TECTONIC GEOMORPHIC AND QUATERNARY GEOLOGIC INVESTIGATION OF THE QUIÉN SABE FAULT, SANTA CLARA AND SAN BENITO COUNTIES, CALIFORNIA

The Quién Sabe fault (QSF) is a historically active fault in the southern San Francisco Bay Region (SFBR) and California and is part of a complex intersection within the San Andreas (SAF) and Calaveras faults (CF). Despite its inclusion in statewide fault hazard models (UCERF3), the QSF has not been the focus of research for over 40 years, the paleoseismic history of the fault is entirely unknown, and it’s role as a transfer structure is purely speculative. Thus, all fault parameters remain poorly constrained or incompletely defined, such as slip rate, sense of slip, fault length and width, event history, displacement per event, recurrence, and potential structural linkages.

The purpose of this ongoing research of the QSF is to: (1) evaluate and refine the location of it’s many fault strands; (2) constrain fault length and width and evaluate potential connections with adjacent active faults; and (3) evaluate fault activity and better constrain poorly understood fault parameters via paleoseismic investigation. The first phase of this study utilized new LiDAR data, historical aerial photos, and other geospatial resources, to produce 1:6,000-scale maps of the QSF and the areal Quaternary geology within a 160 km² mapping area, verified through extensive field reconnaissance.

We interpret the QSF to consist of multiple potentially Holocene-active fault strands that support (1) a revised width of 1.8 km and length of 29 km for the QSF, and (2) potential linkages to the northwest with the Central CF and to the south with the Paicenes fault via the Tres Pinos fault. These postulated linkages may support the QSF as an alternative or additional pathway to the Southern CF for the northward transfer of slip. Additionally, the revised QSF map provides a highly resolved geologic framework to investigate the most active strands of the QSF via paleoseismic trenching. This study has identified multiple high-quality paleoseismic sites that are the focus of ongoing work funded by the USGS.