EVIDENCE OF SHALLOW FAULT ZONE STRENGTHENING AFTER THE 1992 M7.5 LANDERS, CALIFORNIA, EARTHQUAKE

Repeated seismic surveys of the Landers, California, fault zone that r uptured in the magnitude (M) 7.5 earthquake of 1992 reveal an increase in seismic velocity with time, P, S, and fault zone trapped waves wer e excited by near-surface explosions in two locations in 1994 and 1996 , and were recorded on two linear, three-component seismic arrays depl oyed across the Johnson Valley fault trace, The travel times of P and S waves for identical shot-receiver pairs decreased by 0.5 to 1.5 perc ent from 1994 to 1996, with the larger changes at stations located wit hin the fault zone. These observations indicate that the shallow Johns on Valley fault is strengthening after the main shock, most likely bec ause of closure of cracks that were opened by the 1992 earthquake, The increase in velocity is consistent with the prevalence of dry over we t cracks and with a reduction in the apparent crack density near the f ault zone by approximately 1.0 percent from 1994 to 1996.