Premonitory vertical migration of microearthquakes in central California-Evidence of dilatancy biasing?

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 V-p/ V-s preceding moderate earthquakes at Garm, U.S.S.R. A less well-defined depth anomaly occurred in October-Decembe1r 971, preceding the Limekiln Roads warmin Decembe1r 971 and the 5.0 Melendy Ranch earthquake in February 1972. The observed depth anomalies can be attributed to dilatancy biasing of althought rue vertical migrationo seismicity cannot be ruled out.