adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Earthquakes occur when energy stored in elastically strained rocks is
suddenly released. This release of energy causes intense ground shaking in
the area near the source of the earthquake and sends waves of elastic
energy, called seismic waves, throughout the Earth. Earthquakes can be
generated by bomb blasts, volcanic
eruptions, and sudden slippage along faults. Earthquakes are
definitely a geologic hazard for those living in earthquake prone areas,
but the seismic
waves generated by earthquakes are invaluable for studying the
interior of the Earth.
Origin of Earthquakes
Most natural earthquakes are caused by sudden slippage along a fault
zone. The elastic rebound theory suggests that if slippage along a fault is
hindered such that elastic strain energy builds up in the deforming rocks
on either side of the fault, when the slippage does occur, the energy
released causes an earthquake. This theory was discovered by making
measurements at a number of points across a fault. Prior to an earthquake
it was noted that the rocks adjacent to the fault were bending. These
bends disappeared after an earthquake suggesting that the energy stored in
bending the rocks was suddenly released during the earthquake.
Seismology, The Study of Earthquakes
When an earthquake occurs, the elastic energy is released and sends out
vibrations that travel throughout the Earth. These vibrations are called
seismic waves. The study of how seismic waves behave in the Earth is
Seismographs - Seismic waves travel through the Earth as vibrations. A
seismometer is an instrument used to record these vibrations and the
resulting graph that shows the vibrations is called a seismograph. The
seismometer must be able to move with the vibrations, yet part of it must
remain nearly stationary.
This is accomplished by isolating the recording device (like a pen) from
the rest of the Earth using the principal of inertia. For example, if the
pen is attached to a large mass suspended by a spring, the spring and the
large mass move less than the paper which is attached to the Earth, and on
which the record of the vibrations is made.
Seismic Waves - The source of an earthquake is called the focus,
which is an exact location within the Earth were seismic waves are
generated by sudden release of stored elastic energy. The epicenter is the
point on the surface of the Earth directly above the focus. Sometimes the
media get these two terms confused. Seismic waves emanating from the focus
can travel in several ways, and thus there are several different kinds of
Where, Vp is the velocity of the P-wave, K is the incompressibility
of the material, m is the rigidity of the material, and r is the
density of the material.
- Secondary waves, also called shear waves. They travel with a
velocity that depends only on the rigidity and density of the
material through which they travel:
Vs = Ö [( m )/r ]
S-waves travel through material by shearing it or changing its shape in
the direction perpendicular to the direction of travel. The resistance to
shearing of a material is the property called the rigidity. It is notable
that liquids have no rigidity, so that the velocity of an S-wave is zero
in a liquid. (This point will become important later). Note that S-waves
travel slower than P-waves, so they will reach a seismograph after the
- Surface waves differ from body waves in that they do not travel through
the Earth, but instead travel along paths nearly parallel to the surface
of the Earth. Surface waves behave like S-waves in that they cause up and
down and side to side movement as they pass, but they travel slower than
S-waves and do not travel through the body of the Earth.
The record of an earthquake, a seismograph, as recorded by a seismometer,
will be a plot of vibrations versus time. On the seismograph, time is
marked at regular intervals, so that we can determine the time of arrival
of the first P-wave and the time of arrival of the first S-wave.
P-waves are the same thing as sound waves. They move through the material
by compressing it, but after it has been compressed it expands, so that
the wave moves by compressing and expanding the material as it travels.
Thus the velocity of the P-wave depends on how easily the material can be
compressed (the incompressibility), how rigid the material is (the
rigidity), and the density of the material. P-waves have the highest
velocity of all seismic waves and thus will reach all seismographs first.
Because P-waves have a higher velocity than S-waves, the P-waves arrive at
the seismographic station before the S-waves.
Location of Earthquakes
Location of Earthquakes - In order to determine the location of an
earthquake, we need to have recorded a seismograph of the earthquake from
at least three
seismographic stations at different distances from the epicenter of
the quake. In addition, we need one further piece of information - that is
the time it takes for P-waves and S-waves to travel through the Earth and
arrive at a seismographic station. Such information has been collected
over the last 80 or so years, and is available as travel time curves.
