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[Illustration: _Many buildings in Charleston, South Carolina, were
damaged or destroyed by the large earthquake that occurred August 31,
1886._]

        U.S. Department of the Interior / U.S. Geological Survey

                              Earthquakes

                        _by Kaye M. Shedlock and
                           Louis C. Pakiser_

            For sale by the U.S. Government Printing Office
                      Superintendent of Documents
               Mail Stop: SSOP, Washington, DC 20402-9328

One of the most frightening and destructive phenomena of nature is a
severe earthquake and its terrible aftereffects. An earthquake is a
sudden movement of the Earth, caused by the abrupt release of strain
that has accumulated over a long time. For hundreds of millions of
years, the forces of plate tectonics have shaped the Earth as the huge
plates that form the Earth’s surface slowly move over, under, and past
each other. Sometimes the movement is gradual. At other times, the
plates are locked together, unable to release the accumulating energy.
When the accumulated energy grows strong enough, the plates break free.
If the earthquake occurs in a populated area, it may cause many deaths
and injuries and extensive property damage.

Today we are challenging the assumption that earthquakes must present an
uncontrollable and unpredictable hazard to life and property. Scientists
have begun to estimate the locations and likelihoods of future damaging
earthquakes. Sites of greatest hazard are being identified, and definite
progress is being made in designing structures that will withstand the
effects of earthquakes.

[Illustration: _USGS scientist uses portable seismic recording equipment
near Mount St. Helens, Washington._]

                         Earthquakes in History

The scientific study of earthquakes is comparatively new. Until the 18th
century, few factual descriptions of earthquakes were recorded, and the
natural cause of earthquakes was little understood. Those who did look
for natural causes often reached conclusions that seem fanciful today;
one popular theory was that earthquakes were caused by air rushing out
of caverns deep in the Earth’s interior.

The earliest earthquake for which we have descriptive information
occurred in China in 1177 B.C. The Chinese earthquake catalog describes
several dozen large earthquakes in China during the next few thousand
years. Earthquakes in Europe are mentioned as early as 580 B.C., but the
earliest for which we have some descriptive information occurred in the
mid-16th century. The earliest known earthquakes in the Americas were in
Mexico in the late 14th century and in Peru in 1471, but descriptions of
the effects were not well documented. By the 17th century, descriptions
of the effects of earthquakes were being published around the
world—although these accounts were often exaggerated or distorted.

The most widely felt earthquakes in the recorded history of North
America were a series that occurred in 1811-12 near New Madrid, Mo. A
great earthquake, whose magnitude is estimated to be about 8, occurred
on the morning of December 16, 1811. Another great earthquake occurred
on January 23, 1812, and a third, the strongest yet, on February 7,
1812. Aftershocks were nearly continuous between these great earthquakes
and continued for months afterwards. These earthquakes were felt by
people as far away as Boston and Denver. Because the most intense
effects were in a sparsely populated region, the destruction of human
life and property was slight. If just one of these enormous earthquakes
occurred in the same area today, millions of people and buildings and
other structures worth billions of dollars would be affected.

[Illustration: _The great 1906 San Francisco earthquake and fire
destroyed most of the city and left 250,000 people homeless._]

The San Francisco earthquake of 1906 was one of the most destructive in
the recorded history of North America—the earthquake and the fire that
followed killed nearly 700 people and left the city in ruins. The Alaska
earthquake of March 27, 1964, was of greater magnitude than the San
Francisco earthquake; it released perhaps twice as much energy and was
felt over an area of almost 500,000 square miles. The ground motion near
the epicenter was so violent that the tops of some trees were snapped
off. One hundred and fourteen people (some as far away as California)
died as a result of this earthquake, but loss of life and property would
have been far greater had Alaska been more densely populated.

                        Where Earthquakes Occur

The Earth is formed of several layers that have very different physical
and chemical properties. The outer layer, which averages about 70
kilometers in thickness, consists of about a dozen large, irregularly
shaped plates that slide over, under, and past each other on top of the
partly molten inner layer. Most earthquakes occur at the boundaries
where the plates meet. In fact, the locations of earthquakes and the
kinds of ruptures they produce help scientists define the plate
boundaries.

[Illustration: Diagram of plate boundaries and earthquake locations.]

There are three types of plate boundaries: spreading zones, transform
faults, and subduction zones. At _spreading zones_, molten rock rises,
pushing two plates apart and adding new material at their edges. Most
spreading zones are found in oceans; for example, the North American and
Eurasian plates are spreading apart along the mid-Atlantic ridge.
Spreading zones usually have earthquakes at shallow depths (within 30
kilometers of the surface).

