17 A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara Valley, California: A Showcase of Urban Earth Science I

Saturday, 30 April 2005
7:55 AM - 12:00 PM

STRUCTURE AND POTENTIAL EARTHQUAKE SOURCES, SANTA CLARA VALLEY, CALIFORNIA

Paper number: 17-10
Program: THEME SESSIONS
Time: 10:40 AM – 11:00 AM

Authors

Abstract

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.