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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.

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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 FAULT G •....••• GARLOCK FAULT WW •.. WHITE WOLF FAULT E .••.••.•. ELSINORE FAULT S G .•. SAN GABRIEL FAULT S J ••• SAN JACINTO FAULT D V ... DEATH VALLEY FAULT H •....••• HAYWARD 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? -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.

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. . . . . SAN

' FRANCISCO

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. . . . . .

-------- ...... . FAULT

Dashed where approximately located. Dotted where concealed.

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5

0

MILES 0

KILOMETERS

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. .

· .

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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.

BERKELEY

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0

·. ·. ·. ·. ·.

·.

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 1 00 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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• • • • . . ..

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7~ e1836 ~ .... ~ '0 1838 ~

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• APPROXIMATE MAGNITUDE

• 6 - 6.9 • 7 - 7.9 • 8.3 (1906)

SURFACE DISPLACEMENT AND DATE ........_ 1952

1954 J ••• "'"'1954

~1932

. . 187~~

. 1952

~· . .· . . \ . • ••

Fig. 4-Epicenters of some large earthquakes in the Cali­fornia-Nevada region. Heavy lines show where surface of ground was broken (after Richter, 1958).

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.

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As a result of the historic San Francisco earthquake of April 18, 1906, the fence was offset 8V2 feet.

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 190-mile 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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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.

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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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.

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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.

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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.

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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 agency, the Deport­ment of the In terio r has responsibility for most o f our notiona lly owned public 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 ional parks and h isto rical places, and provid ­ing for the enjoyment of life through outdoor recreation. The Deportment assesses our energy and mineral resources and works to assure that their development is in the best interests o f all ou r people. The Deportment a lso has a major responsi ­bil ity for Am eri can Indian reservation communiti es and for peo­ple who li ve in Island Territories under U.S. adm in istration.

Cec il D. Andru s. Secretary

U.S. Deparlmenl ol the lnlerior

V.E. McKelvey , Direclor

Geological Survey