abstract

Analysis of contemporary accounts indicates that several small to moderate central California earthquakes preceded the great 1857 earthquake by 1 to 9 hr. The earliest events apparently were felt only in the San Francisco area or the Sacramento and Sierran Foothills region. Two later and much more widely felt foreshocks were experienced within the region bounded by San Francisco, Visalia, Fort Tejon, and Santa Barbara. A comparison with felt areas and intensity distributions of modern events of known source and magnitude indicates that these later two shocks were 5 ≦M ≲ 6 and probably originated at some point within an area of radius ≈60 km that includes the southeastern 100 km of the historically creeping segment of the San Andreas fault. The northwestern terminus of the 1857 rupture is probably located along this segment.

If the location of these foreshocks is indicative of the epicenter of the main event, then the several-hundred-kilometer main-event rupture propagated principally in a unilateral fashion toward the southeast. This implies that, like many great earthquakes, the 1857 rupture originated on a fault segment historically characterized by moderate activity and propagated into an historically quiet segment.

There is a strong possibility that the foreshock activity represents a moderate Parkfield-Cholame sequence similar to those of 1901, 1922, 1934, and 1966. To the extent that such premonitory activity is characteristic of the failure of the 1857 segment of the fault, studies of the creeping segment of the fault may be relevant to the prediction of large earthquakes in central and southern California.

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