Study of the 1999 M 7.1 Hector Mine, California, Earthquake Fault Plane by Trapped Waves

Abstract

We recorded fault-zone trapped waves from aftershocks on portable seismometers in a tight linear array across the Lavic Lake fault, which was one of several faults that ruptured in the M 7.1 Hector Mine, California, earthquake on 16 October 1999. Trapped waves with large amplitudes and long duration at 4 to 7 Hz produced by aftershocks occurring within the rupture zone were recorded at stations close to the fault trace. However, the S waves registered at stations farther from the rupture zone for the same events were much briefer. Trapped waves recorded at the Hector Mine rupture zone are similar to those observed in the Landers rupture zone (Li et al., 1994a,b), but show higher frequencies. Simulations of these trapped waves indicate a 75 to 100-m-wide low-velocity and low-Q waveguide along the Hector Mine rupture zone in which the S velocity is reduced by about 40% to 50% from wall-rock velocities, and Q is 10 to 60 in the depth range from the surface to 10 km. We interpret this low-velocity waveguide as being a remnant of the process zone formed by inelastic deformation around the propagating crack tip during dynamic rupture in the 1999 Hector Mine earthquake. The reductions of velocities and Q within the Hector Mine rupture zone are similar to those within the Landers rupture zone, suggesting that the fault-zone rock was damaged to the same degree in the two earthquakes. The wave-guide width (75-100 m) on the Hector Mine rupture zone (40 km in the total length) is half that (150-250 m) of the Landers rupture zone (80 km in the total length), consistent with the scaling of process zone size to rupture length as predicted in some published dynamic rupture models. Locations of aftershocks for which we observed trapped waves show bifurcation of the northern Hector Mine rupture at depth, although only the west rupture branch broke to the surface.