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Home , Cape Mendocino, San Andreas Fault

U.S. Department of the Interior I U.S. Geological Survey The San Andreas

The by Sandra S. Schulz and Robert E. Wallace

Housing development along the San Andreas fault near , (photo by Robert E. Wallace). The presence of the San Andreas , however, was but one fault was brought dramatically to of many that have resulted from world attention on April18, 1906, episodic displacement along the when sudden displacement along fault" throughout its life of about the fault produced the great San 15-20 million years. Francisco earthquake and fire. This

San Andreas fault in the , (photo by Robert E. Wallace).

2 What Is It? fault network that cuts through rocks of the California coastal Scientists have learned that the region. The entire San Andreas Earth's crust is fractured into a fault system is more than 800 miles series of "plates" that have been long and extends to depths of at moving very slowly over the Earth's least 10 miles within the Earth. In surface for millions of years. detail, the fault is a complex zone Two of these moving plates meet of crushed and broken rock from a in western California; the boundary few hundred feet to a mile wide. between them is the San Andreas Many smaller faults branch from fault. The (on the and join the San Andreas fault west) moves northwestward rela­ zone. Almost any road cut in the tive to the zone shows a myriad of small frac­ (on the east), causing tures, fault gouge (pulverized along the fault. The San Andreas is rock), and a few solid pieces of the "master" fault of an intricate rock.

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Pacific Plate

l'ntarcttlc Plate Antarctic Plate

A simplified map of the Earth's crustal plates

3 Where Is It? The San Andreas fault forms a continuous narrow break in the The figure below shows the Earth's crust that extends from general location of the San Andreas southward to fault and several other major faults near San Bernardino. in California. The complex network Southeastward from Cajon Pass of fau Its that make up the San several branching faults, including Andreas fault system in central the San Jacinto and Banning California is shown in the next faults, share the movement of the figure. crustal plates. In this stretch of the fault zone, the name "San Andreas" generally is applied to the north­ easternmost branch.

Explanation j Segments on which slip occurred during Great Earthquakes of 1857, 1872 and 1906 Segments on which slip occurred during Smaller Earthquakes

1836 Oates of Earthquakes of Magnitudes 7·8

The San Andreas fault system and other large faults in California: different segments of the fault display different behavior.

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Sacramento •

Stockton e

The branches of the San Andreas fault system in central California (from a map by Darrell G. Herd, USGS) 5 What Surface Features , or through Cajon or Characterize It? Tejon Passes may not realize that they are within the San Andreas fault zone. On the ground, the fault Over much of its length, a linear can be recognized by carefully trough reveals the presence of the inspecting the landscape. The fault San Andreas fault; from the air, the zone is marked by distinctive land­ linear arrangement of lakes, bays, forms that include long straight and valleys in this trough is strik­ escarpments, narrow ridges, and ing. Viewed from the ground, how­ small undrained ponds formed by ever, the features are more subtle. the settling of small blocks within For example, many people driving the zone. Many stream channels near Crystal Springs Reservoir characteristically jog sharply to the near San Francisco, or along right where they cross the fault.

A stream channel offset by the San Andreas fault, Carrizo Plain, central California (photo by Robert E. Wallace).

6 What Kind of Movement great earthquakes remain "locked" Has Occurred Along the and quiet over a hundred or more years while strain builds up; then, Fault? in great lurches, the strain is re­ Blocks on opposite sides of the leased, producing great earth­ San Andreas fault move horizon­ quakes. Other stretches of the tally. If a person stood on one side fault, however, apparently accom­ of the fault and looked across it, modate movement more by con­ the block on the opposite side stant creep than by sudden offsets would appear to have moved to the that generate great earthquakes right. Geologists refer to this type (see the illustration on page 4). of fault displacement as right­ In historical times, these creeping lateral strike-slip. sections have not generated earth­ During the 1906 earthquake in quakes of the magnitude seen on the San Francisco region, roads, the "locked" sections. fences, and rows of trees and Geologists believe that the total bushes that crossed the fault were accumulated displacement from offset several yards, and the road earthquakes and creep is at least across the head of Tomales Bay 350 miles along the San Andreas was offset almost 21 feet, the fault since it came into being about maximum offset recorded. In each 15-20 million years ago. Studies of case, the ground west of the fault a segment of the fault between moved relatively northward. and the re­ Sudden offset that initiates a vealed geologically similar terranes great earthquake occurs on only on opposite sides of the fault now one section of the fault at a time. separated by 150 miles, and some Total offset accumulates through crustal blocks may have moved time in an uneven fashion, through more than 20° of latitude. primarily by movement on first one Although it is difficult to imagine and then another section of the this great amount of shifting of the fault. The sections that produce Earth's crust, the rate represented by these ancient offsets is con­ sistent with the rate measured in historical time. Surveying shows a drift at the rate of as much as 2 inches per year.

