Earthquakes
11
Earthquakes begins with a discussion of earthquakes, faults, and origination of earthquakes including elastic
rebound. Following a detailed look at the San Andreas Fault, seismology is introduced with a presentation of
the types of seismic waves, their propagation, and how they appear on a typical seismic traces. This is
followed by a discussion of earthquake epicenters – how they are located and their worldwide distribution.
Earthquake intensity and magnitude are also explained. The destruction caused by seismic vibrations and their
associated perils introduces a discussion of earthquake prediction. The chapter closes with an examination of
whether earthquakes can be predicted and the use of earthquakes as evidence for plate tectonics.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Explain the origin of earthquakes, including their relationship to faults.
Briefly discuss elastic rebound and the accumulation of strain in rocks.
Discuss seismology, including the characteristics and recording of earthquake waves.
Explain how to locate the source of an earthquake.
Compare and contrast shallow- versus deep-focus earthquakes.
Discuss the measurement of earthquake intensity and magnitude.
Explain the use and limitations of the modified Mercalli Intensity scale.
Compare and contrast earthquake magnitude measurements using the Richter scale with the newer
concept of moment magnitude.
Discuss the various aspects of earthquake destruction, including some of the factors that determine
the extent of damage produced by a given earthquake.
Explain earthquake prediction in terms of both short-range and long-range forecasting.
Understand the occurrence of earthquakes in relation to tectonic plate boundaries.
Chapter Outline___________________________________________________________________
I.
What is an earthquake?
A. An earthquake is the vibration of Earth
produced by the rapid release of energy
1. Energy released radiates in all
directions from its source, the focus
2. Energy is in the form of waves
3. Sensitive instruments around the
world record the event
B. Earthquakes and faults
1.
Movements that produce earthquakes
are usually associated with large
fractures in Earth’s crust called faults
2. Most of motion along faults can be
explained by the plate tectonics
theory
C. Elastic rebound
1. Mechanism for earthquakes was first
explained by H.F. Reid
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a.
D.
Rocks on both sides of an existing
fault are deformed by tectonic
forces
b. Rocks bend and store elastic
energy
c. Frictional resistance holding the
rocks together is overcome
d. Slippage at the weakest point (the
focus) occurs
e. Earthquakes occur as the
deformed rock “springs back” to
its original shape (elastic rebound)
f. Earthquakes most often occur
along existing faults whenever the
frictional forces on the fault
surfaces are overcome
Foreshocks and aftershocks
1.
2.
II.
Adjustments that follow a major
earthquake often generate smaller
earthquakes called aftershocks
Small earthquakes, called foreshocks,
often precede a major earthquake by
days or, in some cases, by as much as
several years
San Andreas Fault: an active earthquake
zone
A. San Andreas is the most studied fault
system in the world
B. Displacement occurs along discrete
segments 100 to 200 kilometers long
1. Some portions exhibit slow, gradual
displacement known as fault creep
2. Other segments regularly slip
producing small earthquakes
3. Still other segments store elastic
energy for hundreds of years before
rupturing in great earthquakes
a. Process described as stick-slip
motion
b. Great earthquakes should occur
about every 50 to 200 years along
these sections
III.
Seismology
A. The study of earthquake waves,
seismology, dates back almost 2000
years to the Chinese
B. Seismographs, instruments that record
seismic waves
1. Records the movement of Earth in
relation to a stationary mass on a
rotating drum or magnetic tape
2. More than one type of seismograph is
needed to record both vertical and
horizontal ground motion
3. Records obtained are called
seismographs
C. Types of seismic waves
1. Surface waves
a. Travel along outer part of Earth
b. Complex motion
c. Cause greatest destruction
d. Waves exhibit greatest amplitude
and slowest velocity
e. Waves have the greatest periods
(time interval between crests)
f. Often referred to as long waves,
or L waves
2. Body waves
a. Travel through Earth’s interior
b. Two types based on mode of
travel
1. Primary (P) waves
a. Push-pull (compress and
expand) motion, changing
the volume of the
intervening material
b. Travel through
1. Solids
2. Liquids
3. Gases
c. Generally, in any solid
material, P waves travel
about 1.7 times faster than
S waves
2. Secondary (S) waves
a.
"Shake" motion at right
angles to their direction of
travel
Earthquakes
b.
c.
d.
e.
IV.
