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

Geological Survey

Earthquakes
 Earthquakes
by Louis C. Pakiser

' ^^H J.*4»» - . "

This aerial view shows Turnagain Heights

in Anchorage, Alaska, shortly after the
March 27, 1964, earthquake.
 Earthquakes in History
One of the most frightening and The scientific study of earth-
destructive phenomena of nature quakes is comparatively new.
that man experiences is a severe Until the 18th century few factual
earthquake and its terrible after- descriptions of earthquakes were
effects. Since long before man recorded, and the natural cause of
existed, probably for more than 4 earthquakes was little understood.
billion years, the forces within the Many people believed that an
Earth that produce earthquakes earthquake was a massive punish-
have played an active role in ment and a warning to the un-
shaping the Earth's surface. These repentant. A 16th century scholar,
unpredictable catastrophic move- for example, suggested that
ments of the Earth have caused a statues of Mercury and Saturn be
great number of human casualties placed on each wall of buildings
and extensive property damage. to protect against earthquakes.
Today man is challenging the Those who did look for natural
assumption that earthquakes must causes often reached fanciful con-
present an uncontrollable and clusions; one popular theory was
unpredictable hazard to life and that earthquakes were caused by
property. Scientists have begun to air rushing out of caverns deep
explore the means of predicting in the Earth's interior.
earthquakes. Sites of greatest An early earthquake for which
hazard are being identified, and we have detailed descriptive infor-
definite progress is being made mation occurred on November 1,
in designing structures that 1755, in the vicinity of Lisbon,
will withstand the effects of Portugal. Shocks from the quake
earthquakes. were felt in many parts of the world.
In some parts of the United States
chandeliers rattled, and in Europe
buildings trembled.
After the quake, Portuguese
priests were asked to document
their observations. Their records,
still preserved, represent the first
systematic attempt to investigate
an earthquake and its effects.
Since then, detailed records have
been kept of almost every major
earthquake.
The most widely felt earth-
USGS scientist uses portable seismic
quakes in the recorded history of
recording equipment near Mount St. North America were a series that
Helens, Washington. occurred in 1811-12 near New
"The sea rose boiling in the harbour and broke up all the craft harboured there; the city
burst into flames, and ashes covered the streets and squares; the houses came crashing
down, roofs piling up on foundations, and even the foundations were smashed to pieces.
Thirty thousand inhabitants of both sexes and all ages were crushed to death under
the ruins."
from Candide; Voltaire

Madrid, Mo. The shocks started region, the destruction of human

December 16, 1811, and continued life and property was slight. If this
intermittently for 2 days. Large earthquake occurred in the same
shocks followed on January 23, area today, it probably would cause
1812, and again on February 7. The severe damage to many cities in
largest of these quakes was felt the central Mississippi Valley.
over an area of 2 million square The San Francisco earthquake
miles from Canada to the Gulf of of 1906 was one of the most de-
Mexico and from the Rocky Moun- structive in the recorded history of
tains to the Atlantic Ocean. North America the earthquake
Because the most intense effects and the fire that followed killed
were in a sparsely populated nearly 700 people and left the city
3
 Locations of Earthquakes
in ruins. The Alaska earthquake of Most earthquakes occur in areas
March 27, 1964, however, was of bordering the Pacific Ocean. This
greater magnitude than the San circum-Pacific belt, called the "ring
Francisco earthquake; it released of fire," includes the Pacific coasts
perhaps twice as much energy and of North and South America, the
was felt over an area of almost Aleutians, Japan, Southeast Asia,
500,000 square miles. The ground and Australasia. Half a million
motion near the epicenter was so people within this area have lost
violent that the tops of some trees their lives because of earthquakes,
were snapped off. One hundred and property valued in billions of
and fourteen people (some as far dollars has been severely damaged
away as California) died as a result or destroyed.
of this earthquake, but loss of life
and property would have been far
greater had Alaska been more Earthquakes occur most frequently in
densely populated. those zones shown in yellow below.

