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