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High-resolution structures of the Landers fault zone inferred from aftershock waveform data
Authors:Hongyi Li  Lupei Zhu  Hongfeng Yang
Institution:Department of Earth and Atmospheric Sciences, Saint Louis University, St Louis, MO 63108;, USA. E-mail:
Abstract: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.
Keywords:fault model  fracture zone  ray theory  reflection seismology  seismic modelling  seismic-wave propagation
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