Graphics and Text Version
The San Andreas Fault
by Sandra S. Schulz and Robert E. Wallace
The presence of the San Andreas fault was brought
dramatically to world attention on April 18, 1906, when sudden
displacement along the fault produced the great San Francisco
earthquake and fire. This earthquake, however, was but one of
many that have resulted from episodic displacement along the
fault throughout its life of about 15-20 million years.
What Is It?
Scientists have learned that the Earth's crust is fractured
into a series of "plates" that have been moving very slowly over
the Earth's surface for millions of years.
Two of these moving plates meet in western California; the
boundary between them is the San Andreas fault. The Pacific
Plate (on the west) moves northwestward relative to the North
American Plate (on the east), causing earthquakes along the
fault. The San Andreas is the "master" fault of an intricate
fault network that cuts through rocks of the California coastal
region. The entire San Andreas fault system is more than 800
miles long and extends to depths of at least 10 miles within the
Earth. In detail, the fault is a complex zone of crushed and
broken rock from a few hundred feet to a mile wide. Many smaller
faults branch from and join the San Andreas fault zone. Almost
any road cut in the zone shows a myriad of small fractures, fault
gouge (pulverized rock), and a few solid pieces of rock.
Where Is It?
The San Andreas fault forms a continuous narrow break in the
Earth's crust that extends from northern California southward to
Cajon Pass near San Bernardino. Southeastward from Cajon Pass
several branching faults, including the San Jacinto and Banning
faults, share the movement of the crustal plates. In this
stretch of the fault zone, the name "San Andreas" generally is
applied to the northeastern most branch.
What Surface Features Characterize It?
Over much of its length, a linear trough reveals the
presence of the San Andreas fault; from the air, the linear
arrangement of lakes, bays, and valleys in this trough is
striking. Viewed from the ground, however, the features are more
subtle. For example, many people driving near Crystal Springs
Reservoir, near San Francisco, or along Tomales Bay, or through
Cajon or Tejon Passes may not realize that they are within the
San Andreas fault zone. On the ground, the fault can be
recognized by carefully inspecting the landscape. The fault zone
is marked by distinctive landforms that include long straight
escarpments, narrow ridges, and small undrained ponds formed by
the settling of small blocks within the zone. Many stream
channels characteristically jog sharply to the right where they
cross the fault.
What Kind of Movement Has Occurred Along the
Fault?
Blocks on opposite sides of the San Andreas fault move
horizontally. If a person stood on one side of the fault and
looked across it, the block on the opposite side would appear to
have moved to the right. Geologists refer to this type fault
displacement as right-lateral strike-slip.
During the 1906 earthquake in the San Francisco region,
roads, fences, and rows of trees and bushes that crossed the
fault were offset several yards, and the road across the head of
Tomales Bay was offset almost 21 feet, the maximum offset
recorded. In each case, the ground west of the fault moved
relatively northward.
Sudden offset that initiates a great earthquake occurs on
only one section of the fault at a time. Total offset
accumulates through time in an uneven fashion, primarily by
movement on first one, and then another section of the fault.
The sections that produce great earthquakes remain "locked" and
quiet over a hundred or more years while strain builds up; then,
in great lurches, the strain is released, producing great
earthquakes. Other stretches of the fault, however, apparently
accommodate movement more by constant creep than by sudden
offsets that generate great earthquakes. In historical times,
these creeping sections have not generated earthquakes of the
magnitude seen on the "locked" sections.
Geologists believe that the total accumulated displacement
from earthquakes and creep is at least 350 miles along the San
Andreas fault since it came into being about 15-20 million years
ago. Studies of a segment of the fault between Tejon Pass and
the Salton Sea revealed geologically similar terranes on opposite
sides of the fault now separated by 150 miles, and some crustal
blocks may have moved through more than 20 degrees of
latitude.
Although it is difficult to imagine this great amount of
shifting of the Earth's crust, the rate represented by these
ancient offsets is consistent with the rate measured in
historical time. Surveying shows a drift at the rate of as much
as 2 inches per year.
What Is an Earthquake?
The crustal plates of the Earth are being deformed by
stresses from deep within the Earth. The ground first bends,
then, upon reaching a certain limit, breaks and "snaps" to a new
position. In the process of breaking or "faulting," vibrations
are set up that are the earthquakes. Some of the vibrations are
of very low frequency, with many seconds between waves, whereas
other vibrations are of high enough frequency to be in the
audible range.
The vibrations are of two basic types, compression waves and
transverse or shear waves. Since the compression waves travel
faster through the Earth, they arrive first at a distant point;
they are known as primary or "P" waves. The transverse waves
arriving later are referred to as shear or "S" waves. In an
earthquake, people may note first a sharp thud, or blast-like
shock, that marks the arrival of the P wave. A few seconds
later, they may feel a swaying or rolling motion that marks the
arrival of the S wave.
What Do Earthquake "Magnitude" and "Intensity"
Mean?
Magnitude is a measure of the size of an earthquake. The
Richter Scale, named after Charles F. Richter of the California
Institute of Technology, is the best known scale for the
measuring of magnitude (M) of earthquakes. The scale is
logarithmic; a recording of 7, for example, signifies a
disturbance with ground motion 10 times as large as a recording
of 6. The energy released by an earthquake of M 7, however, is
approximately 30 times that released by an earthquake of M 6; an
earthquake of M 8 releases 900 times (30x30) the energy of an
earthquake of M 6. An earthquake of magnitude 2 is the smallest
earthquake normally felt by humans. Earthquakes with a Richter
value of 5 or higher are potentially damaging. Some of the
world's largest recorded earthquakes--on January 31, 1906, off
the coast of Colombia and Ecuador, and on March 2, 1933, off the
east coast of Honshu, Japan--had magnitudes of 8.9 on this scale,
which is open ended.
