LIDAR MAPPING OF THE BENNETT VALLEY FAULT—DEFINING A CONNECTION BETWEEN MAJOR PLATE-BOUNDARY FAULTS IN NORTHERN CALIFORNIA

The Rodgers Creek and Maacama faults form a major strand of the San Andreas plate boundary system north of San Francisco, accommodating ~6-12 mm/yr of dextral shear, but how and where slip transfers between these subparallel, overlapping faults has remained unclear. The Bennett Valley fault (BVF) is the structural continuation of the Maacama fault southward, and geologic mapping shows that strands of the BVF approach to within 0.5 km of the Rodgers Creek fault. However, aside from a relatively short section at its north end, the BVF has not been recognized as active during the Holocene. In this study, we use lidar-derived high-resolution topography to search for fine-scale geomorphic evidence of recent faulting. The BVF traverses the flanks of Sonoma and Bennett Mountains, where possible expression of surface faulting may be obscured by vegetation. Through careful inspection of topographic slope- and hillshade images, we have identified a broadly distributed network of fault strands, several of which appear to connect directly to the Rodgers Creek fault. At its north end, the BVF connects to a section of the Maacama fault designated as late Quaternary in age; in this area, we have mapped a similarly distributed array of subtle fault traces that link up with more clearly expressed Holocene traces of the Maacama fault to the north.

The finding of an active-fault link between the Rodgers Creek and Maacama faults identifies an avenue of slip-rate transfer—and the potential for complex earthquake-rupture pathways— between these major neighboring faults. Our mapping also highlights the significance of distributed zones of surface faulting, which have mostly escaped detection, but which may be identifiable with high-resolution imaging such as provided by airborne lidar. Distributed fault zones have recently been host to significant earthquakes in the Western U.S. (in particular, 2014 Napa, 2019 Ridgecrest, and 2020 Monte Cristo Range) and thus have gained attention, but because such faults are not well mapped and studied, their contribution to hazard is poorly constrained and may be underappreciated.