Temporarily change the
shape of the material that
transmits them
Travel only through solids
Slower velocity than P
waves
Slightly greater amplitude
than P waves
Locating the source of earthquakes
A. Terms
1. Focus – the place within Earth where
earthquake waves originate
2. Epicenter – location on the surface
directly above the focus
B. Epicenter is located using the difference
in velocities of P and S waves
1. Three station recordings are needed to
locate an epicenter
2. Each station determines the time
interval between the arrival of the
first P wave and the first S wave at
their location
3. A travel-time graph is used to
determine each station’s distance to
the epicenter
4. A circle with a radius equal to the
distance to the epicenter is drawn
around each station
5. The point where all three circles
intersect is the earthquake epicenter
C. Earthquake belts
1. About 95 percent of the energy
released by earthquakes originates in
a few relatively narrow zones that
wind around the globe
2. Major earthquake zones include the
a. Circum-Pacific belt
b. Mediterranean Sea region,
through Iran, and on past the
Himalayan complex
c. Oceanic ridge system
D. Earthquake depths
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1.
Earthquakes originate at depths
ranging from 5 to nearly 700
kilometers
2. Earthquake foci arbitrarily classified
as
a. Shallow – within 70 kilometers of
the surface
b. Intermediate – between 70 and
300 kilometers
c. Deep – greater than 300
kilometers
3. Definite patterns exist
a. Shallow focus occur along the
oceanic ridge system
b. Almost all deep-focus earthquakes
occur in the circum-Pacific belt,
particularly in regions situated
landward of deep-ocean trenches
1. In the Tonga trench region,
foci depths increase with
distance from the trench
2. Called Wadati-Benioff zones
V.
Measuring the size of earthquakes
A. Two measurements that describe the size
of an earthquake are
1. Intensity – a measure of the degree of
earthquake shaking at a given locale
based on the amount of damage
2. Magnitude – estimates the amount of
energy released at the source of the
earthquake
B. Intensity scales
1. Modified Mercalli Intensity Scale was
developed using California buildings
as its standard
2. The drawback of intensity scales is
that destruction may not be a true
measure of the earthquakes actual
severity
C. Magnitude scales
1. Richter magnitude
a. Concept introduced by Charles
Richter in 1935
b. Richter scale
1.
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Based on the amplitude of the
largest seismic wave recorded
Accounts for the decrease in
wave amplitude with increased
distance
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3.
Largest magnitude recorded on
a Wood-Anderson
seismograph was 8.9
4. Magnitudes less than 2.0 are
not felt by humans
5. Each unit of Richter magnitude
increase corresponds to
a. A tenfold increase in wave
amplitude
b. About a 32-fold energy
increase
2. Other magnitude scales
a. Several “Richter-like” magnitude
scales have been developed
b. Moment magnitude
1. Developed because none of the
“Richter-like” magnitude
scales adequately estimates the
size of very large earthquakes
2. Derived from the amount of
displacement that occurs along
a fault
3. Calculated using
a. Average amount of
displacement along the fault
b. Area of the rupture surface
c. Shear strength of the
faulted block
4. Has gained wide acceptance
among seismologists and
engineers
VI.
2.
Earthquake destruction
A. Amount of structural damage
attributable to earthquake vibrations
depends on
1. Intensity and
2. Duration of the vibrations
3. Nature of the material upon which
the structure rests
4. Design of the structure
B. Destruction from seismic vibrations
1. Ground shaking
a.
Regions within 20 to 50
kilometers of the epicenter will
experience about the same
intensity of ground shaking
b. However, destruction varies
considerably mainly due to the
nature of the ground on which the
structures are built
1. Soft sediments generally
amplify vibrations more than
solid bedrock
2. Regions farther from the
epicenter may experience more
damage than near locations
because of amplified ground
motion attributed to soft
sediments
2. Liquefaction of the ground
a. Unconsolidated materials
saturated with water turn into a
mobile fluid
b. Underground objects may float
toward the surface
3. Seiches
a. The rhythmic sloshing of water in
lakes, reservoirs, and enclosed
basins
b. Waves can weaken reservoir walls
and cause destruction
C. Tsunamis, or seismic sea waves
1. Destructive waves that are often
inappropriately called “tidal waves”
2. Result from
a. Vertical displacement along a
fault located on the ocean floor or
b. From a large undersea landslide
triggered by an earthquake
3. In the open ocean, height is usually
less than 1 meter
4. In shallower coastal waters the water
piles up to heights that occasionally
exceed 30 meters
5. Can be very destructive
D.
