A Caltech Library Service

Foreshocks and Aftershocks of the Great 1857 California Earthquake

Meltzner, Aron J. and Wald, David J. (1999) Foreshocks and Aftershocks of the Great 1857 California Earthquake. Bulletin of the Seismological Society of America, 89 (4). pp. 1109-1120. ISSN 0037-1106.

PDF - Published Version
See Usage Policy.


Use this Persistent URL to link to this item:


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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Meltzner, Aron J.0000-0002-2955-0896
Additional Information:© 1999, by the Seismological Society of America. Manuscript received 15 August 1998. Many people contributed to the completion of this study. We benefited greatly from discussion with or help from William Deverell, Jim O'Donnell, and Kerry Sieh at Caltech; William Bakun and James Dewey of the U.S. Geological Survey; Tousson Toppozada of the California Division of Mines and Geology; and Duncan Agnew at U.C. San Diego. We also thank William Bakun, Lucile Jones, and Paul Reasenberg of the USGS for very useful reviews, which improved this manuscript. Maps in this report have been created using GMT (Wessel and Smith, 1991). This work has been supported with funds provided by the Summer Undergraduate Research Fellowship program of the California Institute of Technology, and by the U.S. Geological Survey.
Funding AgencyGrant Number
Caltech Summer Undergraduate Research Fellowship (SURF)UNSPECIFIED
Issue or Number:4
Record Number:CaltechAUTHORS:20140806-113310221
Persistent URL:
Official Citation:Aron J. Meltzner and David J. Wald Foreshocks and aftershocks of the great 1857 California earthquake Bulletin of the Seismological Society of America, August 1999, v. 89, p. 1109-1120
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48079
Deposited By: Ruth Sustaita
Deposited On:06 Aug 2014 18:48
Last Modified:09 Mar 2020 13:19

Repository Staff Only: item control page