Deep San Jacinto fault Zone Geometric and Bimaterial Properties from Analysis of Fault Zone Head Waves Recorded by Several Dense Across-fault Arrays

Abstract:

A critical research direction in crustal seismology is better quantifying the geometric and material properties along deep fault interfaces in complex geological/tectonic terrains. A representative case study of such complexity is the San Jacinto fault zone (SJFZ) Trifurcation Area (TFA) in Southern California. In this study, we use fault zone head waves (FZHW) detected at three surface locations ~10 km apart along the SJFZ and northwest of the TFA using several dense across-fault seismic arrays to help constrain deep TFA fault geometry and bimaterial properties.

We use FZHW (radiating from fault) and direct P (propagating from event) wavefront moveouts across large-N arrays at the three sites as templates and phase-weighted stacking to; (1) detect all other events with similar FZHW and direct P moveouts during the ~month-long deployments of the large-N arrays, and (2) identify the same wavefronts recorded by spatiotemporally overlapping shorter linear arrays at two of the sites. In turn, the moveout templates from the short linear arrays are then used to detect all other candidate FZHW events within their ~10-year recording windows. These analyses show, as expected, a general increase in differential time between FZHW and trailing direct waves with increasing propagation distance along the SJFZ through the TFA (~0.1-0.4 s), especially around the source regions of the 2016 M 5.2 and 2020 M 4.9 events that ruptured the central Clark fault branch. In the same area though, the short arrays’ results show for events >10 km deep a migration of maximum differential time to the northeast of the Clark fault surface trace. This suggests with increasing depth the dip of this major bimaterial fault becomes progressively shallower to the northeast. Further analysis, including spatial gradients in the differential times of these on-fault events, which reveal changing along fault bimaterial contrasts, will be presented at the meeting.