Movement of blocks along the San Andreas fault. 7 What Is an Earthquake? The vibrations are of two basic types, compression waves and The crustal plates of the Earth transverse or shear waves. Since are being deformed by stresses the compression waves travel from deep within the Earth. The faster through the Earth, they ground first bends, then, upon arrive first at a distant point; they reaching a certain limit, breaks are known as primary or "P" and "snaps" to a new position. In waves. The transverse waves arriv­ the process of breaking or "fault­ ing later are referred to as shear or ing," vibrations are set up that are "S" waves. In an earthquake, the earthquakes. Some of the people may note first a sharp thud, vibrations are of very low frequen­ or blastlike shock, that marks the cy, with many seconds between arrival of the P wave. A few waves, whereas other vibrations seconds later, they may feel a are of high enough frequency to be swaying or rolling motion that in the audible range. marks the arrival of the S wave.

Focus of Earthquake

Outer core

Cross section of the Earth showing the paths of some compression (" P," or primary) and transverse (" S," or shear) waves generated by earthquakes.

8 What Do Earthquake the smallest earthquake normally "Magnitude" and felt by humans. Earthquakes with a Richter value of 5 or higher are "Intensity" Mean? potentially damaging. Some of the Magnitude is a measure of the world's largest recorded earth­ size of an earthquake. The Richter quakes-on January 31, 1906, off Scale, named after Charles F. the coast of Colombia and Richter of the California Institute Ecuador, and on March 2, 1933, off of Technology, is the best known the east coast of Honshu, Japan­ scale for measuring the magnitude had magnitudes of 8.9 on this (M) of earthquakes. The scale is scale, which is open ended. logarithmic; a recording of 7, for As the Richter scale does not example, signifies a disturbance adequately differentiate between with ground motion 10 times as the largest earthquakes, a new large as a recording of 6. The "moment magnitude" scale is energy released by an earthquake being used by seismologists to of M 7, however, is approximately provide a better measure. On the 30 times that released by an earth­ , the San quake of M 6; an earthquake of Francisco earthquake is estimated M 8 releases 900 times (30 x 30) the at magnitude 7.7 compared to an energy of an earthquake of M 6. estimated Richter magnitude of 8.3. An earthquake of magnitude 2 is

Strainmeter in a San Francisco tunnel monitors the nearby San Andreas fault.

9 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" re- ported at different points generally 0 0 decreases away from the earth- ~ 00 f} 0 0 quake epicenter. The intensity c o o o~ • ~Qo range, from I to XII , is expressed in 6) i£[""'0~~~. 09 0 Roman numerals. For example, an 00 , 9; earthquake of intensity II barely 0 ~· ' would be felt by people favorably ~ situated, while intensity X would produce heavy damage, especially to unreinforced masonry. Local geologic conditions strongly in- fl uence the intensity of an earth- quake. Commonly, sites on soft ground or alluvium have intensities 2 to 3 units higher than sites on bedrock. Earthquakes Along the Fault

The map shows the loca- tion of all earthquakes of magnitude 1.5 and larger in the California-Nevada region during Earthquakes of magnitude 1.5 and 1980. Literally thousands of small larger recorded in earthquakes occur in California 1980 on the San each year, providing scientists with Andreas and other large faults in clear indications of places where California and Nevada faults cut the Earth's crust. (data compiled by The largest historical earthquakes Jerry P. Eaton, USGS, and others) that occurred along the San Andreas fault were those in 1857 and 1906 (see the map on page 4 ). The earthquake of January 9, 1857, in ap- parently was about the same mag- nitude as the San Francisco earth- I quake of 1906. According to

