Seismic Imaging Across the San Gregorio Fault Zone at Pescadero, California

The right-lateral San Gregorio-Hosgri Fault (SGHF) is a major strand of the San Andreas fault system that forms the boundary between the Pacific and North American plates. The SGHF extends mainly offshore for approximately 400 km along the central California coast, merges with the San Andreas Fault at Bolinas, California to the north, and terminates at the western Transverse Ranges near at Point Arguello to the south. Prior studies provide highly variable estimates of slip, which indicates uncertainty about the seismic hazard associated with the SGHF. To better understand the structure and geometry of the Frijoles Fault strand of SGHF, we conducted a 585-m-long, active-source, high-resolution (5-m shot and receiver spacing) seismic imaging survey across the fault where it is onshore at Pescadero, California. We recorded both P- and S-wave seismic data along the linear profile and used 2-D refraction tomography to develop VP, VS, VP/VS, and Poissons ratio tomographic models. Additionally, we used the Multichannel Analysis of Surface Waves (MASW) method to develop a second VS model. Our VP model, with maximum velocities of 4200 m/s at 60 m depth, shows a prominent low-velocity zone with velocities between 2600 and 2800 m/s slightly west of the mapped surface trace of the fault. We also observe a small increase in the depth of the 1500 m/s VP contour (groundwater) slightly west of the near-surface trace of the fault, suggesting the fault acts as a groundwater barrier. Our VS models show velocities range from about 120 to 500 m/s in the upper 30 m, with a lower velocity west of the surface trace of the fault. Based on our VP and VS models, we interpret a slightly west-dipping main fault and possibly a minor splay approximately 100 to 150 m to the east of the main surface fault trace. This faulting geometry suggest a component of down-to-the-west normal faulting associated with the dominantly strike-slip San Gregorio fault.