Continuing Study of the Post-Seismic Fault Healing of the 1992 M7.5 Landers, California, Earthquake

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Abstract for proposal EAR0105508 (PH # 25x)Title: Continuing Study of the Post-Seismic Fault Healing of the 1992 M7.5 Landers, California, EarthquakePI's: Yong-Gang Li, University of Southern California, and John E. Vidale, University of California, Los Angeles We propose to continue our study of the post-seismic healing process on Landers, California, rupture zone using fault-zone seismic waves excited by explosions detonated within the rupture zone. Repeated surveys in 1994, 1996, 1998, and 2000 have revealed 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 velocity increase between 1994 and 1998 is consistent with a decrease of ~0.03 in apparent crack density within the rupture zone. We also found that cracks near the fault zone was partially water-filled and became more wet with time. However, the data recorded in the most recent experiment showed that wave velocities in the shallow rupture zone did not increase further more but with a slight decrease for shear velocity between 1998 and 2000, inferring that the fault healing probably became invisible since 6 to 8 years after the 1992 earthquake. However, we speculate that the healing process on Landers rupture zone might have been affected by the M7.1 Hector Mine earthquake on October 25, 1999, occurring only ~25 km east of Landers. The Hector Mine mainshock triggered slip on the neighboring faults as shown in the InSAR data and could cause crack opening or coalescence in the shallow crust due to strong shaking and change in the mean stress in the region. We shall conduct a repeated experiment in 2001-2002. The new data will help us to determine if the healing process on the Landers rupture zone was affected by the Hector Mine earthquake or has become invisible. We shall compare the healing process on the Landers rupture zone with that on the Hector Mine rupture zone for which we are monitoring from November of 2000. We shall develop a mechanical model based on crack dilatancy theory to interpret the fault healing quantitatively.