move
together.
Figure 10.1
Figure 10.2
Figure B10.01
How Earthquakes Are Studied (2)
 Three inertial seismometers are commonly used
in one instrument housing to measure up-down,
east-west, north-south motions simultaneously.
Earthquake Focus And Epicenter
 The earthquake focus is the point where
earthquake starts to release the elastic strain of
surrounding rock.
 The epicenter is the point on Earth’s surface that
lies vertically above the focus of an earthquake.
 Fault slippage begins at the focus and spreads
across a fault surface in a rupture front.
 The rupture front travels at roughly 3 kilometers
per second for earthquakes in the crust.
Figure 10.3
Seismic Waves (1)
 Vibrational waves spread outward initially
from the focus of an earthquake, and continue
to radiate from elsewhere on the fault as
rupture proceeds.
Seismic Waves (2)
 There are two basic families of seismic waves.
 Body waves can transmit either:


Compressional motion (P waves), or
Shear motion (S waves).
 Surface
waves are vibrations that are trapped near
Earth’s surface. There are two types of surface waves:


Love waves, or
Rayleigh waves.
Body Waves (1)
 Body waves travel outward in all directions from
their point of origin.
 The first kind of body waves, a compressional
wave, deforms rocks largely by change of
volume and consists of alternating pulses of
contraction and expansion acting in the direction
of wave travel.
 Compressional
waves are the first waves to be
recorded by a seismometer, so they are called P (for
“primary”) waves.
Figure 10.4
Body Waves (2)
 The second kind of body waves is a shear wave.
 Shear waves deform materials by change of shape,
 Because shear waves are slower than P waves and
reach a seismometer some time after P waves arrives,
they are called S (for “secondary”) waves.
Body Waves (3)
 Compressional (P) waves can pass through solids,
liquid, or gases.
 P waves move more rapidly than other seismic
waves:


6 km/s is typical for the crust.
8 km/s is typical for the uppermost mantle.
Body Waves (4)
 Shear (S) waves consist of an alternating series
of side-wise movements.
 Shear
waves can travel only within solid matter.
 A typical speed for a shear wave in the crust is 3.5
km/s, 5 km/s in the uppermost mantle.
 Seismic body waves, like light waves and sound
waves, can be reflected and refracted by change
in material properties.
 When change in material properties results in a
change in wave speed, refraction bends the
direction of wave travel.
Figure 10.5
Body Waves (5)
 For seismic waves within Earth, the changes in
wave speed and wave direction can be either
gradual or abrupt, depending on changes in
chemical composition, pressure, and mineralogy.
 If Earth had a homogeneous composition and
mineralogy, rock density and wave speed would
increase steadily with depth as a result of increasing
pressure (gradual refraction).

Measurements reveal that the seismic waves are refracted
and reflected by several abrupt changes in wave speed.
Figure 10.6
Surface Waves (1)



Surface waves travel more slowly than P waves and
S waves, but are often the largest vibrational signals
in a seismogram.
Love waves consist entirely of shear wave
vibrations in the horizontal plane, analogous to an S
wave that travels horizontally.
Rayleigh waves combine shear and compressional
vibration types, and involve motion in both the
vertical and horizontal directions.
Figure 10.7
Surface Waves (2)

The longer the wave length of a surface wave, the
deeper the wave motion penetrates Earth. Surface
waves of different wave lengths develop different
velocities. This Behavior is called Dispersion
Determining The Epicenter (1)

An earthquake’s epicenter can be calculated from
the arrival times of the P and S waves at a
seismometer.

The farther a seismometer is away from an epicenter, the
greatest the time difference between the arrival of the P
and S waves.
Determining The Epicenter (2)

The epicenter can be determined when data from
three or more seismometers are available.


It lies where the circles intersect (radius = calculated
distance to the epicenter).
The depth of an earthquake focus below an
epicenter can also be determined, using P-S time
intervals.
Figure 10.8
Figure 10.9
Earthquake Magnitude


The Richter magnitude scale is divided into steps
called magnitudes with numerical values M.
Each step in the Richter scale, for instance, from
magnitude M = 2 to magnitude M = 3, represents
approximately a thirty fold increase in earthquake
energy.
Earthquake Frequency (1)



Each year there are roughly 200 earthquakes
worldwide with magnitude M = 6.0 or higher.
Each year on average, there are 20 earthquakes with
M = 7.0 or larger.
Each year on average, there is one “great”
earthquake with M = 8.0 or larger.
Earthquake Frequency (2)

Four earthquakes in the twentieth century met or
exceeded magnitude 9.0.




