Characterization of fault zones by analysis of aftershock waveform data

Large property contrasts between materials in a fault zone and the surrounding rock are often produced by repeating earthquakes. Fault zones are usually characterized by fluid concentration, clay-rich fault gouge, increased porosity, and dilatant cracks. Thus, fault zones are thought to have reduced seismic velocities than the surrounding rocks. In this article, we first investigated the synthetic waveforms at a linear array across a vertical fault zone by using 3D finite difference simulation. Synthetic waveforms show that when sources are close to, inside, or below the fault zone, both arrival times and waveforms of P- and S-waves vary systematically across the fault zone due to reflections and transmissions from boundaries of the low-velocity fault zone. The arrival-time patterns and waveform characteristics can be used to determine the fault zone structure. Then, we applied this method to the aftershock waveform data of the 1992 Landers M7.4 and the 2008 Wenchuan (汶川) M8.0 earthquakes. Landers waveform data reveal a low-velocity zone with a width of approximately 270-370 m, and P- and S-wave velocity reductions relative to the host rock of approximately 35%–60%; Wenchuan waveform data suggest a low-velocity zone with a width of approximately 220–300 m, and P- and S-wave velocities drop relative to the host rock of approximately 55%.