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Structural Properties of the San Jacinto Fault Zone at Blackburn Saddle from Seismic Data of a Dense Linear Array

Pieter-Ewald Share, Amir A. Allam, Yehuda Ben-Zion, Fan-Chi Lin, & Frank L. Vernon

Published September 18, 2018, SCEC Contribution #9073

We image the San Jacinto fault zone at Blackburn Saddle using earthquake waveforms recorded by a ~2 km across-fault linear array with 108 three-component sensors separated by ~10-30 m. The length and spatiotemporal sampling of the array allow us to derive high-resolution information on the internal fault zone structure with spatial extent that can be merged with regional tomography models. Across-fault variations in polarization, amplitude, and arrival time of teleseismic P waves indicate abrupt changes in subsurface structure near the surface trace of the fault (sensor BS55) and ~270 m to the northeast (sensor BS34). Analysis of fault zone head waves from local events corroborate the teleseismic results and indicates further a broad damage zone primarily northeast of the fault bounded by a bimaterial interface near BS34. Arrival time differences between head and direct P waves with changing event distance show that segments of this bimaterial interface have >10% velocity contrast and continues at depth from the array to at least 50 km southeast. Detection and waveform inversions of Love-type fault zone trapped waves generated by local events indicate a trapping structure within the broader damage zone with width of ~150 m, velocity reduction of ~55% from the surrounding rock and depth extent of ~2 km. The performed analyses provide consistent results on the subsurface location of the main seismogenic fault and properties of a major bimaterial interface and damage structure. The imaged fault zone properties are consistent with preferred propagation direction of earthquakes in the area to the northwest.

Share, P., Allam, A. A., Ben-Zion, Y., Lin, F., & Vernon, F. L. (2018). Structural Properties of the San Jacinto Fault Zone at Blackburn Saddle from Seismic Data of a Dense Linear Array. Pure and Applied Geophysics, 176(3), 1169-1191. doi: 10.1007/s00024-018-1988-5.