INDIAN
OCEAN

Earthquake movement during the San

Francisco earthquake caused the tracks
to buckle.
 The United States has experi- more information on the nature,
enced less destruction than other causes, and effects of earthquakes.
countries located in this ring of Scientists, including those of the
fire, but millions of Americans live U.S. Geological Survey's National
in potential quake areas. Large Center for Earthquake Research in
parts of the Western United States Menlo Park, Calif., are studying the
are known to be particularly vul- causes of earthquakes in hopes of
nerable. Nuclear reactors, great finding methods of prediction and
dams, schools, and high-rise apart- of developing practices that will
ments and other housing develop- reduce the destructive effects of
ments are being planned and built earthquakes.
in places where the danger of
major earthquakes is ever present.
This has created an urgent need for
i ~v_'.
Nature of Earthquakes
An earthquake is the oscillatory, while others are of very low fre-
sometimes violent movement of the quency actually many seconds
Earth's surface that follows a or minutes between swings. These
release of energy in the Earth's vibrations cause the entire planet
crust. This energy can be gener- to quiver or ring like a bell or a
ated by a sudden dislocation of tuning fork.
segments of the crust, by a vol- A fault is a fracture in the Earth's
canic eruption, or even by man- crust along which two blocks of
made explosions. Most of the the crust have slipped with respect
destructive quakes, however, are to each other. One crustal block
caused by dislocations of the may move horizontally in one direc-
crust. When subjected to deep- tion while the block facing it moves
seated forces (whose origins and in the opposite direction, or one
natures are largely unknown) the block may move upward while the
crust may first bend and then, other moves downward. Faults are
when the stress exceeds the distinguished by the kinds of
strength of the rocks, break and movements that characterize them.
"snap" to a new position. In the Movement along California's fa-
process of breaking, vibrations mous San Andreas Fault is pre-
called "seismic waves" are gen- dominantly horizontal, and the fault
erated. These waves travel from is called a strike-slip fault. A fault
the source of the earthquake to in which the movement is vertical
more distant places along the is called a dip-slip fault. Along
surface and through the Earth at many faults, movement is both
varying speeds depending on the horizontal and vertical.
medium through which they move.
Some of the vibrations are of high
enough frequency to be audible,

Movement along strike-slip fault (left).

Movement along dip-slip fault (right).
 Distance from Earth's
surface to center is
U PP er «pyp
approximately 4,000 miles.
Mantle Lower
If Mantle

Thickness of mantle
is approximately
1.800 miles.

