Improved Mapping of Active Faults Near Recent California Earthquakes: Napa Valley and Ridgecrest Region

Belle Philibosian, Jessica Thompson Jobe, Colin Chupik, & Kiara Broudy

Published September 10, 2023, SCEC Contribution #12990, 2023 SCEC Annual Meeting Poster #069

Several recent significant earthquakes in California occurred along relatively slow-slipping faults that had been little-studied prior to those events. These faults likely have earthquake recurrence intervals in the thousands of years and are thus far less frequently active than the major faults of California. Nevertheless, the wide distribution of slow-slipping faults such as these makes them collectively a significant contributor to regional seismic hazard.

The West Napa Fault (WNF), which generated the 2014 South Napa earthquake, was thought to be ~45 km long based on aerial photography and limited field investigations (but no lidar data). The previously mapped fault zone covered the 2014 rupture area, but missed subtle geomorphic features associated with some strands. We use airborne lidar topography and field observations to refine and expand the fault mapping, revealing a previously unrecognized northern section and a likely southward continuation, for a total fault length of ~70 km. Thus, the WNF may be capable of producing earthquakes significantly larger than the 2014 event.

The area surrounding the 2019 Ridgecrest earthquakes had never been comprehensively assessed for active faulting; prior to the 2019 events, that region of the Quaternary Fault and Fold Database (QFFD) had been assembled piecemeal from a variety of sources at various scales. Only ~35% of the 2019 rupture occurred on previously mapped faults. We use high-resolution satellite imagery and derived topography along with field observations to produce a more complete fault map, documenting a distributed network of NW-trending dextral and NE-trending sinistral faults, not all of which ruptured in 2019. We estimate that 50–70% of the 2019 surface ruptures could have been recognized as active faults with detailed inspection of pre-event data.

Whereas it is unlikely that the recently ruptured fault sections will rupture again soon, both fault zones contain additional un-ruptured sections and strands that may have been brought closer to failure by the nearby earthquakes. We intend that our improved mapping be used to update the QFFD and other hazard assessment tools. Furthermore, our observations serve to motivate more detailed study of other relatively slow-slipping faults that nevertheless present significant seismic hazard.

Key Words
Ridgecrest, Napa, fault mapping

Citation
Philibosian, B., Thompson Jobe, J., Chupik, C., & Broudy, K. (2023, 09). Improved Mapping of Active Faults Near Recent California Earthquakes: Napa Valley and Ridgecrest Region. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology