Healing of the shallow fault zone from 1994–1998 After the 1992 <i>M</i>7.5 Landers, California, Earthquake

Abstract

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 ∼1.2% between 1994 and 1996, and increased further by ∼0.7% between 1996 and 1998. This trend indicates the Landers rupture zone has been healing by strengthening after the mainshock, most likely due to the closure of cracks that opened during the 1992 earthquake. The observed fault‐zone strength recovery is consistent with a decrease of ∼0.03 in the apparent crack density within the fault zone. The ratio of decrease in travel time for P to S waves changed from 0.75 in the earlier two years to 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.