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

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 reveals the existence of a deep bimaterial interface that extends from the array to at least 50 km southeast and has a section with > 10% velocity contrast. This analysis also corroborates the teleseismic results and indicates a broad damage zone primarily northeast of the fault bounded by a shallow bimaterial interface near BS34 that merges with the deep interface. 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 earthquake ruptures in the area to the northwest.