Structure and Velocities of the Northeastern Santa Cruz Mountains and the Western Santa Clara Valley, California, from the SCSI-LR Seismic Survey

Introduction: The Santa Clara Valley is located in the southern San Francisco Bay area of California and generally includes the area south of the San Francisco Bay between the Santa Cruz Mountains on the southwest and the Diablo Ranges on the northeast. The area has a population of approximately 1.7 million including the city of San Jose, numerous smaller cities, and much of the high-technology manufacturing and research area commonly referred to as the Silicon Valley. Major active strands of the San Andreas Fault system bound the Santa Clara Valley, including the San Andreas fault to the southwest and the Hayward and Calaveras faults to the northeast; related faults likely underlie the alluvium of the valley. This report focuses on subsurface structures of the western Santa Clara Valley and the northeastern Santa Cruz Mountains and their potential effects on earthquake hazards and ground-water resource management in the area. Earthquake hazards and ground-water resources in the Santa Clara Valley are important considerations to California and the Nation because of the valley's preeminence as a major technical and industrial center, proximity to major earthquakes faults, and large population.

To assess the earthquake hazards of the Santa Clara Valley better, the U.S. Geological Survey (USGS) has undertaken a program to evaluate potential earthquake sources and potential effects of strong ground shaking within the valley. As part of that program, and to better assess water resources of the valley, the USGS and the Santa Clara Valley Water District (SCVWD) began conducting collaborative studies to characterize the faults, stratigraphy, and structures beneath the alluvial cover of the Santa Clara Valley in the year 2000. Such geologic features are important to both agencies because they directly influence the availability and management of groundwater resources in the valley, and they affect the severity and distribution of strong shaking from local or regional earthquakes sources. As one component of these joint studies, the U. S. Geological Survey acquired more than 28 km of combined seismic reflection/refraction data from the Santa Cruz Mountains to the central Santa Clara Valley in December 2000. The seismic investigation included both high-resolution (~5-m shot and sensor spacing) and relatively lower-resolution (~50-m sensor) seismic surveys from the central Santa Cruz Mountains to the central part of the valley. Collectively, we refer to these seismic investigations as the 2000 western Santa Clara Seismic Investigations (SCSI).