From the seismographs at each station one determines the S-P interval (the
difference in the time of arrival of the first S-wave and the time of
arrival of the first P-wave. Note that on the travel time curves, the S-P
interval increases with increasing distance from the epicenter. Thus the
S-P interval tells us the distance to the epicenter from the seismographic
station where the earthquake was recorded. Thus at each station we can
draw a circle on a map that has a radius equal to the distance from the
Three such circles will intersect in a point that locates the epicenter of
Magnitude of Earthquakes
Magnitude of Earthquakes - Whenever a large destructive earthquake
occurs in the world the press immediately wants to know where the
earthquake occurred and how big the earthquake was (in California the
question is usually - Was this the Big One?). The size of an earthquake is
usually given in terms of a scale called the Richter Magnitude. Richter
Magnitude is a scale of earthquake size developed by a seismologist named
Conrad Richter. The Richter Magnitude involves measuring the amplitude
(height) of the largest recorded wave at a specific distance from the
earthquake. While it is correct to say that for each increase in 1 in the
Richter Magnitude, there is a tenfold increase in amplitude of the wave,
it is incorrect to say that each increase of 1 in Richter Magnitude
represents a tenfold increase in the size of the Earthquake (as is
commonly incorrectly stated by the Press).
A better measure of the size of an earthquake is the amount of energy
released by the earthquake. The amount of energy released is related to
the Richter Scale by the following equation:
Log E = 11.8 + 1.5 M
Where Log refers to the logarithm to the base 10, E is the energy
released in ergs, and M is the Richter Magnitude. Anyone with a hand
calculator can solve this equation by plugging in various values of M
and solving for E, the energy released. I've done the calculation for
you in the following table:
2.0 x 1013
6.3 x 1014
2.0 x 1016
6.3 x 1017
2.0 x 1019
6.3 x 1020
2.0 x 1022
6.3 x 1023
From these calculations you can see that each increase in 1 in Richter
Magnitude represents a 31 fold increase in the amount of energy
released. Thus, a magnitude 7 earthquake releases 31 times more energy
than a magnitude 6 earthquake. A magnitude 8 earthquake releases 31 x 31
or 961 times more energy than a magnitude 6 earthquake.
The Hiroshima atomic bomb released an amount of energy equivalent to a
magnitude 5.5 earthquake. The largest earthquake recorded, the Alaska
earthquake in 1964, had a Richter Magnitude of about 8.6. Note that
larger earthquakes are possible, but have not been recorded by humans.
Ground Shaking - Shaking of the ground caused by the passage of
seismic waves near the epicenter of the earthquake is responsible
for the collapse of most structures. The intensity of ground shaking
depends on distance from the epicenter and on the type of bedrock
underlying the area.
In general, loose unconsolidated sediment is subject to more
intense shaking than solid bedrock.
Damage to structures from shaking depends on the type of
construction. Concrete and masonry structures, because they are
brittle are more susceptible to damage than wood and steel
structures, which are more flexible.
Ground Rupture - Ground rupture only occurs along the fault zone
that moves during the earthquake. Thus structures that are built
across fault zones may collapse, whereas structures built adjacent
to, but not crossing the fault may survive.
Fire - Fire is a secondary effect of earthquakes. Because power
lines may be knocked down and because natural gas lines may rupture
due to an earthquake, fires are often started closely following an
earthquake. The problem is compounded if water lines are also broken
during the earthquake since there will not be a supply of water to
extinguish the fires once they have started. In the 1906 earthquake
in San Francisco more than 90% of the damage to buildings was caused
Rapid Mass-Wasting Processes - In mountainous regions subjected to
earthquakes ground shaking may trigger rapid mass-wasting events like
rock and debris falls, rock and debris slides, slumps, and debris
Liquefaction - Liquefaction is a processes that occurs in
water-saturated unconsolidated sediment due to shaking. In areas
underlain by such material, the groundshaking causes the grains to loose
grain to grain contact, and thus the material tends to flow. You can
demonstrate this process to yourself next time your go the beach. Stand
on the sand just after an incoming wave has passed. The sand will easily
support your weight and you will not sink very deeply into the sand if
you stand still. But, if you start to shake your body while standing on
this wet sand, you will notice that the sand begins to flow as a result
of liquefaction, and your feet will sink deeper into the sand.
Tsunamis are giant ocean waves that can rapidly travel across oceans, as
we discussed in the Oceans and Their Margins. Earthquakes that occur
along coastal areas can generate tsunamis, which can cause damage
thousands of kilometers away on the other side of the ocean.
