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Time variance of seismic velocity from multiple explosive sources southeast of Hollister, California

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94525

Abstract

As part of an intensive effort to monitor the section of the San Andreas fault south of Hollister, California, for any seismic velocity time variance associated with earthquakes, 17 explosive charges were fired between August 1974 and May 1975, at a point 35 km east of the fault zone and recorded by a special network of 51 seismometers within and west of the San Andreas fault zone. The shot point was selected to minimize differences in rock properties for successive shot locations, and all the data were recorded on a single tape recorder dedicated to the experiment. By cross-correlating the records from successive shots, it was possible to determine differential travel times to an accuracy of 4 msec or better using a 250-kg explosive charge recorded in the distance range between 30 to 50 km. No definitive changes in seismic velocity were detected during the time covered by this experiment. The spatial distribution of stations and the temporal distribution of shots were designed to reveal anomalies that might be associated with a magnitude 4 or greater earthquake. Nine small earthquakes (M = 2.9 to 3.3) did occur during the experiment, but anomalies associated with these earthquakes could have escaped detection. This work demonstrates the feasibility of monitoring large regions with a high density of instruments to detect systematic changes in arrival times from artifical sources with an accuracy approaching 1 msec and a cost commensurate with the task of earthquake prediction.

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