SANTA CLARA VALLEY 3D GROUND MOTION STUDIES

Ground motion prediction for seismically at-risk urban areas is an important objective of the USGS. To advance this objective, the USGS has conducted ground motion studies for the past several years in the Santa Clara Valley. This work is directed toward constructing a 3D P- and S-wave velocity model capable of predicting strong-ground motions for future postulated earthquakes to 1 Hz. During the course of this study we investigate the relationship between observed ground motion and structural features such as faults, sedimentary basins, and areas of lower-rigidity fill. Velocity information is collected from a variety of sources, including reflection/refraction, boreholes, travel time picks, and active and passive methods based on the measurement of surface wave phase velocity curves. Ground motion records are collected from a 41-element portable array, recording local and regional earthquakes, including the 2004, M6 Parkfield earthquake. These records are used to investigate site response and wave propagation characteristics across the valley. The above information is combined with a structural model, produced by colleagues at the USGS, including the depth to basement and fault locations, to obtain a velocity model that is consistent with all the available data. The velocity model is then adjusted and validated by 3D finite-difference modeling of waveforms from local and regional earthquakes. Frequency-wavenumber analysis has shown the importance of surface-waves generated at the edges of the Santa Clara Valley in the composition of valley ground motion records. The largest site amplifications at frequencies of 1 Hz and lower lie over the western side of the Evergreen Basin. This basin is bounded on the west by the Silver Creek fault, which is also the boundary of large amplification. To the east, the high amplification does not extend over the deepest part of the basin, and may be diminished by thrusting of a wedge of Great Valley sequence or other higher-velocity strata into the Evergreen Basin. Site amplification in the Cupertino Basin is low to moderate compared with values for the Evergreen Basin. At frequencies above 1 Hz, the shallow basement region between the two basins is observed to have high amplification.