Multiple-Fault Rupture of the M7.1 Hector Mine, California, Earthquake from Fault-Zone Trapped Waves Generated by Explosions and Aftershocks

We carried out an experiment using 3 tight linear seismic arrays deployed across the rupture zone of the 1999 M7.1 Hector Mine earthquake to record fault-zone trapped waves generated by explosions and aftershocks within the rupture zone. We delineated the complex multiple-faulting pattern of the 40-km-long rupture zone with trapped waves. 3-D FD simulations of trapped waves indicate a 75 to 100-m-wide low-velocity and low-Q zone (waveguide) along the rupture surface on the Lavic Lake fault (LLF) in the Bullion Mountains. The S velocity within the waveguide varies from 1.0 to 2.5 km/s at depths of 0-10 km, reduced by 40-50% from the wall-rock velocity, and Q is 10-60. The pattern of aftershocks for which we observed trapped waves shows that this low-velocity waveguide bifurcated in the northern and southern portions of the rupture zone, indicating a multiple-fault rupture at seismogenic depth. North of the Bullion Mountains, although only the rupture segment on the northwest LLF broke to the surface, a rupture segment on a buried fault also extended ~15 km in the more northerly direction from the mainshock epicenter. To south, the rupture on the LLF intersected the Bullion fault and bifurcated while there was minor rupture on the southeast BF. Thus, the analysis of fault-zone trapped waves helps delineate a more complex set of rupture planes than the surface breakage, in accord with the complex pattern of aftershock distribution and geodetic evidence that the Hector Mine event involved several faults. Simulations of dynamic rupture using a finite-element code [Oglesby et al., 2001 AGU Fall] show that such a faulting pattern is physically plausible and consistent with observations. We shall repeat the explosions in the Fall of 2001 to monitor temporal changes in physical properties of the rupture zone.