Near-surface structure of the San Andreas fault, San Francisco Peninsula segment

Abstract

High-resolution seismic images were developed using both seismic refraction and reflection profiling from coincident P- and S-wave seismic data acquired near Woodside, California, in June 2012. A 60-m-long seismic profile was approximately centered on the 1906 surface rupture of the Peninsula segment of the San Andreas Fault (SAF). The data were acquired and processed with methods previously used by the U.S. Geological Survey in other successful studies. The seismic images suggest the presence of possibly three near-surface fault traces within about 25 m of the main 1906 surface rupture. A Pwave, high-velocity zone relates to a significant fault trace observed southwest of the main 1906 fault trace that does not appear to break the surface. This trace may be associated with long-term movement prior to the main 1906 break, and the fault traces observed may merge at depth. The reflection image displays strong, near-vertical diffractions, particularly beneath the P-wave, high-velocity zone, consistent with fault traces inferred from the seismic images. A borehole augered into this P-wave, highvelocity zone revealed bright blue clay, possibly originating from weathered serpentinite. Because this study found evidence for multiple fault traces in the SAF zone, it is possible that slip histories for the Peninsula segment of the SAF may have been miscalculated. Further work in geophysics and especially paleoseismology is needed to better constrain the geometry and slip history of the Peninsula segment of the SAF.