Fault Continuity and Rupture Branching of the West Napa Fault Resulting from the 2014 Mw 6.0 South Napa Earthquake, Inferred by Fault-Zone Trapped Waves

We use fault-zone trapped waves (FZTWs) to evaluate continuity between the West Napa (WNF) and Franklin (FF) faults and fault branching within the WNF zone in northern California. We evaluated FZTWs generated by 55 on-fault aftershocks of the 24 August 2014 Mw 6.0 South Napa earthquake and recorded on three recording arrays deployed across the resulting surface ruptures. Our 3D finite-difference simulations indicate the WNF zone consists of a 400- to 500-m-wide, 5- to 7-km-deep, low-velocity waveguide, with seismic velocities reduced by 40 to 50%. Post-S coda durations of the FZTWs increase with distance between the aftershocks and the recording arrays, demonstrating that the low-velocity waveguide extends southward to the FF, within which seismic velocities are reduced by 30-35%. FZTWs indicate that the combined WNF-FF zone is at least ~75 km long, but potential fields data indicate that the WNF extends to the Maacama fault, suggesting a WNF-FF zone that is at least 110 km long. The presence of multiple FZTWs, generated by two explosions detonated within the main surface rupture of the WNF and recorded by a 15-km-long seismic array across the fault zone, demonstrate that the faulting is highly distributed. Within 1.5 km of the main 2014 surface rupture, there are at least two subordinate fault traces that formed 3- to 6-km-long surface ruptures. Finite-difference modeling suggests these subordinate rupture zones form low-velocity waveguides and connect with the main rupture at ~2-3 km depth. FZTWs recorded at the Carneros Fault (CF), ~1-km west of the WNFZ, suggest that the CF connects with the WNFZ at shallow depths, even though there was not surface rupture on the CF during the 2014 event. We suggest that the continuous and widely distributed WNF-FF may pose significant regional hazards from amplification and extended ground shaking along the fault-zone waveguides, even if surface rupture is limited to only a segment of the overall fault zone.