As the Richter scale does not adequately differentiate
between the largest earthquakes, a new "moment magnitude" scale
is being used by seismologists to provide a better measure. On
the moment magnitude scale, the San Francisco earthquake is
estimated at magnitude 7.7 compared to an estimated Richter
magnitude of 8.3.
Intensity is a measure of the strength of shaking
experienced in an earthquake. The Modified Mercalli Scale
represents the local effect or damage caused by an earthquake;
the "intensity" reported at different points generally decreases
away from the earthquake epicenter. The intensity range, from I -
XII, is expressed in Roman numerals. For example, an earthquake
of intensity II barely would be felt by people favorably
situated, while intensity X would produce heavy damage,
especially to unreinforced masonry. Local geologic conditions
strongly influence the intensity of an earthquake. Commonly,
sites on soft ground or alluvium have intensities 2 to 3 units
higher than sites on bedrock.
Earthquakes Along the Fault
Literally thousands of small earthquakes occur in California
each year, providing scientists with clear indications of places
where faults cut the Earth's crust. The largest historical
earthquakes that occurred along the San Andreas fault were those
in 1857 and 1906.
The earthquake of January 9, 1857, in southern California
apparently was about the same magnitude as the San Francisco
earthquake of 1906. According to newspaper accounts, ground
movement in both cases was roughly the same type. An account of
the 1857 earthquake describes a sheep corral cut by the fault
that was changed from a circle to an "S"-shape--movement clearly
representative of right-lateral strike-slip. Studies of offset
stream channels indicate that as much as 29 feet of movement
occurred in 1857.
The San Francisco earthquake and fire of April 18, 1906,
took about 700 lives and caused millions of dollars worth of
damage in California from Eureka southward to Salinas and beyond.
The earthquake was felt as far away as Oregon and central Nevada.
The 1906 earthquake, which has been estimated at a magnitude 8.3
on the Richter Scale, caused intensities as high as XI on the
Modified Mercalli Scale. Surface offsets occurred along a 250-
mile length of the fault from San Juan Bautista north past Point
Arena and offshore to Cape Mendocino.
On May 18, 1940, an earthquake of magnitude 7.1 occurred
along a previously unrecognized fault in the Imperial Valley.
Similar movement on the Imperial fault occurred during an
earthquake in November 1979. The greatest surface displacement
was 17 feet of right-lateral strike-slip in the 1940 earthquake.
Clearly, this fault is part of the San Andreas system. Other
earthquakes of probable magnitudes of 7 or larger occurred on the
Hayward fault in 1836 and 1868 and on the San Andreas fault in
1838.
When Could the Next Large Earthquake Occur Along
the San Andreas Fault?
Along the Earth's plate boundaries, such as the San Andreas
fault, segments exist where no large earthquakes have occurred
for long intervals of time. Scientists term these segments
"seismic gaps" and, in general, have been successful in
forecasting the time when some of the seismic gaps will produce
large earthquakes. Geologic studies show that over the past
1,400 to 1,500 years large earthquakes have occurred at about
150-year intervals on the southern San Andreas fault. As the
last large earthquake on the southern San Andreas occurred in
1857, that section of the fault is considered a likely location
for an earthquake within the next few decades. The San Francisco
Bay area has a slightly lower potential for a great earthquake,
as less than 100 years have passed since the great 1906
earthquake; however, moderate-sized, potentially damaging
earthquakes could occur in this area at any time.
A great earthquake very possibly will not occur unannounced.
Such an earthquake may be preceded by an increase in seismicity
for several years, possibly including several foreshocks of about
magnitude 5 along the fault. Before the next large earthquake,
seismologists also expect to record changes in the Earth's
surface, such as a shortening of survey lines across the fault,
changes in elevation, and effects on strainmeters in wells. A
key area for research on methods of earthquake prediction is the
section of the San Andreas fault near Parkfield in central
California, where a moderate-size earthquake has occurred on the
average of every 20-22 years for about the last 100 years. Since
the last sizeable earthquake occurred in 1966, Parkfield has a
high probability for a magnitude 5-6 earthquake before the end of
this century and possibly one may occur within a few years of
1988. The U.S. Geological Survey has placed an array of
instruments in the Parkfield area and is carefully studying the
data being collected, attempting to learn what changes might
precede an earthquake of about that size.
What Can Be Done About the Faults and
Earthquakes?
Even though people cannot stop earthquakes from happening,
they can learn to live with the problems caused by earthquakes.
Three major lines of defense against earthquake hazards are being
developed. Buildings in earthquake-prone areas should be
designed and constructed to resist earthquake shaking. Building
codes that require attention to earthquake shaking have been
improving in recent decades and constitute a first line of
defense. In some cities, programs are underway to strengthen or
tear down older buildings most likely to collapse during
earthquakes. A second line of defense involves the selective use
of land to minimize the effects of hazardous ground. High-
occupancy or critical structures, for example, should not be
placed astride the San Andreas fault or on landslide-prone areas.
The third line of defense will be the accurate prediction of
earthquakes. When such prediction becomes possible, it will
permit timely evacuation of the most hazardous buildings. A
major program aimed at learning how to predict earthquakes and to
assess and minimize their hazards was initiated following the
Earthquake Hazards Reduction Act of 1977 and is being carried
out by the U.S. Geological Survey, other Federal Agencies,
universities, and private groups.
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 Interest Publications of the U.S. Geological
Survey," write:
U.S. Geological Survey
Information Services
P.O. Box 25286
Denver, CO 80225
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