E.
Landslides and ground subsidence
Fire
VII. Can earthquakes be predicted?
A. Short-range predictions
Earthquakes
1.
Goal is to provide a warning of the
location and magnitude of a large
earthquake within a narrow time
frame
2. Research has concentrated on
monitoring possible precursors –
phenomena that precede a
forthcoming earthquake such as
measuring
a. Uplift
b. Subsidence
c. Strain in the rocks
3. Currently, no reliable method exists
for making short-range earthquake
predictions
B. Long-range forecasts
1. Give the probability of a certain
magnitude earthquake occurring on a
time scale of 30 to 100 years, or more
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2.
Based on the premise that
earthquakes are repetitive or cyclical
a. Using historical records or
b. Paleoseismology
3. Are important because they provide
information used to
a. Develop the Uniform Building
Code and
b. Assist in land-use planning
VIII. Earthquakes: Evidence for plate tectonics
A. A good fit exists between the plate
tectonics model and the global
distribution of earthquakes
B. The connection of deep-focus earthquakes
and oceanic trenches is further evidence
C. Only shallow-focus earthquakes occur
along divergent and transform fault
boundaries
Answers to the Review Questions
1. An earthquake is ground shaking caused by a sudden cracking and rupturing of highly strained rock and
quick, lateral and/or vertical movements of the blocks on either side of the rupture surface. Failure occurs
when the cumulative strains finally exceed the internal, cohesive bonds that hold the rock together.
Failure (rupture and block movements) is catastrophic, usually taking only a few tens of seconds to a
minute, and a high percentage of the strain energy is converted to ground vibrations. Following the
earthquake, strain begins gradually building again toward a future quake.
2. A fault is the plane or zone of fracture separating two blocks that are abruptly displaced during an
earthquake. The focus is the point at depth, usually in a fault zone, where the displacement and sudden
release of elastic energy initiate. It marks the initial rupture site associated with the earthquake. The
epicenter is the point on the surface directly above the focus. These relationships are nicely shown in
Figure 11.2.
3. H. F. Reid, a professor at Johns Hopkins University, based his elastic rebound idea on studies of the 1906
San Francisco earthquake.
4. Reid concluded that the quake was due to excess, elastic strain energy being suddenly (catastrophically)
released as the highly overstrained rocks snapped back (rebounded) to a state of much lower strain. Cool
lithospheric rocks have elastic limits large enough to support earthquake-causing, elastic strains. Hence,
most earthquakes originate in the lithosphere. Because they are much warmer, asthenospheric rocks begin
deforming by flowage (plastic deformation) at much lower stress magnitudes. Therefore, any stored,
elastic strain energies in the asthenosphere are too small in magnitude to produce a strong earthquake.
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5. Creep movements should reduce the likelihood of an earthquake by reducing the level of strain
accumulated in the fault-zone rocks. A fault without creep may reflect two fundamentally different
conditions. First, the stresses may be too small to move the blocks and there is no accumulation of elastic
strain. In this case the fault is clearly inactive. In the other case, the fault is active but locked; the blocks
cannot respond to tectonic stresses by moving so elastic strain accumulates. A locked fault has a high
probability for hosting a damaging future quake and such areas, known as seismic gaps, are of special
interest in earthquake forecasting.
6. A seismograph produces scaled and timed records of ground motion, ground velocity, and ground
acceleration that result from earthquake (seismic) waves moving through the instrument site. The sensing
part of the instrument is usually placed underground in a rigid, concrete vault. The rigid structure vibrates
in concert with the passing seismic waves and ground motions (displacement, velocity, and acceleration)
are sensed by optical or electromagnetic sensors set in motion by the seismic vibrations. These variations
are converted to electrical signals, amplified, and recorded as a function of time, giving an accurate record
of the seismic wave patterns.
7. P waves (primary or compressional waves) travel though matter as particle vibrations along lines parallel
to the ray path or to the path traced by a point traveling with the wave front. P waves move through
solids, liquids, and gases, the “vibrations” being in the form of “push-pull” waves. S waves (secondary or
shear waves) move by particle vibrations at right angles to the ray path and they are transmitted only by
solids, not by liquids or gases. Also, P waves always travel faster than S waves in a given material.
8. Compressional waves (P waves) are transmitted through solids and fluids because the transmitting
material need not have any shear strength. S waves are shear waves and can propagate only in materials
with a finite, shear strength. Thus S waves are transmitted only by solids, not by liquids and gases.