_Transform faults_ are found where plates slide past one another. An
example of a transform-fault plate boundary is the San Andreas fault,
along the coast of California and northwestern Mexico. Earthquakes at
transform faults tend to occur at shallow depths and form fairly
straight linear patterns.

[Illustration: Map explanation: yellow lines, plate Boundary.]

_Subduction zones_ are found where one plate overrides, or subducts,
another, pushing it downward into the mantle where it melts. An example
of a subduction-zone plate boundary is found along the northwest coast
of the United States, western Canada, and southern Alaska and the
Aleutian Islands. Subduction zones are characterized by deep-ocean
trenches, shallow to deep earthquakes, and mountain ranges containing
active volcanoes.

Earthquakes can also occur within plates, although plate-boundary
earthquakes are much more common. Less than 10 percent of all
earthquakes occur within plate interiors. As plates continue to move and
plate boundaries change over geologic time, weakened boundary regions
become part of the interiors of the plates. These zones of weakness
within the continents can cause earthquakes in response to stresses that
originate at the edges of the plate or in the deeper crust. The New
Madrid earthquakes of 1811-12 and the 1886 Charleston earthquake
occurred within the North American plate.

[Illustration: Damaged house.]

                         How Earthquakes Happen

An earthquake is the vibration, sometimes violent, of the Earth’s
surface that follows a release of energy in the Earth’s crust. This
energy can be generated by a sudden dislocation of segments of the
crust, by a volcanic eruption, or even by manmade explosions. Most
destructive quakes, however, are caused by dislocations of the crust.
The crust may first bend and then, when the stress exceeds the strength
of the rocks, break and “snap” to a new position. In the process of
breaking, vibrations called “seismic waves” are generated. These waves
travel outward from the source of the earthquake along the surface and
through the Earth at varying speeds depending on the material through
which they move. Some of the vibrations are of high enough frequency to
be audible, while others are of very low frequency. These vibrations
cause the entire planet to quiver or ring like a bell or a tuning fork.

A _fault_ is a fracture in the Earth’s crust along which two blocks of
the crust have slipped with respect to each other. Faults are divided
into three main groups, depending on how they move. _Normal faults_
occur in response to pulling or tension; the overlying block moves down
the dip of the fault plane. _Thrust (reverse) faults_ occur in response
to squeezing or compression; the overlying block moves up the dip of the
fault plane. _Strike-slip (lateral) faults_ occur in response to either
type of stress; the blocks move horizontally past one another. Most
faulting along spreading zones is normal, along subduction zones is
thrust, and along transform faults is strike-slip.

[Illustration: _Normal Fault. Blocks are pulled apart_]

[Illustration: _Thrust Fault. Blocks are pushed together_]

[Illustration: _Strike-Slip Fault. Blocks slide past each other_]

Geologists have found that earthquakes tend to reoccur along faults,
which reflect zones of weakness in the Earth’s crust. Even if a fault
zone has recently experienced an earthquake, however, there is no
guarantee that all the stress has been relieved. Another earthquake
could still occur. In New Madrid, a great earthquake was followed by a
large aftershock within 6 hours on December 16, 1811. Furthermore,
relieving stress along one part of the fault may increase stress in
another part; the New Madrid earthquakes in January and February 1812
may have resulted from this phenomenon.

[Illustration: Diagram of Earth’s layers and seismic wave propagation.]

The _focal depth_ of an earthquake is the depth from the Earth’s surface
to the region where an earthquake’s energy originates (the _focus_).
Earthquakes with focal depths from the surface to about 70 kilometers
(43.5 miles) are classified as shallow. Earthquakes with focal depths
from 70 to 300 kilometers (43.5 to 186 miles) are classified as
intermediate. The focus of deep earthquakes may reach depths of more
than 700 kilometers (435 miles). The focuses of most earthquakes are
concentrated in the crust and upper mantle. The depth to the center of
the Earth’s core is about 6,370 kilometers (3,960 miles), so even the
deepest earthquakes originate in relatively shallow parts of the Earth’s
interior.

The _epicenter_ of an earthquake is the point on the Earth’s surface
directly above the focus. The location of an earthquake is commonly
described by the geographic position of its epicenter and by its focal
depth.

Earthquakes beneath the ocean floor sometimes generate immense sea waves
or tsunamis (Japan’s dread “huge wave”). These waves travel across the
ocean at speeds as great as 960 kilometers per hour (597 miles per hour)
and may be 15 meters (49 feet) high or higher by the time they reach the
shore. During the 1964 Alaska earthquake, tsunamis engulfing coastal
areas caused most of the destruction at Kodiak, Cordova, and Seward and
caused severe damage along the west coast of North America, particularly
at Crescent City, Calif. Some waves raced across the ocean to the coasts
of Japan.

