Earthquakes - San Andreas Fault

An earthquake of probable magnitude of 7 or larger occurred on the San Andreas fault in 1838.

January 9, 1857 Fort Tejon earthquake — 350 kilometers were ruptured in central and southern California. Though it is known as the Fort Tejon earthquake, the epicenter is thought to have been located far to the north, just south of Parkfield. Only two deaths were reported. The magnitude was about 8.0. 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.

April 18, 1906 San Francisco Earthquake — 430 kilometers were ruptured in Northern California. The epicenter was near San Francisco. The quake 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.

1989 Loma Prieta earthquake — 40 kilometers were ruptured (although the rupture did not reach the surface) near Santa Cruz, California, causing 63 deaths and moderate damage in certain vulnerable locations in the San Francisco Bay Area. Magnitude this time was about 7.1. The earthquake also postponed game 3 of the 1989 World Series at Candlestick Park.

2004 Parkfield earthquake — on 28 September 2004 at quarter past 10 in the morning, a magnitude 6.0 earthquake struck California on the San Andreas Fault. This earthquake was originally expected in 1993 based on the latest earthquake prediction theories of the time. Eleven years passed before this prediction finally came to pass. Despite the extra time between events the magnitude of the earthquake was no larger than originally expected.

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