Time Variations in Teleseismic Residuals Prior to the Magnitude 5.1 Bear Valley Earthquake of February 24, 1972

Summary

Teleseismic arrivals for large earthquakes occurring in the Circum-Pacific seismic source regions have been analysed for P- velocity variations prior to the February 24, 1972, Bear Valley earthquake at several stations in the vicinity of Bear Valley, California. The teleseismic arrivals have been analysed by the method of two-station residuals and corrected for observed azimuthal variations. The data covers the time period of July, 1971, through April, 1972, and suggests that during part of January, 1972, a P-velocity anomaly occurred beneath station BVL 2 km from the epicenter of the magnitude 5.1 Bear Valley event. A maximum vertical travel time delay of 0.15 seconds is observed. No other anomalous behavior associated with this event is suggested by the data for the other stations ranging from 7 to 19 km from the event’s epicenter. These results support an anomalous zone of limited size with a maximum horizontal extent of less than 5 km perpendicular and about 10 km parallel to the San Andreas fault relative to the epicenter and confined within a 5 to 10 km portion of the uppermost crust. Also a P-velocity delay for waves travelling essentially along the intermediate stress axis would imply in this case that the P-velocity anomaly is caused by a bulk modulus mechanism such as that proposed by the dilatancy-fluid flow theory.