[Illustration: _Tsunami destruction on Kamehameha Avenue on Hilo’s
waterfront, 1946._ (Photograph provided by the U.S. Army Corps of
Engineers.)]

_Liquefaction_, which happens when loosely packed, water-logged
sediments lose their strength in response to strong shaking, causes
major damage during earthquakes. During the 1989 Loma Prieta earthquake,
liquefaction of the soils and debris used to fill in a lagoon caused
major subsidence, fracturing, and horizontal sliding of the ground
surface in the Marina district in San Francisco.

[Illustration: _Liquefaction of sands and debris caused major damage
throughout the Marina district in San Francisco during the Loma Prieta
earthquake._]

Landslides triggered by earthquakes often cause more destruction than
the earthquakes themselves. During the 1964 Alaska quake, shock-induced
landslides devastated the Turnagain Heights residential development and
many downtown areas in Anchorage. An observer gave a vivid report of the
breakup of the unstable earth materials in the Turnagain Heights region:
_I got out of my car, ran northward toward my driveway, and then saw
that the bluff had broken back approximately 300 feet southward from its
original edge. Additional slumping of the bluff caused me to return to
my car and back southward approximately 180 feet to the corner of
McCollie and Turnagain Parkway. The bluff slowly broke until the corner
of Turnagain Parkway and McCollie had slumped northward._

[Illustration: _Many homes were damaged by landslides triggered by the
1964 Alaska earthquake (above) and the 1989 Loma Prieta shock (below)._]

[Illustration: Home damaged by landslide triggered by the 1989 Loma
Prieta shock.]

[Illustration: Scientist examining seismographic equipment.]

                         Measuring Earthquakes

The vibrations produced by earthquakes are detected, recorded, and
measured by instruments called seismographs. The zig-zag line made by a
seismograph, called a “seismogram,” reflects the changing intensity of
the vibrations by responding to the motion of the ground surface beneath
the instrument. From the data expressed in seismograms, scientists can
determine the time, the epicenter, the focal depth, and the type of
faulting of an earthquake and can estimate how much energy was released.

The two general types of vibrations produced by earthquakes are _surface
waves_, which travel along the Earth’s surface, and _body waves_, which
travel through the Earth. Surface waves usually have the strongest
vibrations and probably cause most of the damage done by earthquakes.

Body waves are of two types, _compressional_ and _shear_. Both types
pass through the Earth’s interior from the focus of an earthquake to
distant points on the surface, but only compressional waves travel
through the Earth’s molten core. Because compressional waves travel at
great speeds and ordinarily reach the surface first, they are often
called “primary waves” or simply “P” waves. P waves push tiny particles
of Earth material directly ahead of them or displace the particles
directly behind their line of travel.

Shear waves do not travel as rapidly through the Earth’s crust and
mantle as do compressional waves, and because they ordinarily reach the
surface later they are called “secondary” or “S” waves. Instead of
affecting material directly behind or ahead of their line of travel,
shear waves displace material at right angles to their path and are
therefore sometimes called “transverse” waves.

[Illustration: Diagram of propagation of seismic waves.]

The first indication of an earthquake is often a sharp thud, signaling
the arrival of compressional waves. This is followed by the shear waves
and then the “ground roll” caused by the surface waves. A geologist who
was at Valdez, Alaska, during the 1964 earthquake described this
sequence: _The first tremors were hard enough to stop a moving person,
and shock waves were immediately noticeable on the surface of the
ground. These shock waves continued with a rather long frequency, which
gave the observer an impression of a rolling feeling rather than abrupt
hard jolts. After about 1 minute the amplitude or strength of the shock
waves increased in intensity and failures in buildings as well as the
frozen ground surface began to occur.... After about 3½ minutes the
severe shock waves ended and people began to react as could be
expected._

[Illustration: _Large earthquakes cause more damage east of the Rocky
Mountains; this map shows areas that suffered major architectural damage
(striped areas) and minor damage (dotted areas) during the magnitude-8
earthquakes in New Madrid and San Francisco and the smaller but still
damaging quakes in Charleston and San Fernando._]

The severity of an earthquake can be expressed in several ways. The
_magnitude_ of an earthquake, usually expressed by the _Richter Scale_,
is a measure of the amplitude of the seismic waves. The _moment
magnitude_ of an earthquake is a measure of the amount of energy
released—an amount that can be estimated from seismograph recordings.
The _intensity_, as expressed by the _Modified Mercalli Scale_, is a
subjective measure that describes how strong a shock was felt at a
particular location.

