Multiple-fault rupture of the M7.1 Hector Mine, California, earthquake from fault zone trapped waves

[1] We studied the complex multiple-faulting pattern of the 40-km-long rupture zone of the 1999 M7.1 Hector Mine, California, earthquake with fault zone trapped waves generated by near-surface explosions and aftershocks, and recorded by linear seismic arrays deployed across the surface rupture. The explosion excited trapped waves, with relatively large amplitudes at 3–5 Hz and a long duration of S coda waves, are similar to those observed for aftershocks but have lower frequencies and travel more slowly. Threedimensional finite difference simulations of fault zone trapped waves indicate a 75to 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 0–8 km, reduced by 35–45% 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 has two branches 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 main shock epicenter. To the south, the rupture on the LLF intersected the Bullion fault (BF) and bifurcated. The rupture on the south LLF extended 10 km from the intersection and diminished while there was minor rupture on the southeast BF, which dips to the northeast and disconnects from the LLF at depth. 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 which may also rupture individually. Our simulations of dynamic rupture using a finite element code show that generic models are able to produce the general features of the northern part of the rupture, including slips on subparallel fault segments. The models indicate that such a faulting pattern is physically plausible and consistent with observations.