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Division of Geological and Planetary Sciences California Institute of Technology, Pasadena, California 91125meltzner{at}gps.caltech.edu
U.S. Geological Survey, 525 S. Wilson Avenue, Pasadena, California, 91106wald{at}usgs.gov
Abstract
The San Andreas fault is the longest fault in California and one of the longest strike-slip faults anywhere in the world, yet we know little about many aspects of its behavior before, during, and after large earthquakes. We conducted a study to locate and to estimate magnitudes for the largest foreshocks and aftershocks of the 1857 M 7.9 Fort Tejon earthquake on the central and southern segments of the fault. We began by searching archived first-hand accounts from 1857 through 1862, by grouping felt reports temporally, and by assigning modified Mercalli intensities to each site. We then used a modified form of the grid-search algorithm of Bakun and Wentworth, derived from empirical analysis of modern earthquakes, to find the location and magnitude most consistent with the assigned intensities for each of the largest events. The result confirms a conclusion of Sieh that at least two foreshocks ("dawn" and "sunrise") located on or near the Parkfield segment of the San Andreas fault preceded the mainshock. We estimate their magnitudes to be M 
6.1 and M 5.6, respectively. The aftershock rate was below average but within one standard deviation of the number of aftershocks expected based on statistics of modern southern California mainshock-aftershock sequences. The aftershocks included two significant events during the first eight days of the sequence, with magnitudes M 6.25 and M 6.7, near the southern half of the rupture; later aftershocks included a M 6 event near San Bernardino in December 1858 and a M 6.3 event near the Parkfield segment in April 1860. From earthquake logs at Fort Tejon, we conclude that the aftershock sequence lasted a minimum of 3.75 years.
This article has been cited by other articles:
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  N. A. Toke, J R. Arrowsmith, J. J. Young, and C. J. Crosby Paleoseismic and Postseismic Observations of Surface Slip along the Parkfield Segment of the San Andreas Fault Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S221 - S238. [Abstract] [Full Text] [PDF]
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N. A. Toke and J R. Arrowsmith Reassessment of a Slip Budget along the Parkfield Segment of the San Andreas Fault Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S339 - S348. [Abstract] [Full Text] [PDF]
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T. Toppozada and D. Branum San Andreas M ~6 Earthquakes within 40 km of the Priest Valley End Zone of the 1857 Faulting Bulletin of the Seismological Society of America, September 1, 2006; 96(4B): S385 - S396. [Abstract] [Full Text] [PDF]
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W. H. Bakun Estimating Locations and Magnitudes of Earthquakes in Southern California from Modified Mercalli Intensities Bulletin of the Seismological Society of America, August 1, 2006; 96(4A): 1278 - 1295. [Abstract] [Full Text] [PDF]
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The Tejon Pass Earthquake of 22 October 1916: An M 5.6 Event on the Lockwood Valley and San Andreas Faults, Southern California Bulletin of the Seismological Society of America, August 1, 2004; 94(4): 1293 - 1304.
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Aftershocks and Triggered Events of the Great 1906 California Earthquake Bulletin of the Seismological Society of America, October 1, 2003; 93(5): 2160 - 2186.
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San Andreas Fault Zone, California: M >=5.5 Earthquake History Bulletin of the Seismological Society of America, October 1, 2002; 92(7): 2555 - 2601.
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