The Richter Scale, named after Dr. Charles F. Richter of the California
Institute of Technology, is the best known scale for measuring the
magnitude of earthquakes. The scale is logarithmic so that a recording
of 7, for example, indicates a disturbance with ground motion 10 times
as large as a recording of 6. A quake of magnitude 2 is the smallest
quake normally felt by people. Earthquakes with a Richter value of 6 or
more are commonly considered major; great earthquakes have magnitudes of
8 or more on the Richter scale.

The Modified Mercalli Scale expresses the intensity of an earthquake’s
effects in a given locality in values ranging from I to XII. The most
commonly used adaptation covers the range of intensity from the
condition of “I—Not felt except by a very few under especially favorable
conditions,” to “XII—Damage total. Lines of sight and level are
distorted. Objects thrown upward into the air.” Evaluation of earthquake
intensity can be made only after eyewitness reports and results of field
investigations are studied and interpreted. The maximum intensity
experienced in the Alaska earthquake of 1964 was X; damage from the San
Francisco and New Madrid earthquakes reached a maximum intensity of XI.

[Illustration: _The January 17, 1994, earthquake at Northridge,
California, caused this collapse of a major highway interchange._
(Photograph by James W. Dewey, USGS.)]

Earthquakes of large magnitude do not necessarily cause the most intense
surface effects. The effect in a given region depends to a large degree
on local surface and subsurface geologic conditions. An area underlain
by unstable ground (sand, clay, or other unconsolidated materials), for
example, is likely to experience much more noticeable effects than an
area equally distant from an earthquake’s epicenter but underlain by
firm ground such as granite. In general, earthquakes east of the Rocky
Mountains affect a much larger area than earthquakes west of the
Rockies.

An earthquake’s destructiveness depends on many factors. In addition to
magnitude and the local geologic conditions, these factors include the
focal depth, the distance from the epicenter, and the design of
buildings and other structures. The extent of damage also depends on the
density of population and construction in the area shaken by the quake.

The Loma Prieta earthquake of 1989 demonstrated a wide range of effects.
The Santa Cruz mountains suffered little damage from the seismic waves,
even though they were close to the epicenter. The central core of the
city of Santa Cruz, about 24 kilometers (15 miles) away from the
epicenter, was almost completely destroyed. More than 80 kilometers (50
miles) away, the cities of San Francisco and Oakland suffered selective
but severe damage, including the loss of more than 40 lives. The
greatest destruction occurred in areas where roads and elevated
structures were built on unstable ground underlain by loose,
unconsolidated soils.

The Northridge, California, earthquake of 1994 also produced a wide
variety of effects, even over distances of just a few hundred meters.
Some buildings collapsed, while adjacent buildings of similar age and
construction remained standing. Similarly, some highway spans collapsed,
while others nearby did not.

[Illustration: _A sudden increase in earthquake tremors signaled the
beginning of a series of eruptions at Redoubt Volcano in 1989-90._]

                       Volcanoes and Earthquakes

Earthquakes are associated with volcanic eruptions. Abrupt increases in
earthquake activity heralded eruptions at Mount St. Helens, Washington;
Mount Spurr and Redoubt Volcano, Alaska; and Kilauea and Mauna Loa,
Hawaii. The location and movement of swarms of tremors indicate the
movement of magma through the volcano. Continuous records of seismic and
tiltmeter (a device that measures ground tilting) data are maintained at
U.S. Geological Survey volcano observatories in Hawaii, Alaska,
California, and the Cascades, where study of these records enables
specialists to make short-range predictions of volcanic eruptions. These
warnings have been especially effective in Alaska, where the imminent
eruption of a volcano requires the rerouting of international air
traffic to enable airplanes to avoid volcanic clouds. Since 1982, at
least seven jumbo jets, carrying more than 1,500 passengers, have lost
power in the air after flying into clouds of volcanic ash. Though all
flights were able to restart their engines eventually and no lives were
lost, the aircraft suffered damages of tens of millions of dollars. As a
result of these close calls, an international team of volcanologists,
meteorologists, dispatchers, pilots, and controllers have begun to work
together to alert each other to imminent volcanic eruptions and to
detect and track volcanic ash clouds.

                         Predicting Earthquakes

The goal of earthquake prediction is to give warning of potentially
damaging earthquakes early enough to allow appropriate response to the
disaster, enabling people to minimize loss of life and property. The
U.S. Geological Survey conducts and supports research on the likelihood
of future earthquakes. This research includes field, laboratory, and
theoretical investigations of earthquake mechanisms and fault zones. A
primary goal of earthquake research is to increase the reliability of
earthquake probability estimates. Ultimately, scientists would like to
be able to specify a high probability for a specific earthquake on a
particular fault within a particular year. Scientists estimate
earthquake probabilities in two ways: by studying the history of large
earthquakes in a specific area and the rate at which strain accumulates
in the rock.

