STRUCTURE AND POTENTIAL EARTHQUAKE SOURCES, SANTA CLARA VALLEY, CALIFORNIA

Multiple seismic imaging surveys across and within the Santa Clara Valley (SCV) provide detailed information on the structure, composition, hazards, and ground-water resources of the SCV. Two separate ~20-km-long, combined seismic reflection and refraction profiles extend from the San Andreas fault , across the SCV, to the Calaveras fault . These profiles were acquired with 50-m seismograph and ~1-km shot spacing, yielding the most detailed compressional velocities (Vp) and reflection data across the valley to date. We processed the data from these profiles as both seismic-velocity and seismic-reflection images. The images show that the SCV is flanked by deep sedimentary basins (Vp < 4 km/s) on its southwest (Cupertino Basin) and northeast (Evergreen Basin) ends, with a topographic high in Franciscan basement (Vp~4 km/s) near the center of the valley. The seismic images suggest that the sediment thickness in the Cupertino and Evergreen basins is about 1.6-1.8 km deep and 2.5-3.0 km deep, respectively, where the seismic profiles cross them. High-velocity basement rocks (Vp > 6.4 km/s) underlie much of the area at about 5 km depth or less, suggesting that Franciscan rocks are less than 5 km thick. We also acquired an ~10-km-long, high-resolution (5-m shot and sensor spacing) reflection and refraction profile that crosses the southwestern half of the valley. The high-resolution images show evidence for numerous faults within the Santa Cruz Mountains and beneath the western SCV. Offset post-Miocene strata, including layers of the ground-water aquifer system, and seismicity recorded over the past 30 years suggest that some of the faults buried beneath the alluvium of the SCV are currently active. The high-population density within the valley and the proximity of such buried faults makes the earthquake hazard in the western SCV potentially high. This hazard is compounded by the thick sequence (~1.6 km) of low-velocity (< 4 km/s) sediments within the Cupertino Basin that may amplify seismic waves generated by movement on regional and/or local faults. The high-resolution images, combined with borehole data, show primary ground-water aquifers gently dip and thicken from the central Cupertino Basin toward the center of the SCV.