Abstract
Mean apparent focal depths of microearthquakes occurring along a 20‐km stretch of the San Andreas fault southeast of Hollister, California, increased by 25 percent some 60 days before the magnitude 4.6 Stone Canyon earthquake of September 4, 1972. The shape of the time‐depth anomaly is virtually identical to the time plots of Vp/Vs preceding moderate earthquakes at Garm, U.S.S.R. A less well‐defined depth anomaly occurred in October‐December 1971, preceding the Limekiln Road swarm in December 1971 and the magnitude 5.0 Melendy Ranch earthquake in February 1972. The observed depth anomalies can be attributed to dilatancy biasing of hypocenters, although true vertical migration of seismicity cannot be ruled out.
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