LOCATIONS OF THE SERRA AND RELATED FAULTS ALONG THE SAN FRANCISCO PENINSULA IN THE CITY OF SAN CARLOS FROM MULTI-METHOD, ACTIVE-SOURCE SEISMIC IMAGING

Quaternary deformation along the San Francisco (SF) Peninsula, east of the San Andreas fault (SAF), is concentrated along the Foothills thrust fault system, a principal member of which is the Serra fault. The Serra fault is a Holocene-active, multi-strand, reverse fault that trends subparallel to (within 5 km of) the SAF along the SF Peninsula between Daly City and Millbrae, California. The Serra fault is a significant range-building structure of the Santa Cruz Mountains that likely roots into the SAF. The length of the Serra and related faults, which affects the maximum magnitude earthquakes that can be generated, is unknown, as fault locations have been obscured in many places by urbanization. To assess the existence, location, and structure of the Serra and related faults of the Foothill thrust system in the City of San Carlos, we conducted a high-resolution (5 m shot and sensor spacing) seismic imaging campaign. Because of the complexity of the geology, which makes standard reflection imaging exceedingly difficult, we used a multi-method approach that includes (a) tomographic VIP, Vs, VIP/Vs, and Poisson’s ratios, (b) the Multi-Channel Analysis of Surface Waves (MASW) method, and (c) reflection imaging. These combined methods have proven highly effective in locating faults in other complex geologic settings, including other parts of the SF Peninsula. We located several structures of the Foothills thrust fault system, including possible traces of the Serra fault. We can use these fault locations to plan future guided-wave studies to better assess the length and connectivity of structures both north and south of the study area. These results, combined with other geologic studies, will help to better assess the location, length, slip history, and seismic hazards associated with the Foothills thrust fault system.