Near-Surface Seismic Images and Geometry of the San Andreas Fault, Santa Cruz Mountains, California

In July 2005, the US Geological Survey and the University of Nevada-Las Vegas acquired a 305-m-long, high-resolution, combined seismic reflection and refraction profile across the San Andreas fault within the Los Trancos Open Space Preserve in Palo Alto, California. The objective of the seismic investigation was to determine the geometry and seismic velocities of the San Andreas fault along a part of the surface rupture area of the 1906 M 7.9 San Francisco earthquake. Seismic sources (sledge hammer blows) and geophones (40-Hz single-element verticals) were co-located (1-m lateral offsets) and were spaced at 5-m increments along the profile. The data were recorded with two 60-channel seismographs without acquisition filters. From the resulting seismic data, we developed tomographic P-wave velocity models of the upper 80 m and stacked and migrated reflection images of the upper few hundred meters along the profile. Seismic P-wave velocities range from about 800 m/s to 4500 m/s in the upper 80 m. For rocks with velocities greater than 2500 m/s, the San Andreas fault is expressed as a well-defined low-velocity zone that is about 60 to 80 m wide. In stacked reflection images, the near-surface (upper 100 m) San Andreas fault zone includes multiple vertically offset reflectors with varying dips. On the basis of offset reflectors and lateral variations in the velocity structure, we suggest that the active fault zone is wider than previously indicated by some geologic maps. Surface ruptures from earthquakes prior to the 1906 rupture have likely involved some or all of the imaged near-surface splays. Other more populated areas along the San Andreas fault probably have similar near-surface geometries, and land-use planning in those areas should assume similar complex geometries for the San Andreas fault.