Thickness of
Earth's crust is
5 to 40 miles

Focus of most earthquakes

is in crust or upper mantle

EPICENTER
ace

Seismic waves

Mantle
 Geologists have found that in California along the San Andreas
earthquakes tend to reoccur along and associated faults have shallow
faults, which reflect zones of focal depths; for most the depth
weakness in the Earth's crust. The is less than 10 miles. During the
fact that a fault zone has recently past 100 years, Earth movements
experienced an earthquake offers have occurred along more than
no assurance that enough stress half the entire length of the San
has been relieved to prevent Andreas Fault and the rupture
another quake. itself is visible at the land surface
The focal depth of an earthquake in many places.
is the depth from the Earth's sur- Very shallow earthquakes are
face to the region (focus) where an probably caused by fracturing of
earthquake's energy originates. the brittle rock in the crust or by
Earthquakes with focal depths internal stresses that overcome the
from the surface to about 60 frictional resistance locking oppo-
kilometers (38 miles) are classified site sides of a fault. The immediate
as shallow. Earthquakes with focal cause of intermediate and deep
depths from 60 to 300 kilometers earthquakes is not yet fully under-
(38 to 188 miles) are classified as stood.
intermediate. The focus of deep The epicenter of an earthquake
earthquakes may reach depths of is the point on the Earth's surface
700 kilometers (440 miles). directly above the focus. The loca-
The focuses of most earth- tion of an earthquake is commonly
quakes are concentrated in the described by the geographic posi-
crust and upper mantle. Compared tion of its epicenter and by its
to a depth of about 4,000 miles to focal depth.
the center of the Earth's core, Earthquakes beneath the ocean
earthquakes can be considered to floor sometimes generate immense
originate in relatively shallow parts sea waves or "tsunamis" (Japan's
of the Earth's interior. Earthquakes dread "huge wave"). These waves
travel across the ocean at speeds
as great as 960 kilometers per hour
(600 miles per hour) and may be 15
meters (50 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 Landslides triggered by earth-
and caused severe damage along quakes often cause more destruc-
the west coast of North America, tion than the earthquake shocks
particularly at Crescent City, Calif. themselves. During the 1964 Alaska
Some waves raced across the quake, shock-induced landslides
ocean to the coasts of Japan. devastated the Turnagain Heights
Water levels in artesian wells residential development and many
fluctuate as seismic waves travel downtown areas in Anchorage. An
through the rock layers that hold observer gave a vivid report of the
the water. During passage of breakup of the unstable Earth
seismic waves from a large earth- materials in the Turnagain Heights
quake, water levels in some wells region:
fluctuate wildly, not only in the / got out of my car, ran
immediate vicinity of the earth- northward toward my drive-
quake but also at great distances way, and then saw that the
from it. The water level change bluff had broken back ap-
may be long lasting or even per- proximately 300 feet south-
manent. The Alaska quake appears ward from its original edge.
to have caused changes in wells in Additional slumping of the
many areas, both local and remote. bluff caused me to return to
Water levels in wells in New my car and back southward
Orleans, La., for example, rose and approximately 180 feet to
fell as a result of the quake. the corner of McCollie and
Turnagain Parkway. The
bluff slowly broke until the
corner of Turnagain Park-
way and McCollie had
slumped northward.

Background: Fracture and pressure ridges

developed ir» ice on a lake in northern
0, Kenai Lowlands.
f

\
10
 * Ifi .'I

Seismic seawaves (tsunamis) damaged these railroad tracks.

The Turnagain Heights landslide shortly after the earthquake.

 *«~J
.^x
I
Measurement of
Earthquakes
The vibrations produced by Body waves are of two types,
earthquakes are detected, re- compressional and shear. Both
corded, and measured by instru- types of body waves pass through
ments called seismographs. The the Earth's interior from the focus
zig-zag line made by a seismo- of an earthquake to distant points
graph, called a "seismogram," on the surface, but only compres-
reflects the varying amplitude of sional waves travel through the
the vibrations by responding to the Earth's molten core. Because
motion of the ground surface be- compressional waves travel at
neath the instrument. From the great speeds and ordinarily reach
data expressed in seismograms, the surface first, they are often
the time, the epicenter, and the called "primary waves" or simply
focal depth of an earthquake can "P" waves. P waves push tiny
be determined, and estimates can particles of Earth material directly
be made of the amount of energy ahead of them or displace the
that was released. particles directly behind their line
The two general types of vibra- of travel.
tions produced by earthquakes are Shear waves do not travel as
surface waves which travel along rapidly through the Earth's crust
the Earth's surface and body waves and mantle as do compressional
which travel through the Earth. waves, and because they ordinar-
Surface waves usually have the ily reach the surface later they are
strongest vibrations and probably called "secondary" or "S" waves.
cause most of the damage done by Instead of affecting material
earthquakes. directly behind or ahead of their
line of travel, shear waves displace

