Postseismic Fault Healing on the Rupture Zone of the 1999 M 7.1 Hector Mine, California, Earthquake

Abstract

We probed the rupture zone of the October 1999 M 7.1 Hector Mine earthquake using repeated near-surface explosions in October 2000 and November 2001. Three dense linear seismic arrays were deployed across the north and south Lavic Lake faults (LLFs) that broke to the surface in the mainshock and across the Bullion fault (BF) that experienced minor slip in that event. Two explosions each year were detonated in the rupture zone, one on the middle and one on the south LLF. We found that P and S velocities of fault-zone rocks increased by 0.7%-1.4% and 0.5%-1.0% between 2000 and 2001, respectively. In contrast, the velocities for P and S waves in surrounding rocks increased much less. This trend indicates that the Hector Mine rupture zone has been healing by strengthening after the main- shock, most likely due to the closure of cracks that opened during the 1999 earth- quake. The observed fault-zone strength recovery is consistent with an apparent crack density decrease of 1.5% within the rupture zone. The ratio of travel-time decrease for P to S waves was 0.72, consistent with partially fluid-filled cracks near the fault zone. This restrengthening is similar to that observed after the 1992 M 7.4 Landers earthquake, which occurred 25 km to the west (Li and Vidale, 2001). We also find that the velocity increase with time varies from one fault segment to another at the Hector Mine rupture zone. We see greater changes on the LLFs than on the BF, and the greatest change is on the middle LLF at shallow depth. We tentatively conclude that greater damage was inflicted, and thus greater healing is observed, in regions with larger slip in the mainshock.