9. Surface waves have much higher amplitudes than body waves (P and S waves) and account for nearly all
of the dangerous ground displacements and accelerations associated with earthquakes. The horizontally
vibrating surface waves generally present more danger to buildings and other structures than surface
waves with vertical motion. Closer to the epicenter of a shallow-focus quake, the various waves more or
less arrive at the same time giving a cumulative effect to the first strong ground motions.
10. The travel time versus distance curves for P and S waves are shown in the figure. To answer the question,
you must find the point on the distance axis that corresponds to a difference in arrival times between the P
and S waves of 3 minutes. On tracing paper make a line whose length is equal to a time interval of 3
minutes (0 to 3 minutes, 3 to 6 minutes, etc.) on the vertical or time axis. Then move the line onto the
graph and find the distance on the horizontal axis where the time interval between the first P and S
arrivals is 3 minutes. The correct distance is about 1900 kilometers.
11. This is the circum-Pacific belt. The earthquakes here are associated with active subduction zones around
the rim of the Pacific Ocean (Fig. 11.13) where the oceanic crust is sinking into the mantle beneath island
arcs or continental margins.
12. Deep-focus quakes occur below oceanic trenches and, like the trenches, are directly associated with
subducting slabs of oceanic lithosphere. With increasing depth, the intermediate and deep-focus
epicenters migrate farther toward the interior of the upper plate in correspondence to the dip of the
Earthquakes
95
subduction zone. The earthquakes are thought to occur in the sunken oceanic plate. Long after sinking
into the mantle, interior parts of the slab stay cool enough to maintain some rigidity and the minerals
present may be “destabilized” and subjected to very fast phase changes. Mantle rocks surrounding the
slab are too plastic to accumulate elastic strain energy and mineral phases there are more or less in
equilibrium with respect to temperatures and pressures at those depths. Thus deep-focus earthquakes are
associated only with subduction zones.
13. The Richter earthquake-magnitude scale is based on standardized measurements of ground vibration
amplitudes and the total energy released during the earthquake. It is a logarithmic numerical scale and a
theoretical consideration of rock strength suggests that 9 is about the highest magnitude possible. The
Richter scale is quantitative because magnitudes are based on measured wave amplitudes and a physically
valid relationship between amplitude and wave energy.
The Mercalli earthquake-intensity scale is based on visual observations. It gives an estimate of the ground
shaking intensity in terms of human perceptions, eyewitness accounts, and damage to buildings or other
property. As such, it is a subjective scale and the ratings are strongly dependent on local site
characteristics such as foundation stability and building design. The scale (from I to XII) utilizes Roman
numerals.
14. For each increase of one on the Richter scale, wave amplitude increases (10) times.
15. An earthquake measuring 7 on the Richter scale releases about (30) times more energy than an earthquake
with a magnitude of 6.
16. Moment magnitude has gained popularity among seismologists because: (1) it is the only magnitude scale
that correctly estimates the size of very large earthquakes; (2) it is determined by the size of the rupture
surface and the amount of displacement, thus it is a better reflection of the total energy released during a
quake; and (3) it can be verified by both field studies (measurement of fault displacements) and by
seismographic methods.
17. Many factors can be noted, particularly the amplitude of the ground displacement or acceleration, the
length of time that shaking occurs, and the character of the ground shaking. In general, vertical ground
motion is not so dangerous as lateral or horizontal shaking, and short period (high-frequency) vibrations
are less dangerous than longer period vibrations. Stability of the foundation material, building design, and
construction quality are also important factors.
18. Weak foundation material and poor building design both contributed to the unusually severe damage at a
site over 200 miles from the epicenter. The city is built on old lakebeds and marshlands known to have
existed when Cortez and the Spaniards were there early in the sixteenth century. Soft, unconsolidated,
water-saturated sediments are extremely unstable with respect to vibrations; and ground shaking is greatly
intensified over that in underlying solid rock. In the Mexico City case, intensified shaking caused slides,
slumps, and liquefaction, leading to numerous foundation failures.
19. Two factors account for the greater loss of life from the Armenian earthquake. First, the damaged cities in
Armenia were densely populated and very near the epicenter. Secondly, most people lived in large,
multifamily, concrete slab apartment buildings that collapsed or were heavily damaged. A 5.8 magnitude
aftershock finished off many of the damaged or weakened buildings. Although the Northridge, California
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earthquake was larger in terms of moment magnitude, poor construction practices in Armenia resulted in
a much higher number of deaths.