Focus
material at right angles to their path The severity of an earthquake
and are therefore sometimes called can be expressed in several ways.
"transverse" waves. The magnitude of an earthquake, as
The first indication of an earth- expressed by the Richter Scale, is a
quake will often be a sharp thud- measure of the amplitude of the
signaling the arrival of compres- seismic waves and is related to the
sional waves. This will be followed amount of energy released an
by the shear waves and then the amount that can be estimated from
"ground roll" caused by the surface seismograph recordings. The inten-
waves. A geologist who was at sity, as expressed by the Modified
Valdez, Alaska, during the 1964 Mercalli Scale, is a subjective
earthquake described this phe-. measure which describes how
nomenon: severe a shock was felt at a par-
The first tremors were hard ticular location. Damage or loss of
enough to stop a moving life and property is another, and
person and shock waves ultimately the most important,
were immediately notice- measure of an earthquake's
able on the surface of the severity.
ground. These shock waves The Richter Scale, named after
continued with a rather long Dr. Charles F. Richter of the Cali-
frequency which gave the fornia Institute of Technology, is the
observer an impression of a best known scale for measuring the
rolling feeling rather than magnitude of earthquakes. The
abrupt hard jolts. After about scale is logarithmic so that a re-
1 minute the amplitude or cording of 7, for example, indicates
strength of the shock waves a disturbance with ground motion
increased in intensity and 10 times as large as a recording of
failures in buildings as well 6. A quake of magnitude 2 is the
as the frozen ground sur- smallest quake normally felt by
face began to occur . . . humans. Earthquakes with a Richter
Alter about 3 1/2 minutes value of 6 or more are commonly
the severe shock waves considered major in magnitude.
ended and people began to The Modified Mercalli Scale ex-
react as could be expected. presses the intensity of an earth-
The times of arrival of compres- quake's effects in a given locality in
sional and shear waves at selected values ranging from I to XII. The
seismograph stations throughout the most commonly used adaptation
world indicate where and when the covers the range of intensity from
earthquake occurred and, some- the condition of "I Not felt except
times, its focal depth. The recorded by a very few under especially
amplitudes of seismic waves indi- favorable conditions," to "XII
cate the amount of energy released Damage total. Lines of sight and
by the quake. level are distorted. Objects thrown

14
upward into the air." Evaluation of focal depth, the distance from the
earthquake intensity can be made epicenter, local geologic conditions,
only after eyewitness reports and and the design of buildings and
results of field investigations are other works of man. The extent of
studied and interpreted. The max- damage also depends on the densi-
imum intensity experienced in the ty of population and construction in
Alaska earthquake of 1964 was X; the area shaken by the quake.
the San Francisco earthquake of The Alaska earthquake of 1964
1906 reached a maximum intensity demonstrated wide variations in its
of XI. effects. The town of Whittier, built
Earthquakes of large magnitude on firm granite, suffered little
do not necessarily cause the most damage from the seismic waves
intense surface effects. The effect despite the fact that it was close to
in a given region depends to a the earthquake's epicenter. Whittier
large degree on local surface and did experience great destruction
subsurface geologic conditions. An and the loss of 13 lives from the
area underlain by unstable ground enormous seawaves produced by
(sand, clay, or other unconsolidated submarine landslides, however. In
materials), for example, is likely to Anchorage, much farther from the
experience much more noticeable earthquake's epicenter, damage
effects than an area equally distant was selective; the greatest destruc-
from an earthquake's epicenter but tion occurred in areas where homes
underlain by firm ground such as and other buildings were con-
granite. structed near unstable slopes
An earthquake's destructiveness underlain by clay and other uncon-
depends on many factors. In addi- solidated materials.
tion to magnitude, these include the

Government Hill Elementary School in Anchorage, Alaska, destroyed in the 1964 earthquake.

