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Group B, Poster #046, Seismology

The variable continuous bimaterial interface in the San Jacinto fault zone revealed by dense seismic array analysis of fault zone head waves

Pieter-Ewald Share, Frank L. Vernon, & Yehuda Ben-Zion
Poster Image: 

Poster Presentation

2022 SCEC Annual Meeting, Poster #046, SCEC Contribution #12318 VIEW PDF
Key factors controlling earthquake ruptures include fault geometry, continuity and seismic velocity structure around the fault. We present a novel tool that better informs deep bimaterial fault geometry embedded in distributed damage and seismicity, associated velocity contrasts across the fault, and correlations with surface complexities. The method employs fault zone head and direct body waves and is applied to recordings from five spatiotemporally different seismic arrays along the complex San Jacinto fault zone (SJFZ) in southern California. We detect and distinguish these signals based on instantaneous phase coherence and relative energy in a cascading manner from one scale array to ano...ther. The analysis reveals a >70 km long continuous bimaterial interface within the SJFZ with several deep northeast dipping fault segments. The northern SJFZ, for instance, locates ~7 km northeast of its surface expression at 18 km depth. P-wave velocity contrasts range from 0% to >15%, consistent with other bimaterial faults, and differ by a few % depending on fault-array azimuth, implying velocity contrast anisotropy. S-wave head waves and velocity contrasts are also imaged for the first time at the southern SJFZ, averaging to 2% (+/- 1.8%) in agreement with tomography results. The imaged geometry and continuity suggest the SJFZ initiated along remnant tectonic structures and translates to a rupture potential of M>7.2, i.e., the sizes of its largest paleo-earthquakes. The P and S contrasts, and their ratios, have important implications for earthquake rupture speed, mode, directivity and frictional heating along the SJFZ and other major faults globally.