Download The San Andreas Fault

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The
San Andreas
Fault
IJ. S. DEPOSITORY
The
San Andreas
Fault
by Robert Wallace
The presence of the San Andreas fault was
dramatically brought to the attention of the world
on April 18, 1906, when displacement along the
fault resulted in the great San Francisco earthquake and fire. This, however, was but one of
many, many earthquakes that have resulted from
displacement along the fault throughout its life of
possibly 100 million years.
What is it?
The San Andreas fault is a fracture in the Earth's
crust along which two parts of the crust have
slipped with respect to each other. It is the
"master" fault of an intricate network of faults
that cuts through rocks of the coastal region of
California (fig. 1).
SA ••• SAN ANDREAS
G •....••• GARLOCK
WW •.. WHITE WOLF
E .••.••.•. ELSINORE
S G .•. SAN GABRIEL
S J ••• SAN JACINTO
D V ... DEATH VALLEY
H •....••• HAYWARD
FAULT
FAULT
FAULT
FAULT
FAULT
FAULT
FAULT
FAULT
Fig. 1-The network of faults in coastal California (after
Crowell, 1962).
The fault is a huge fracture some 600 or more
miles long, extending almost vertically into the
Earth to a depth of at least 20 miles. In detail it
is a complex zone of crushed and broken rock
from from a few hundred feet to a mile wide.
Many smaller faults branch from and join the San
Andreas fault zone, and if one examines almost
any road cut in the zone, he will find a myriad
of small fractures, fault gouge (pulverized rock),
and few solid pieces of rock.
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Where is it? -
From Cajon Pass southeastward the identity of
the fault becomes confused, because several
branching faults such as the San Jacinto, Mission
Creek, and Banning faults have similar characteristics. Nevertheless, the San Andreas type of
faulting continues unabated southward to and
under the Gulf of Lower California.
Figure 1 shows the general location of the San
Andreas fault and some other major faults in
California, and figure 2 shows its location in more
detail in northern California.
The San Andreas fault forms a continuous break
from northern California southward to Cajon Pass.
·.
..
BERKELEY
..
.
..
SAN
' FRANCISCO
..
·.
...
.
..
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0
0
-------- ...... .
·.
FAULT
Dashed where approximately located.
Dotted where concealed.
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·.
5
0
MILES
5
0
·.·.
5
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KILOMETERS
Fig . 2-The San Andreas and other faults in the San Francisco Bay area (after California Division of Mines and
Geology, Geologic Map of Calif., 1961).
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·.
What surface features
characterize it?
Over much of its length a linear trough reveals
the presence of the fault, and from an airplane
the linear arrangement of the lakes, bays, and
valleys appears striking. Undoubtedly, however,
many people driving near Crystal Springs Reservoir, along Tomales Bay, through Cajon or Tejon
Passes, do not realize they are on the San Andreas
fault zone. On the ground, the fault zone can be
recognized by long straight escarpments, narrow
ridges, and small undrained ponds formed by the
settling of small blocks within the fault zone.
Characteristically, stream channels jog sharply
along the fault trace.
What type and amount
of movement has there been
along the fault?
Essentially, blocks on opposite sides of the
San Andreas fault move horizontally (fig. 3), and
if one were to stand on one side of the fault and
look across it, the block on the opposite side
would appear to be moved to the right. Geologists
Fig. 3-The stream channel has been moved from A to C
by repeated movements over thousands of years and from
A to B by recent movements.
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refer to this as a right-lateral strike-slip fault, or
wrench fault.
During the 1906 earthquake, roads, fences, and
rows of trees and bushes that crossed the fault
were offset several feet, and the road across the
head of Tomales Bay was offset 21 feet, the maximum offset recorded. In each case the ground
west of the fault moved relatively northward.
Geologists who have studied in detail the fault
between Los Angeles and San Francisco have
suggested that the total accumulated displacement along the fault may be as much as 350 miles.
Similarly, geologic study of a segment of the fault
between Tejon Pass and the Salton Sea revealed
geologically similar terrains on opposite sides of
the fault now separated by 150 miles, indicating
that the separation is a result of movement along
the San Andreas and branching San Gabriel faults.
