Distributed Faulting of the Northern West Napa Fault Zone in Napa Valley, California

Abstract Mapped surface ruptures from the 24 August 2014 Mw 6.0 South Napa earthquake in the Napa Valley, California, show a 2-km-wide zone of distributed faulting in the southern and central West Napa fault zone (WNFZ). In the northern WNFZ at Hendry Winery (HW), however, the mapped 2014 surface ruptures encompass an ∼100-m-wide zone, implying significant narrowing of the near-surface fault zone to the north. We present a tomographic shear-wave velocity (VS) model and guided-wave data that indicate the northern WNFZ is at least 400-m wide, with multiple near-surface fault traces. Our VS model shows that the 2014 surface ruptures are underlain by discrete low-velocity zones (LVZs), and coincident guided-wave data show that the LVZs carry fault-zone guided waves. If nearby (