Scientists study the past frequency of large earthquakes in order to
determine the future likelihood of similar large shocks. For example, if
a region has experienced four magnitude 7 or larger earthquakes during
200 years of recorded history, and if these shocks occurred randomly in
time, then scientists would assign a 50 percent probability (that is,
just as likely to happen as not to happen) to the occurrence of another
magnitude 7 or larger quake in the region during the next 50 years.

But in many places, the assumption of random occurrence with time may
not be true, because when the strain is released along one part of the
fault system, it may actually increase on another part. Four magnitude
6.8 or larger earthquakes and many magnitude 6-6.5 shocks occurred in
the San Francisco Bay region during the 75 years between 1836 and 1911.
For the next 68 years (until 1979), no earthquakes of magnitude 6 or
larger occurred in the region. Beginning with a magnitude 6.0 shock in
1979, the earthquake activity in the region increased dramatically;
between 1979 and 1989, there were four magnitude 6 or greater
earthquakes, including the magnitude 7.1 Loma Prieta earthquake. This
clustering of earthquakes leads scientists to estimate that the
probability of a magnitude 6.8 or larger earthquake occurring during the
next 30 years in the San Francisco Bay region is about 67 percent (twice
as likely as not).

Another way to estimate the likelihood of future earthquakes is to study
how fast strain accumulates. When plate movements build the strain in
rocks to a critical level, like pulling a rubber band too tight, the
rocks will suddenly break and slip to a new position. Scientists measure
how much strain accumulates along a fault segment each year, how much
time has passed since the last earthquake along the segment, and how
much strain was released in the last earthquake. This information is
then used to calculate the time required for the accumulating strain to
build to the level that results in an earthquake. This simple model is
complicated by the fact that such detailed information about faults is
rare. In the United States, only the San Andreas fault system has
adequate records for using this prediction method.

[Illustration: _Using a two-color laser to detect movement along a fault
near Parkfield, California._]

Both of these methods, and a wide array of monitoring techniques, are
being tested along part of the San Andreas fault. For the past 150
years, earthquakes of about magnitude 6 have occurred an average of
every 22 years on the San Andreas fault near Parkfield, California. The
last shock was in 1966. Because of the consistency and similarity of
these earthquakes, scientists have started an experiment to “capture”
the next Parkfield earthquake. A dense web of monitoring instruments was
deployed in the region during the late 1980s. The main goals of the
ongoing Parkfield Earthquake Prediction Experiment are to record the
geophysical signals before and after the expected earthquake; to issue a
short-term prediction; and to develop effective methods of communication
between earthquake scientists and community officials responsible for
disaster response and mitigation. This project has already made
important contributions to both earth science and public policy.

[Illustration: _San Andreas fault in the Carrizo Plain, central
California._]

Scientific understanding of earthquakes is of vital importance to the
Nation. As the population increases, expanding urban development and
construction works encroach upon areas susceptible to earthquakes. With
a greater understanding of the causes and effects of earthquakes, we may
be able to reduce damage and loss of life from this destructive
phenomenon.

[Illustration: _A statue of Christ at Cemetery Hill overlooks the ruined
town of Yungay, Peru. It and a few palm trees were all that remained
standing after the May 31, 1970, earthquake._]

  U.S. Geological Survey
  Information Services
  P.O. Box 25286
  Denver, CO 80225

★ U.S. GOVERNMENT PRINTING OFFICE: 1996—421-205

[Illustration: US DEPARTMENT OF THE INTERIOR; MARCH 3, 1849]

As the Nation’s principal conservation agency, the Department of the
Interior has responsibility for most of our nationally owned public
lands and natural and cultural resources. This includes fostering sound
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biological diversity; preserving the environmental and cultural values
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development is in the best interests of all our people by encouraging
stewardship and citizen participation in their care. The Department also
has a major responsibility for American Indian reservation communities
and for people who live in island territories under U.S. administration.

[Illustration: (_Cover photographs, clockwise from top left_): _Mexico
City, Mexico, 1985_; _Coalinga, Calif., 1983_; _Northridge, Calif.,
1994_; _Anchorage, Alaska, 1964_; _San Francisco, Calif., 1906_; _Loma
Prieta, Calif., 1989_.]

                          Transcriber’s Notes

--Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

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--In the ASCII version only, subscripted numbers are preceded by
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End of Project Gutenberg's Earthquakes, by Kaye M. Shedlock and Louis Pakiser