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Published April 1970 | Published
Journal Article Open

Locations of small earthquakes near the trifurcation of the San Jacinto fault southeast of Anza, California


About 100 small earthquakes (M ≈ 1/2 to 2) which occurred near the trifurcation of the San Jacinto fault southeast of Anza, California, have been accurately located using five- and six-station arrays with dimensions of about 10 km. The pattern of epicenters is complex and extends several km outside of the area outlined by the traces of faulting. Patterns of seismicity observed on opposite sides of the San Jacinto fault are significantly different. On the southwest side, a concentration of foci lies at a depth of about 4-7 km along the projected extension of the Coyote Creek fault a few km northwest of the last surface evidence of faulting. On the northeast side, earthquakes are concentrated at depths between 10 and 15 km. A group of the latter events recorded about 1 week after the magnitude 4.7 earthquake of May 21, 1967 forms a linear pattern parallel to the San Jacinto fault with depths from 3 to 15 km. This pattern may represent the zone of energy release or slip for that earthquake and possibly the plane of the San Jacinto fault at depth, although the epicenters are located about 2 to 3 km to the northeast of the trace of the San Jacinto fault. Most of the earthquakes located in this study are not aftershocks in the usual sense, i.e., easily correlated with a preceding large earthquake. They represent a complex pattern of seismicity which has continued at least for the last 3 years on the micro-earthquake level and for the last 30 years on the macroseismic level.

Additional Information

Copyright © 1970, by the Seismological Society of America. Manuscript received October 20, 1969. The authors are indebted to Prof. Stewart Smith who supervised the field program; to Mr. Francis Lehner and Mr. Dan Tanner who assisted in the operation of the trailers; to Mr. John Nordquist for use of his location program; and to Drs. R. V. Sharp and C. R. Allen who participated in helpful discussions. This research was supported in part by the National Science Foundation Grant GP 2806 (Upper Mantle Project); the National Science Foundation Grant GA 12863 (Earthquake Mechanisms); and the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under AFOSR contract AFOSR-F44620-69-C-0067.

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August 19, 2023
October 17, 2023