10 LEGEND o Moderate to large earthquakes o Small earthquakes Faults

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11 newspaper accounts, ground the Richter Scale, caused inten­ movement in both cases was sities as high as XI on the Modified roughly the same type. An account Mercalli Scale. Surface offsets of the 1857 earthquake describes occurred along a 250-mile length a sheep corral cut by the fault that of the fault from San Juan Bautista was changed from a circle to an north past Point Arena and off­ "S"-shape-movement clearly shore to . representative of right-lateral On May 18, 1940, an earthquake strike-slip. Studies of offset stream of magnitude 7.1 occurred along a channels indicate that as much as previously unrecognized fault in 29 feet of movement occurred in the . Similar move­ 1857. ment on the Imperial fault occurred The San Francisco earthquake during an earthquake in November and fire of April18, 1906, took 1979. The greatest surface dis­ about 700 lives and caused millions placement was 17 feet of right­ of dollars worth of damage in lateral strike-slip in the 1940 California from Eureka southward earthquake. Clearly, this fault is to Salinas and beyond. The earth­ part of the San Andreas system. quake was felt as far away as Other earthquakes of probable and central Nevada. The magnitudes of 7 or larger occurred 1906 earthquake, which has been on the Hayward fault in 1836 and estimated at a magnitude 8.3 on 1868 and on the San Andreas fault in 1838.

A fence, near , California, offset 8.5 teet by displacement on the fault during the 1906 earthquake (photo by G. K. Gilbert)

12 The San Andreas fault in the Mecca Hills. southern California (photo by Robert E. Wallace).

13 When Could the Next that section of the fault is con­ Large Earthquake sidered a likely location for an earthquake within the next few Occur Along the decades. The San Andreas Fault? area has a slightly lower potential for a great earthquake, as less than Along the Earth's plate bound­ 100 years have passed since the aries, such as the San Andreas great 1906 earthquake; however, fault, segments exist where no moderate-sized, potentially damag­ large earthquakes have occurred ing earthquakes could occur in for long intervals of time. Scientists this area at any time. term these segments "seismic A great earthquake very possibly gaps" and, in general, have been will not occur unannounced. Such successful in forecasting the time an earthquake may be preceded by when some of the seismic gaps an increase in seismicity for will produce large earthquakes. several years, possibly including Geologic studies show that over several foreshocks of about magni­ the past 1,400 to 1,500 years large tude 5 along the fault. Before the earthquakes have occurred at next large earthquake, seismolo­ about 150-year intervals on the gists also expect to record changes southern San Andreas fault. As the in the Earth's surface, such as a last large earthquake on the south­ shortening of survey lines across ern San Andreas occurred in 1857, the fault, changes in elevation, and

A devastating fire followed the 1906 earthquake in San Francisco (photo from the P. E. Hotz Collection, USGS Library, Menlo Park, Calif.)

14 effects on strainmeters in wells. A a magnitude 5-6 earthquake before key area for research on methods the end of this century and pos­ of earthquake prediction is the sibly one may occur within a few section of the San Andreas fault years of 1988. The U.S. Geological near Parkfield in central California, Survey has placed an array of where a moderate-size earthquake instruments in the Parkfield area has occurred on the average of and is carefully studying the data every 20-22 years for about the last being collected, attempting to learn 100 years. Since the last sizeable what changes might precede an earthquake occurred in 1966, earthquake of about that size. Parkfield has a high probability for

The San Fernando earthquake of 1971 collapsed fre,eway overpasses in southern California (photo by Robert E. Wallace).

15 What Can Be Done About second line of defense involves the the Faults and selective use of land to minimize the effects of hazardous ground. Earthquakes? High-occupancy or critical struc­ tures, for example, should not be Even though people cannot stop placed astride the San Andreas earthquakes from happening, they fault or on landslide-prone areas. can learn to live with the problems The third line of defense will be caused by earthquakes. Three the accurate prediction of earth­ major lines of defense against quakes. When such prediction be­ earthquake hazards are being comes possible, it will permit time­ developed. Buildings in earth­ ly evacuation of the most hazard­ quake-prone areas should be ous buildings. designed and constructed to resist A major program aimed at learn­ earthquake shaking. Building ing how to predict earthquakes and codes that require attention to to assess and minimize their earthquake shaking have been im­ hazards was initiated following the proving in recent decades and con­ Earthquake Hazards Reduction Act stitute a first line of defense. In of 1977 and is being carried out by some cities, programs are under­ the JJ.S. Geological Survey, other way to strengthen or tear down Federal Agencies, universities, and older buildings most likely to private groups. collapse during earthquakes. A

This. publication is on~ of.a serie~ of general interest publications prepared by the U.S. Ge~log1cal Survey to ~rov1de mformat1on about the earth sciences, natural resources, and the ~nv1ronment. To obta1n a catalog of additional titles in the series "General Interest Publica­ tions of the U.S. Geological Survey," write: Books and Open-File Reports Section U.S. Geological Survey Federal Center, Box 25425 Denver, CO 80255

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