Healing of the shallow fault zone from 1994-1998 after the 1992 M7.5 Landers, California, earthquake

We conducted seismic surveys at the Johnson Valley fault in 1994, 1996, and 1998. We found that the shear velocity of the fault zone rock increased by similar to1.2% between 1994 and 1996, and increased further by similar to0 .7% between 1996 and 1998. This trend indicates the Landers rupture zone ha s been healing by strengthening after the mainshock, most likely due to the closure of cracks that opened during the 1992 earthquake. The observed fau lt-zone strength recovery is consistent with a decrease of similar to0.03 i n the apparent crack density within the fault zone. The ratio of decrease i n travel time for P to S waves changed from 0.75 in the earlier two years t o 0.65 in the later two years between 1994 and 1998, suggesting that cracks near the fault zone are partially fluid-filled and have became more fluid saturated with time.