Stability of P an S velocities from Central California quarry blasts

Travel times of crustal P and S waves from 70 quarry blasts in Central California between July 1961 and June 1973 have been measured for seven paths 46 to 168 km long passing in several cases within 10 km of hypocenters of moderate (ML 4.5 to 5.4) earthquakes. All P and S times and their ratios TS/TP lie within 2.3 per cent of the mean values and over 97 per cent of the TS/TP ratios lie within 1 per cent of the mean values. Variations can be explained by reading errors and uncertainties in source times and locations. There are no indications of velocity variations related to earthquake occurrences. Mean VP/VS ratios range from 1.73 to 1.84 for the various paths. The phenomenon of premonitory dilatancy accompanied by a 10 to 20 per cent reduction in VP or VP/VS cannot be reconciled with these observations unless the affected source regions have lateral dimensions less than about 5 km for these moderate strike-slip earthquakes.