9404762 Li This research is to study the postseismic healing of the fault zone ruptured in the M=7.4 Landers earthquake of 1992 by monitoring propagation characteristics of fault zone trapped waves using portable high-capability seismometers. Previous work showed evidence for seismic guided waves trapped within the fault zone of the Landers earthquake. Based on trapped mode data, it resolved a low velocity, high-attenuation layer with a width of 180 m along the rupture plane. This layer extends from the surface to a depth of at least 10 km in the seismogenic zone. It was also found that the width of this layer varies vertically and laterally along the fault zone, and it becomes very narrow at the fault bend. Observations and simulation of fault zone trapped waves show that the amplitudes and frequency contents of these waves are very sensitive to the velocity structure and properties of the fault zone. Therefore, the subsurface fault variability in space and time can be detected through monitoring minor changes in amplitude and dispersion characteristics of fault zone trapped waves. Results from this study should advance understanding of fault zone structure, rheology and constitutive relations, the transition from quasistatic nucleation to dynamic rupture, and the models and mechanisms for earthquake recurrence. This research is a component of the National Earthquake Hazard Reduction Program. *** ??