It is difficult to imagine this great amount of
shifting of the Earth's crust; yet the rate represented by these ancient offsets seems consistent
with the rate measured in historical time. Precise
surveying shows a slow drift at the rate of about
2 inches per year. At that rate, if the fault has
been uniformly active during its possible 100
million years of existence, over 300 miles of offset
is indeed a possibility.
Movem~nt
of blocks along the San Andreas fault.
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Major earthquakes along the fault
Figure 4 shows the location of some of the
larger earthquakes in the California-Nevada region. The 1838, 1857, and 1906 earthquakes are
the largest that have occurred along the San
Andreas fault.
.•
.•...
.• .• •.•....
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1954 J
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e1836
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1838
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1952
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APPROXIMATE MAGNITUDE
• 6 - 6.9
•
..
•
••
7 - 7 .9
• 8.3 (1906)
SURFACE DISPLACEMENT
AND DATE ........_ 1952
As a result of the historic San Francisco earthquake of
April 18, 1906, the fence was offset 8V2 feet.
Relatively little is known about the 1838 earthquake in the San Francisco area, and many stories
about it are confused with the 1836 earthquake
along the Hayward fault, which passes through
Oakland and Berkeley. The earthquake of January 9, 1857, apparently was about the same magnitude as that of 1906, and newspaper accounts,
although reported by untrained observers, indicated clearly that ground movement was of the
same type as in 1906. For example, one story relates that a round sheep corral cut by the fault
was changed to an S-shape, clearly representative
of right-lateral strike slip.
The San Francisco earthquake of April 18,
1906, resulted in the loss of possibly 700 lives and
of millions of dollars' worth of damage. Damage
extended from Eureka on the north to Salinas and
beyond on the south, and the earthquake was felt
as far away as Oregon and central Nevada. The
earthquake had a magnitude of 8.3 on the Richter
Scale and an intensity of XI on the Modified Mercalli Scale (see fig. 4). Offset occurred along a 190mile length of the fault from San Juan Bautista to
Point Arena.
On May 18, 1940, an earthquake of magnitude
7.1 occurred along a previously unrecognized
fault in Imperial Valley. Clearly this fault, named
the Imperial fault, is a part of the San Andreas
system. The greatest surface displacement was
19 feet of right-lateral strike slip.
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Fig. 4-Epicenters of some large earthquakes in the California-Nevada region. Heavy lines show where surface of
ground was broken (after Richter, 1958).
What is an earthquake?
The crust of the Earth is commonly subjected
to stresses from deep within the Earth. The crust
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 referred to as earthquakes. Some
of the vibrations are of very low frequency, actually many seconds between swings, whereas
other vibrations are of high enough frequency to
be in the audible range.
The vibrations are also of two basic types, compression waves and transverse or shear waves.
Inasmuch as the compression waves travel faster
through the Earth, they arrive first at a distant
point, and thus are known as primary or "P" waves.
The transverse waves arriving later are referred
to as secondary or "S" waves. If one witnesses
an earthquake, he will very possibly note first a
sharp thud, or blastlike shock, which marks the
arrival of the P wave; then a few seconds later
a swaying or rolling motion may be felt, which
marks the arrival of the S wave.
The Modified Mercalli Scale represents the
local effect or damage caused by an earthquake;
thus the "intensity" reported at a given point decreases away from the earthquake center. The
range, from I to XII, is expressed in Roman
numerals. For example, an earthquake of intensity
11 would be barely felt by people favorably situated, and X would produce general panic, destroy
or heavily damage masonry, and produce conspicuous cracks in the ground.
The San Andreas fault cuts through the southwestern suburbs
of San Francisco. San Andreas Lake (in center of photo)
is in the trough along the fault.
What do "magnitude" and "intensity" of an earthquake mean?
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, signifies 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 humans.
