High-resolution seismic images and seismic velocities of the San Andreas fault zone at Burro Flats, Southern California

To better understand the structure of the San Andreas fault (SAF) at Burro Flats in southern California, we acquired a three-dimensional combined set of seismic reflection and refraction profiles centered on the main active trace at Burro Flats. In this article, we discuss the variation in shallow-depth velocities along each seismic profile, with special emphasis on the 1500 m/sec P-wave velocity contour, which can be an indicator of shallow-depth water-saturated unconsolidated sediments. Along the four seismic profiles, minimum depths of the groundwater table, as inferred from 1500 m/sec velocity contour, range from 10 to about 20 m. The largest variations in depth to the top of the groundwater table occur in areas near mapped faults, suggesting that the groundwater flow in Burro Flats is strongly affected by the locations of fault traces. We also used the seismic data to develop seismic reflection images that show multiple strands of the SAF in the upper 60 m. Reflectors above the 10 m depth probably correspond to Holocene alluvial deposits; reflectors below the 15 m depth probably arise from velocity or density variations within the Precambrian gneiss complex, likely due to weathering. Apparent vertical offsets of reflectors are observed along profiles (lines 1 and 2) that are normal to the SAF, indicating minor apparent vertical offsets on the SAF at shallow depths. Along line 2, the apparently vertically offset reflectors correlate with zones of relatively low P-wave velocity. Along the central part of lines 1 and 2, the faults form a flower structure, which is typical of strike-slip faults such as the SAF.