Plate Boundaries. Diverging plate boundaries are zones where
two plates move away from each other, such as at oceanic ridges. In
such areas the lithosphere is in a state of tensional stress and
thus normal faults and rift valleys occur. Earthquakes that occur
along such boundaries show normal fault motion and tend to be
shallow focus earthquakes, with focal depths less than about 20 km.
Such shallow focal depths indicate that the brittle lithosphere must
be relatively thin along these diverging plate boundaries.
Transform Fault Boundaries. Transform fault boundaries are
plate boundaries where lithospheric plates slide past one another in
a horizontal fashion. The San Andreas Fault of California is one of
the longer transform fault boundaries known. Earthquakes along these
boundaries show strike-slip motion on the faults and tend to be
shallow focus earthquakes with depths usually less than about 50 km.
Converging Plate Boundaries - Convergent plate boundaries are
boundaries where two plates run into each other. Thus, they tend to
be zones where compressional stresses are active and thus reverse
faults or thrust faults are common. There are two types of
converging plate boundaries. (1) subduction boundaries, where
oceanic lithosphere is pushed beneath either oceanic or continental
lithosphere; and (2) collision boundaries where two plates with
continental lithosphere collide.
Subduction boundaries -At subduction boundaries cold oceanic
lithosphere is pushed back down into the mantle where two plates
converge at an oceanic trench. Because the subducted lithosphere is
cold it remains brittle as it descends and thus can fracture under
the compressional stress. When it fractures, it generates
earthquakes that define a zone of earthquakes with increasing focal
depths beneath the overriding plate. This zone of earthquakes is
called the Benioff Zone. Focal depths of earthquakes in the Benioff
Zone can reach down to 700 km.
boundaries - At collisional boundaries two plates of
continental lithosphere collide resulting in fold-thrust mountain
belts. Earthquakes occur due to the thrust faulting and range in
depth from shallow to about 200 km.
The Earth's Internal Structure
Much of what we know about the interior
the Earth comes from knowledge of seismic wave velocities and
their variation with depth in the Earth. Recall that body wave
velocities are as follows: Vp = Ö [(K + 4/3m )/r ] Vs = Ö [( m )/r ]
Where K = incompressibility m = rigidity r = density If the properties
of the earth, i.e. K, m, and r where the same throughout, then Vp and Vs
would be constant throughout the Earth and seismic waves would travel
along straight line paths through the Earth. We know however that
density must change with depth in the Earth, because the density of the
Earth is 5,200 kg/cubic meter and density of crustal rocks is about
2,500 kg/cubic meter. If the density were the only property to change,
then we could make estimates of the density, and predict the arrival
times or velocities of seismic waves at any point away from an
earthquake. Observations do not follow the predictions, so, something
else must be happening. In fact we know that K, m, and r change due to
changing temperatures, pressures and compositions of material. The job
of seismology is, therefore, to use the observed seismic wave velocities
to determine how K, m, and r change with depth in the Earth, and then
infer how P, T, and composition change with depth in the Earth. In other
words to tell us something about the internal structure of the Earth.
If the seismic wave velocity in the rock above an interface is
less than the seismic wave velocity in the rock below the
interface, the waves will be refracted or bent upward relative to
their original path.
If the seismic wave velocity decreases when passing into the
rock below the interface, the waves will be refracted down
relative to their original path.
If the seismic wave velocities gradually increase with depth in
the Earth, the waves will continually be refracted along curved
paths that curve back toward the Earth's surface.
One of the earliest discoveries of seismology was a discontinuity at a
depth of 2900 km where the velocity of P-waves suddenly decreases.
This boundary is the boundary between the mantle and the core and was
discovered because of a zone on the opposite side of the Earth from an
Earthquake focus receives no direct P-waves because the P-waves are
refracted inward as a result of the sudden decrease in velocity at the
boundary. This zone is called a P-wave shadow zone.
This discovery was followed by the discovery of an S-wave shadow zone.
The S-wave shadow zone occurs because no S-waves reach the area on the
opposite side of the Earth from the focus. Since no direct S-waves
arrive in this zone, it implies that no S-waves pass through the core.
This further implies the velocity of S-wave in the core is 0. In
liquids m = 0, so S-wave velocity is also equal to 0. From this it is
deduced that the core, or at least part of the core is in the liquid
state, since no S-waves are transmitted through liquids. Thus, the
S-wave shadow zone is best explained by a liquid outer core.
