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.

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

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