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Saturday, July 24, 2004 - Mostly big quakes on San Andreas Fault, study says ASSOCIATED PRESS

LOS ANGELES - A new study concludes that a long stretch of the mighty San Andreas Fault close to highly populated Southern California has usually only had very large earthquakes.
The researchers' finding challenges an idea that frequent small temblors slowly relieve accumulating strain on faults and therefore reduce the likelihood of big quakes.
Digging trenches at a site on the San Andreas 120 miles northwest of Los Angeles, geologists identified six events and found evidence that about 95 percent of slippage there has occurred during big quakes, with magnitudes ranging from 7.5 to 8, according to the study appearing in the current issue of the journal Geology.
The study's authors included professor Kerry Sieh of the California Institute of Technology, his former student and postdoctoral fellow Jing Liu, Charles Rubin of Central Washington University in Ellensburg, Wash., and Yann Klinger of the Institut de Physique du Globe de Paris, France.
The San Andreas Fault system, a huge, visible gash running through western California, is more than 800 miles long and marks the boundary of the Pacific Plate, which is on the west side and is moving northwestward relative to the North American Plate on the east side.
Scientists have long known the average period between big earthquakes on the section of the San Andreas nearest Los Angeles is 130 years. The most recent large earthquake happened 147 years ago, a magnitude-7.9 temblor that struck in 1857.
The researchers dug trenches parallel and perpendicular to a section of the fault lying between the southern San Joaquin Valley city of Bakersfield and the coast.
The trenches allowed them to study how movement of the San Andreas offset gullies that crossed the fault and were subsequently buried by sediment over the centuries.
The researchers found six offsets, the youngest related to the 1857 quake. Older gullies were progressively offset further by the fault.
Of the six, the offsets of five ranged from 5.72 yards to 8.8 yards. The other was about 11/2 yards.
According to scientists, offsets of several yards are typical of earthquakes that are very large and rupture over very long distances. The 1857 quake had a rupture length of 225 miles, extending from Parkfield in Central California to the Cajon Pass east of Los Angeles.
Rubin noted in an interview that the smallest offset was probably not the result of a small quake, but represented the tail end of a rupture that began on a distant section of the fault.

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