abstract

Teleseismic arrivals at 30 stations in the vicinity of Hollister, California, have been analyzed for P-velocity variations prior to the magnitude 5.1 November 28, 1974, Thanksgiving Day earthquake. The teleseismic arrivals for large circum-Pacific source events during the period of May through December, 1974, have been analyzed using the method of two-station relative residuals with a correction for observed azimuthal variations. The data contain no significant travel-time delays associated with this event to within 0.1 sec and all 30 stations analyzed exhibited stable travel-time residuals during the study period. In particular, residuals at station OCR for South and Central American sources whose ray paths pass within 2.5 km of the hypocenter exhibit no time changes. Assuming at least a 10 per cent P-velocity charge for anomalous zones, these observations restrict the maximum possible P-velocity anomaly zone that might have been associated with this particular event to within 3 to 4 km of the hypocenter.

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