High‐resolution structures of the Landers fault zone inferred from aftershock waveform data

SUMMARY High‐frequency body waves recorded by a temporary seismic array across the surface rupture trace of the 1992 Landers, California, earthquake were used to determine fault‐zone structures down to the seismogenic depth. We first developed a technique to use generalized ray theory to compute synthetic seismograms for arbitrarily oriented tabular low‐velocity fault‐zone models. We then generated synthetic waveform record sections of a linear array across a vertical fault zone. They show that both arrival times and waveforms of P and S waves vary systematically across the fault due to transmissions and reflections from boundaries of the low‐velocity fault zone. The waveform characteristics and arrival‐time patterns in the record sections allow us to locate the boundaries of the fault zone and to determine its P ‐ and S ‐wave velocities independently as well as its depth extent. Therefore, the trade‐off between the fault‐zone width and velocities can be avoided. Applying the method to the Landers waveform data reveals a low‐velocity zone with a width of 270–360 m and a 35–60 per cent reduction in P and S velocities relative to the host rock. The analysis suggests that the low‐velocity zone extends to a depth of ∼7 km. The western boundary of the low‐velocity zone coincides with the observed main surface rupture trace.