15
 Reliable earthquake predictions magnitudes and timings of precur-
improve disaster planning and sor events have limited the ability of
preparedness activities, and thus scientists to provide reliable short-
help mitigate earthquakes. Predic- term warnings. Activities and events
tions have the potential for greatly at Parkfield, California, however,
reducing casualties, personal may prove fruitful in research on
property losses, and structural short-term earthquake prediction.
damage. The goal of earthquake
prediction is to give warning of
potentially damaging earthquakes at
a time interval that allows ap-
propriate immediate actions and
with enough precision to avoid any The Parkfield section of the San
sustained disruptive effects on Andreas fault has had remarkably
society. consistent earthquake activity since
The U.S. Geological Survey con- at least 1857. Over the past 130
ducts and supports research on the years, moderate-size earthquakes
prediction of the time, place, and have recurred every 21 to 22 years
magnitude of earthquakes. This on average. Other evidence, in-
research includes field, laboratory, cluding seismograms of the last
and theoretical investigations of three moderate-size earthquakes,
earthquake mechanisms and fault strongly suggest that these seismic
zones. It is aimed at both long-term events were very similar to each
and short-term earthquake other.
predictions. Based on this consistency and
Research on long-term earthquake similarity, scientists at the USGS
prediction (earthquake potential) has and the University of California at
preceded rapidly. Scientists have Berkeley have forecast that another
estimated dates of large prehistoric moderate-size earthquake likely will
earthquakes from detailed studies occur at Parkfield in 1988 ± 5
of fault zones and have combined years. The Parkfield prediction is
these results with seismic data to the first to be endorsed by the Na-
provide estimates of time intervals tional Earthquake Prediction Evalua-
between earthquakes. More than a tion Council.
dozen large earthquakes have been The anticipated Parkfield earth-
sucessfully forecast since 1965 quake provides scientists with a
based on the recognition of seismic unique opportunity to monitor and
gaps, extended periods of earth- study an expected seismic event.
quake quiescence. An important goal of the experiment
Research on short-term prediction is to attempt a short-term (1- to
(within a few hours or days of the 3-day) prediction of the moderate-
event) has proven more challeng- size earthquake. This short-term
ing. Difficulties in establishing warning will be based on precusor

17
indicators, such as unusual in- measures ground tilting) records are
creases in fault deformation and maintained at the Geological
foreshock activity, which are being Survey's Hawaiian Volcano Obser-
continuously measured and record- vatory, located on the rim of Kilauea
ed by monitoring devices in the Volcano, where study of these
area. records enables specialists to make
short-range predictions of volcanic
Volcanoes and eruptions.
Earthquakes
Earthquakes on the island of
Hawaii, the site of active volcanoes
Kilauea and Mauna Loa, appear to
be associated with volcanic activity.
Abrupt increases in quake activity at
times herald an eruption, and the
location of swarms of tremors can
indicate where lava may break out.
Also, prior to an eruption, the The photograph on the top shows how
volcano swells measurably in Mauna Loa Volcano, 30 miles away,
response to the upward movement appears from a window at the Hawaiian
Volcano Observatory. The observatory is
of molten rock. Continuous seismic shown below.
and tiltmeter (a device that

18
 ocation of a seismograph station in a
typical seismograph network in a
California earthquake-prone area

100 miles

19
 Scientific understanding of earth-
quakes 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
cause and effects of earthquakes,
man may be able to reduce damage
and loss of life from this destructive
phenomenon.

This publication is one of a series of general interest

publications prepared by the U.S. Geological Survey
to provide information about the earth sciences,
natural resources, and the environment. To obtain a
catalog of additional titles in the series "General r:
Interest Publications of the U.S. Geological Survey,"
write:
f*~=.
U.S. Geological Survey
Branch of Distribution
P.O. Box 25286
Denver, CO 80225

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.

20 * u - s - GOVERNMENT PRINTING OFFICE: 1991 293-521

 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 wise use of our
land and water resources, and biological diversity; protecting our fish and
wildlife, preserving the environmental and cultural values of our national
parks and historical places, and providing for the enjoyment of life
through outdoor recreation. The Department assesses our energy and
mineral resources and works to assure that their development is in the best interests of all
our people. The Department also has a major responsibility for American Indian reservation
communities and for people who live in Island Territories under U.S. Administration.