The Variable Continuous Bimaterial Interface in the San Jacinto Fault Zone Revealed by Dense Seismic Array Analysis of Fault Zone Head Waves.

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 their 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 another. 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 near 0% to >15%, consistent with other bimaterial faults, and differ by a few % depending on fault‐array azimuth, implying directional‐dependent velocity contrasts. S‐wave head waves and velocity contrasts are also imaged for the first time at the southern SJFZ, averaging to 2.9% 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. Plain Language Summary: Large earthquakes occur on faults that are structurally complex, leading to earthquakes that are themselves structurally and dynamically complex. More accurate information on fault complexities is key to quantifying the potential size and shaking from large earthquakes along them. Here, we introduce a novel tool that resolves the deep geometry, continuity, and contrast in seismic velocity along major transform fault systems using the complex San Jacinto fault zone (SJFZ) in southern California as an exemplary case. It employs unique signals that travel along, and illuminate, continuous faults contrasting rock volumes of different seismic properties. With this tool, we reveal a >70‐km long continuous fault within the SJFZ that is nonsimple and nonplanar, curving laterally, and in depth. For example, a deep northern section of the SJFZ dips extensively toward the northeast. The seismic velocity contrast across the entire fault varies from 0% to >15% and depends on the angle of observation from the surface. This long continuous fault is capable of an M > 7.2 earthquake and likely initiated along older structures that predate the SJFZ. The observed geometries and velocity contrasts have implications for the direction, speed, distribution, and other key defining features of large SJFZ earthquakes. Key Points: Image the San Jacinto fault zone with direct body and fault zone head waves recorded by several dense arrays along the faultReveal a ∼72‐km long continuous bimaterial interface within the core fault zone with variable geometry and P‐ and S‐wave velocity contrastsEstimated bimaterial properties have important implications for earthquake rupture properties along the San Jacinto and other faults [ABSTRACT FROM AUTHOR]