LIDAR MAPPING ILLUMINATES COMPLEXITY AND CONNECTIVITY OF EARTHQUAKE SOURCES: AN EXAMPLE FROM THE SAN ANDREAS PLATE BOUNDARY IN NORTHERN CALIFORNIA

Earthquakes in the past few decades have taught us that faults are more interconnected and spatially complex than anticipated from preexisting maps (for example, 1992 Landers, 2016 Kaikōura, and 2020 Monte Cristo Range earthquakes, to name a few). Recent advances in terrain-imaging technologies, though, are allowing for more detailed and informative mapping. Here we utilize high-resolution topography from a publicly available airborne lidar dataset to interpret geomorphic evidence of recent (Holocene) surface rupture on zones of faults that together form a structural connection between the Rodgers Creek and Maacama faults, overlapping major strands of the San Andreas plate boundary system north of San Francisco Bay. The 53-km-long study area targets the Bennett Valley and southernmost Maacama fault zones (BVFZ-sMFZ), within which latest Quaternary activity had been largely unrecognized. Careful inspection of slope and hillshade images enable recognition of topographic lineaments distributed along and across the entire zone (up to ~4-7 km in width) that are interpreted, with varying levels of confidence, as products of tectonic shear deformation. The most conspicuous lineaments are laterally continuous, cut across topography, and include fine-scale features commonly associated with active faults (such as scarps, sidehill benches, and linear troughs). These “high confidence” strands also bear some resemblance to historical ruptures; in particular, those of the M 6.0 2014 South Napa earthquake, which form a 2-km-wide array on (mostly previously unmapped) faults 15 km east of the study area. Within the BVFZ-sMFZ, the spatial configuration of mapped surface ruptures appears locally to reflect the influence of broader structural fabric, with some strands diverging from the overall zone and converging with potentially active faults beyond the study area. The complex, distributed nature of rupture demonstrates that no single throughgoing fault has been established, suggesting earthquake ruptures may be confined to sections of the zone –or that rupture of the contiguous major faults may nucleate or terminate here. As this study illustrates, improvements in mapping resolution enable a more complete picture of seismic sources in a region, with the promise of more realistic input to hazard assessments.