1952 in Kamchatka (M = 9.0).
1957 in the Aleutian Island (M = 9.1).
1964 in Alaska (M = 9.2).
1960 in Chile (M = 9.5).
Earthquake Frequency (3)

The nuclear bomb dropped in 1945 on the Japanese
city of Hiroshima was equal to an earthquake of
magnitude M = 5.3.

The most destructive man-made devices are small in
comparison with the largest earthquakes.
Earthquake Hazard




Seismic events are most common along plate
boundaries.
Earthquakes associated with hot spot volcanism
pose a hazard to Hawaii.
Earthquakes are common in much of the
intermontane western United States (Nevada, Utah,
and Idaho).
Several large earthquakes jolted central and eastern
North America in the nineteenth century (New
Madrid, Missouri, 1811 and 1812).
Figure 10.10
Earthquake Disasters (1)

In Western nations, urban areas that are known to be
earthquake-prone have special building codes that
require structures to resist earthquake damage.


However, building codes are absent or ignored in many
developing nations.
In the 1976 T’ang Shan earthquake in China,
240,000 people lost their lives.
Earthquake Disasters (2)


Eighteen earthquakes are known to have caused
50,000 or more deaths apiece.
The most disastrous earthquake on record occurred
in 1556, in Shaanxi province, China, where in
estimated 830,000 people died.
Earthquake Damage (1)


Earthquakes have six kinds of destructive effects.
Primary effects:


Ground motion results from the movement of seismic
waves.
Where a fault breaks the ground surface itself, buildings
can be split or roads disrupted.
Earthquake Damage (2)

Secondary effects:




Ground movement displaces stoves, breaks gas lines, and
loosens electrical wires, thereby starting fires.
In regions of steep slopes, earthquake vibrations may
cause regolith to slip and cliffs to collapse.
The sudden shaking and disturbance of water-saturated
sediment and regolith can turn seemingly solid ground to
a liquid mass similar to quicksand (liquefaction).
Earthquakes generate seismic sea waves, called tsunami,
which have been particularly destructive in the Pacific
Ocean.
Modified Mercalli Scale


This scale is based on the amount of vibration
people feel during low-magnitude quakes, and the
extent of building damage during high-magnitude
quakes.
There are 12 degrees of intensity in the modified
Mercalli scale.
World Distribution of Earthquakes



Subduction zones have the largest quakes.
The circum-Pacific belt, where about 80 percent of
all recorded earthquakes originate, follows the
subduction zones of the Pacific Ocean.
The Mediterranean-Himalayan belt is responsible
for 15 percent of all earthquakes.
Figure 10.15
Depth of Earthquake Foci


Most foci are no deeper than 100 km. down in the
Benioff zone, that extends from the surface to as
deep as 700 km.
No earthquakes have been detected at depths below
700 km. Two hypotheses may explain this.


Sinking lithosphere warms sufficiently to become entirely
ductile at 700 km depth.
The slab undergoes a mineral phase change near 670 km
depth and loses its tendency to fracture.
Figure 10.16
First-Motion Studies Of The
Earthquake Source



If the first motion of the arriving P wave pushes the
seismometer upward, then fault motion at the
earthquake focus is toward the seismometer.
If the first motion of the P wave is downward, the
fault motion must be away from the seismometer.
S-waves and surface waves also carry the signature
of earthquake slip and fault orientation and can
provide independent estimates of motion at the
earthquake focus.
Figure 10.17
Figure 10.18
Earthquake Forecasting And Prediction
(1)


Forecasting identifies both earthquake-prone areas
and man-made structures that are especially
vulnerable to damage from shaking.
Earthquake prediction refers to attempts to estimate
precisely when the next earthquake on a particular
fault is likely to occur.
Earthquake Forecasting And Prediction
(2)



Earthquake forecasting is based largely on elastic
rebound theory and plate tectonics.
The elastic rebound theory suggests that if fault
surfaces do not slip easily past one another, energy
will be stored in elastically deformed rock, just as in
a steel spring that is compressed.
Currently, seismologists use plate tectonic motions
and Global positioning System (GPS)
measurements to monitor the accumulation of strain
in rocks near active faults.
Figure 10.19
Earthquake Forecasting And Prediction
(3)


Earthquake prediction has had few successes.
Earthquake precursors:



Suspicious animal behavior.
Unusual electrical signals.
Many large earthquakes are preceded by small
earthquakes called foreshocks (Chinese authorities used
an ominous series of foreshocks to anticipate (the
Haicheng earthquake in 1975).
Figure 10.20
Figure B02
Figure 10.21
Using Seismic Waves As Earth Probes
(1)


Seismic waves are the most sensitive probes we
have to measure the properties of the unseen parts
of the crust, mantle, and core.
Distinct boundaries (or discontinuities) can be
readily detected by refraction and reflection of body
waves deep within Earth.
Figure 10.22
Using Seismic Waves As Earth Probes
(2)



Early in the twentieth century, the boundary
between Earth’s crust and mantle was demonstrated
by a Croatian scientist named Mohorovicic.
A distinct compositional boundary separated the
crust from this underlying zone of different
composition (the Mohorovicic discontinuity).
Seismic wave speeds can be measured for different
rock types in both the laboratory and the field.
Figure 10.23
Using Seismic Waves As Earth Probes
(3)

The thickness and composition of continental crust
vary greatly from place to place.