Earthquakes with a Richter value of 6 or more are
commonly considered major in magnitude. The
largest recorded earthquakes in the world (January 31, 1906, off the coast of Columbia and Ecuador, and March 2, 1933, off the east coast of
Honshu, Japan) had magnitudes of 8.9 on this
scale. Of course, even larger earthquakes are
possible.
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When will the next earthquake
along the San Andreas fault occur?
No one yet knows how to predict exactly when
the next earthquake will occur along the San
Andreas fault, but there is every reason to believe
that the fault will continue to be active as it has
been for millions of years in the past. Another
earthquake as strong as that of 1906 could happen
at any time.
The recorded history of earthquakes along the
San Andreas fault is an extremely small sample
from which, however, a clear pattern of behavior
can be determined. Judging from this short history, great earthquakes seem to occur only a
few times a century, but smaller earthquakes
recorded only on sensitive seismographs occur
much more frequently.
San Francisco City Hall after the earthquake and fire of
April 18, 1906.
It is a popular misconception that once there
has been a small earthquake along a segment of
the fault, strain is released and further earthquakes are not to be expected for many years.
Seismologists have pointed out, however, that the
really great earthquakes have been preceded by
numerous strong shocks and that large earthquakes seem to cluster in periods 10 to 20 years
long. Furthermore, the energy released during
small earthquakes is insignificant compared to that
in earthquakes having the same magnitude as the
one in 1906.
Different segments of the fault also behave differently. For example, in the vicinity of Hollister,
frequent small shocks are recorded, and slow
movement .at the rate of 12 mm per year has been
recorded. In contrast, the segment near San Francisco, except for an earthquake of magnitude 5.3
in 1957, has been relatively quiet since 1906.
Perhaps, as some believe, it is gradually bending
or accumulating strain that will be adjusted all
at once in one large "snap."
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What can be done about the fault?
The best answer to this question is that though
man cannot stop earthquakes from happening,
he can learn to live with the problems they
cause. Of prime importance are adequate building codes, for experience shows that well-constructed buildings greatly lessen the hazards. In
construction projects, greater consideration should
be given to foundation conditions. Degree of
damage will range widely between construction
on bedrock, water-saturated mud, filled ground,
or landslide terrain. For example, in 1906, most
buildings on filled or "made" land near the foot
of Market Street in San Francisco suffered particularly intense damage, whereas buildings on solid
rock suffered little or no damage. Geologists are
horrified to see land developers build rows of
houses straddling the trace of the 1906 break.
Much is yet to be learned about the nature and
behavior of the San Andreas fault and the earthquakes it generates. Some questions geologists
would like to answer are: How old is the fault?
Has movement been uniform? What movement
has there been on branching faults? What is the
fundamental cause of the stresses that produced
the San Andreas fault? The U.S. Geological Survey is one of several organizations actively pursuing answers to these questions.
A housing development near San Francisco, California, that
is sitting on the San Andreas fault.
Photo credits
Photograph of San Francisco City Hall is from the
files of the California Division of Mines and
Geology.
San Andreas fault in the suburbs of San Francisco
is an official photograph released by the U.S.
Navy.
Photograph of San Andreas fault in the Carrizo
Plains is by John Shelton.
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20402
Stock No. 024-ooHJ3015-2
l'rU.S. GOVERNMENT PRINTING OFFICE: 1977-240-966-11
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As the Notion's principal conservation ag e ncy, the Deportment of the In teri o r has responsibility for most o f our notiona lly
own ed pub lic lands and natural resources . Th is includes fosterin g the wisest use of our land and water resources, protecting
our fish and wildlife, preservin g the environmental and cultural
values o f our not i onal parks and h isto rical places, and provid i ng f o r the enj oym e nt of life through outdoor recre ation . The
Deportment assesses our energy and mi neral res ources and
works to assure that their development is in the b est interests
o f all ou r p eople . The Deportment a lso has a major responsi bil ity for Am e ri can Indian r eservation communiti es and for peopl e who li ve in Island Territories under U.S . adm i n istration.
Cec il D. Andru s. Secretary
U.S. Deparlmenl ol the lnlerior
V.E. McKelvey , Direclor
Geological Survey