INVESTIGATION OF THE WEST NAPA FAULT AT EHLERS LANE, NORTH OF SAINT HELENA, CALIFORNIA

The 2014 M 6.0 South Napa earthquake illustrated the need for further investigation of the little-studied West Napa Fault. That event ruptured ~15 km of the ~45-km-long fault zone shown in the USGS Quaternary Faults Database. Based on lidar topography and field examination, we propose additional extensions of the fault zone to the north and south, for a total length of ~70 km. We performed geophysical surveys and excavated two trenches across one strand of our mapped northward continuation of the fault zone in northernmost Napa Valley. While Vp/Vs and guided wave observations suggested a fault near the base of a prominent topographic scarp, our excavations did not reveal any faults within 3–5 m of the ground surface. In the trench exposures we observed three generations of alluvial sedimentary deposits, the oldest (likely ~1 Ma) tilted ~30 degrees west, the next oldest (10–20 ka) potentially tilted a few degrees west, and the youngest (< 10 ka) not visibly deformed. Age control comes from a combination of obsidian hydration, optically stimulated luminescence, and radiocarbon dating. We deduce that a blind or buried fault (with likely oblique thrust/right-lateral slip) is responsible for the observed tilting and uplift, with at least 2 m of local uplift accumulated during the Holocene. Our observations imply that earthquake hazards associated with the West Napa Fault extend farther northward and southward than previously thought, and the WNF may be capable of producing earthquakes significantly larger than the 2014 event. However, the recurrence interval of ruptures on this fault zone is likely in the thousands of years based on the ages of the uplifted sediments and overall subtle geomorphic expression of the fault.