Thickness ranges from 20 to nearly 70 km and tends to be
thickest beneath major continental collision zones, such
as Tibet.
P-wave speeds in the crust range between 6 and 7
km/s. Beneath the Moho, speeds are greater than 8
km/s.
Using Seismic Waves As Earth Probes
(4)


Laboratory tests show that rocks common in the
crust, such as granite, gabbro, and basalt, all have Pwave speeds of 6 to 7 km/s.
Rocks that are rich in dense minerals, such as
olivine, pyroxene, and garnet, have speeds greater
than 8 km/s.

Therefore, the most common such rock, called peridotite,
must be among the principal materials of the mantle.
Using Seismic Waves As Earth Probes
(5)

Some evidence can be obtained from rare samples
of mantle rocks found in kimberlite pipes—narrow
pipe-like masses of intrusive igneous rock,
sometimes containing diamonds, that intrude the
crust but originate deep in the mantle.
Using Seismic Waves As Earth Probes
(6)

Both P and S waves are strongly influenced by a
pronounced boundary at a depth of 2900 km.


Geologists infer that it is the boundary between the
mantle and the core.
Seismic-wave speeds calculated from travel times
indicate that rock density increases from about 3.3
g/cm3 at the top of the mantle to about 5.5 g/cm3 at
the base of the mantle.
Figure 10.25
Using Seismic Waves As Earth Probes
(7)


To balance the less dense crust and the mantle, the
core must be composed of material with a density of
at least 10 to 13 g/cm3.
The only common substance that comes close to
fitting this requirement is iron.
Using Seismic Waves As Earth Probes
(8)



Iron meteorites are samples of material believed to
have come from the core of ancient, tiny planets,
now disintegrated.
All iron meteorites contain a little nickel; thus,
Earth’s core presumably does too.
P-wave reflections indicate the presence of a solid
inner core enclosed within the molten outer core.
Layers of Different Physical Properties
in the Mantle


The P-wave velocity at the top of the mantle is
about 8 km/s and it increases to 14 km/s at the coremantle boundary.
The low-velocity zone can be seen as a small blip in
both the P-wave and S-wave velocity curves.


An integral part of the theory of plate tectonics is the idea
that stiff plates of lithosphere slide over a weaker zone in
the mantle called the asthenosphere.
In the low velocity zone rocks are closer to their melting
point than the rock above or below it.
Figure 10.26
The 400-km Seismic Discontinuity


From the P-and S-wave curves, velocities of both P
and S waves increase in a small jump at about 400
km.
When olivine is squeezed at a pressure equal to that
at a depth of 400 km, the atoms rearrange
themselves into a denser polymorph (polymorphic
transition).
The 670-km Seismic Discontinuity


An increase in seismic-wave velocities occurs at a
depth of 670 km.
The 670-km discontinuity may correspond to a
polymorphic change affecting all silicate minerals
present.
Seismic Waves and Heat (1)


Seismic wave speed is affected by temperature.
Seismologists translate travel-time discrepancies
into maps of ‘fast” and “slow” regions of Earth’s
interior using seismic tomography.
Figure 10.27
Seismic Waves and Heat (2)


Researchers hypothesize that these “slow’’ regions
are the hot source rocks of most mantle plumes.
Near active volcanoes, seismologists have
interpreted travel-time discrepancies to reconstruct
the location of hot and partially molten rock that
supplies lava for eruptions.
Figure 10.28
Earthquakes Influence Geochemical
Cycles (1)



Earthquakes play an important role in the transport
of volatiles through Earth’s solid interior.
Earthquakes facilitate the concentration of many
important metals into ore deposits.
In the mantle, the carbon and hydrologic cycles are
fed when the subducting slab releases water, CO2,
and other volatiles at roughly 100-km depth beneath
the overriding plate.
Earthquakes Influence Geochemical
Cycles (2)

Some seismologists speculate that water released
from the slab helps cause brittle fracture in the slab
itself, and that water may be necessary for deep
earthquakes to occur in the Benioff zone.