Seismic Wave Velocities in the Earth
Over the years seismologists have collected data on how seismic wave
velocities vary with depth in the Earth. Distinct boundaries, called
discontinuities are observed when there is sudden change in physical
properties or chemical composition of the Earth. From these
discontinuities, we can deduce something about the nature of the
various layers in the Earth. As we discussed way back at the beginning
of the course, we can look at the Earth in terms of layers of
differing chemical composition, and layers of differing physical
Layers of Differing Composition
The Crust - Mohorovicic discovered boundary the boundary between
crust and mantle, thus it is named the Mohorovicic Discontinuity
or Moho, for short. 25-30km thick The composition of the crust can
be determined from seismic waves by comparing seismic wave
velocities measured on rocks in the laboratory with seismic wave
velocities observed in the crust. Then from travel times of waves
on many earthquakes and from many seismic stations, the thickness
and composition of the crust can be inferred.
In the ocean basins crust is about 8 to 10 km thick, and has a
composition that is basaltic.
The Mantle - Seismic wave velocities increase abruptly at the
Moho. In the mantle wave velocities are consistent with a rock
composition of peridotite which consists of olivine, pyroxene, and
The Core - At a depth of 2900 Km P-wave velocities suddenly
decrease and S-wave velocities go to zero. This is the top of the
outer core. As discussed above, the outer core must be liquid
since S-wave velocities are 0. At a depth of about 4800 km the
sudden increase in P-wave velocities indicate a solid inner core.
The core appears to have a composition consistent with mostly Iron
with small amounts of Nickel.
At a depth of about 100 km there is a sudden decrease in both P
and S-wave velocities. This boundary marks the base of the
lithosphere and the top of the asthenosphere. The lithosphere is
composed of both crust and part of the upper mantle. It is a
brittle layer that makes up the plates in plate tectonics, and
appears to float and move around on top of the more ductile
At the top of the asthenosphere is a zone where both P- and
S-wave velocities are low. This zone is called the Low-Velocity
Zone (LVZ). It is thought that the low velocities of seismic waves
in this zone are caused by temperatures approaching the partial
melting temperature of the mantle, causing the mantle in this zone
to behave in a very ductile manner.
At a depth of 400 km there is an abrupt increase in the
velocities of seismic waves, thus this boundary is known as the
400 - Km Discontinuity. Experiments on mantle rocks indicate that
this represents a temperature and pressure where there is a
polymorphic phase transition, involving a change in the crystal
structure of Olivine, one of the most abundant minerals in the
Another abrupt increase in seismic wave velocities occurs at a
depth of 670 km. It is uncertain whether this discontinuity, known
as the 670 Km Discontinuity, is the result of a polymorphic phase
transition involving other mantle minerals or a compositional
change in the mantle, or both.
Gravity Anomalies and Isostasy
The shape of earth is not a uniform sphere, but bulges at equator due
to centrifugal force of rotation. Thus radius at equator is about 21
km greater than at poles. Since gravity is a force of attraction
between two bodies and is inversely proportional to the distance
between the two bodies, the pull of gravity is greater at the poles
than at the equator. Thus, a person weighing 90.5 kg (199 lbs.) at the
pole would weigh 90 kg (198 lbs.) at equator. But the variation is not
smooth, there are topographic effects and differences in the mass of
rock bodies near the Earth's surface. Gravity Measurements and Gravity
Gravity is measured with a device known as a gravimeter. A gravimeter
can measure differences in the pull of gravity to as little as 1 part
in 100 million. Measurements of gravity can detect areas where there
is a deficiency or excess of mass beneath the surface of the Earth.
These deficiencies or excesses of mass are called gravity anomalies.
A positive gravity anomaly indicates that an excess of mass exits
beneath the area.
A negative gravity anomaly indicates that there is less mass beneath
Negative anomalies exist beneath mountain ranges, and mirror the
topography and crustal thickness as determined by seismic studies.
Thus, the low density continents appear to be floating on higher
The Principle of isostasy states that there is a flotational balance
between low density rocks and high density rocks. i.e. low density
crustal rocks float on higher density mantle rocks. The height at
which the low density rocks float is dependent on the thickness of the
low density rocks. Continents stand high because they are composed of
low density rocks (granitic composition). Ocean basins stand low,
because they are composed of higher density basaltic and gabbroic
Isostasy is best illustrated by effects of glaciation. During an ice
age crustal rocks that are covered with ice are depressed by the
weight of the overlying ice. When the ice melts, the areas previously
covered with ice have been uplifted.