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A re-examination of historic earthquakes in the San Jacinto fault zone, California

Bent, Allison L. and Helmberger, Donald V. (1991) A re-examination of historic earthquakes in the San Jacinto fault zone, California. Bulletin of the Seismological Society of America, 81 (6). pp. 2289-2309. ISSN 0037-1106.

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The high level of seismic activity and the potential for large earthquakes in the San Jacinto fault zone, southern California, make it desirable to have accurate locations and source parameters for as many previous events as possible. Prior to the installation of a dense seismic network in this region, earthquakes were located using only a few stations with generally poor azimuthal coverage resulting in considerable uncertainty in the locations. We relocate and obtain moment estimates for historic (pre-WWSSN) earthquakes in the western Imperial Valley by comparing the waveforms and travel times with recent earthquakes in the region. All the events are in the M_L 5.5 to 6.5 range. The historic earthquakes of interest occurred in 1937, 1942, and 1954. We use the 1968 Borrego Mountain, 1969 Coyote Mountain, and 1987 Elmore Ranch earthquakes as calibration events. We employ regional and teleseismic data from continuously operating stations, with Pasadena, DeBilt, Berkeley, Ottawa, and St. Louis recording most of the events. The waveforms imply that all the events are almost pure strike-slip events on vertical or near-vertical faults. Approximate values for the strikes were obtained and are within the range of observed strikes for well-studied earthquakes in this region. The earthquakes are relocated by comparing S-P and surface-wave - S travel times of historic events with the presumably well-located recent events. The relocations require only a small change in location for the 1954 event and a larger adjustment in the 1942 epicenter. It also appears that the 1969 earthquake may have been mislocated. The moment estimates are obtained by direct comparison of the maximum amplitudes. The moment estimates imply that the 1968 and not the 1942 earthquake is the largest to have occurred in the region this century. Previous magnitude estimates suggested the 1942 event was larger.

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Additional Information:© 1991 Seismological Society of America. Manuscript received 31 October 1990. This work was supported by USGS contract number 14-08-0001-G1872. California Institute of Technology Division of Geological and Planetary Sciences contribution no. 4941. We would like to thank Hiroo Kanamori for reviewing the manuscript, Lane Johnson for providing the 1954 and 1942 Berkeley records, and Bob Hermann for the St. Louis records. Diane Doser and Chris Sanders provided us with thoughtful reviews that improved the quality of the paper.
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Caltech Division of Geological and Planetary Sciences 4941
Issue or Number:6
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48063
Deposited By: Tony Diaz
Deposited On:06 Aug 2014 18:00
Last Modified:03 Oct 2019 06:59

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