20. Fires, such as those that followed the 1906 San Francisco quake, and secondary effects such as landslides,
dam failures, and tsunamis can be more dangerous than the ground shaking. Damaged utilities, water and
sewer systems, and waste disposal systems leave an earthquake-ravaged area without reliable
communications and electricity and vulnerable to serious public health problems. In addition, hazardous
materials, toxic chemicals, and radioactive substances may be released from broken pipelines, damaged
storage facilities, and in truck and train accidents.
21. Tsunami are very long period, long wavelength surface waves in the oceans generated by sudden
displacement of large volumes of seawater. Most tsunami are caused by sudden vertical motion of the
seafloor during earthquakes. However, tsunami associated with the 1883 Krakatau volcanic eruption are
known to have been triggered by high-volume, extremely fast moving pyroclastic flows entering the
ocean. Sudden caldera collapse would also generate tsunami.
At sea, tsunami are not noticeable; their long wavelengths (kilometers) and modest amplitudes (a few
meters) are easily lost in the normal open-ocean swell. However, as they enter shallower waters along a
coastline, the wave slows, amplitudes grow, and huge waves pile up at the shoreline and surge inland.
Following the surge inland, the water then rushes back out to sea retracing the initial areas of destruction.
Needless to say, tsunami can be very dangerous and destructive.
Tsunami generation and characteristics are very sensitive to the amplitude, period, and duration of seabottom ground shaking, and to coastal bathymetry. Thus occasionally, localized, unusually destructive
tsunami are associated with seemingly modest-magnitude earthquakes.
22. In addition to magnitude, earthquake casualties and damage depend on many other natural and cultural
factors. Aftershocks can slow or stop rescue efforts and cause additional damage and casualties.
Amplified ground shaking and liquefaction intensify damage to structures built on quick clay and soft,
unconsolidated, and/or water-saturated foundation materials. Landslides, rockslides, and rolling boulders
can account for far more deaths and injuries than are attributable directly to the ground shaking. A quake
during the day when most people are out-of-doors results in far fewer injuries and fatalities than one at
night when families are sleeping in their homes. The appalling death tolls associated with earthquakes in
the magnitude 5 to 6 range in countries such as Iran, Afghanistan, India, and Morocco are mainly due to
collapse of weakly bonded mud and rock homes.
As in the 1906 San Francisco earthquake, fire is a constant threat; gas lines and water mains break,
streets, highways, and bridges may be impassable, and there may be no practicable way to fight the fires.
Electrical power failures, damage to sewage plants, unsafe and/or unavailable drinking water, and other
results of infrastructure damage all contribute to poor sanitation and increase the threat of sickness and
epidemics. In more developed countries like the U.S., accidents involving toxic plants, etc., increase the
chances of dangerous secondary effects following a quake. Finally a tsunami may be made more
dangerous by some unusual combination of seafloor displacement, seafloor topography, and coastal
configuration.
Earthquakes
97
23. In recent years, seismologists have concluded that no reliable method exists for making short-range
earthquake predictions. Long-range predictions have been more successful that involve forecasting
magnitudes and locations on time scales of years or perhaps even decades. Such forecasts, while not
always reliable for any short-range planning, are important because they provide information used to
develop building codes and also to assist in land-use planning. Therefore, forecasting today is directed
more toward identifying risks, increasing preparedness, and reducing casualties and damage.
24. Paleoseismicity studies, historical seismicity studies, geologic and geophysical studies, and high and lowtech strain monitoring enable earthquake risks and anticipated maximum ground accelerations to be
determined in a general way. By educating the public, such studies enhance prospects for increased
disaster preparedness and reduced fatalities, injuries, and property damages when the next powerful
earthquake inevitably does strike. They also assist in long-term planning as discussed above in question
number 23.
25. Earthquakes can be used as evidence to support the theory of plate tectonics in several ways. These
include the fit between the plate tectonics model and the global distribution of earthquakes, the
connection of deep-focus earthquakes and oceanic trenches, and the occurrence of only shallow-focus
earthquakes along divergent and transform fault boundaries.
Lecture outline, art-only, and animation PowerPoint presentations for each chapter of Earth,
9e are available on the Instructor’s Resource Center CD (0131566911).
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