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Subsurface Structure of the Santa Rosa Plain, California, from High-Resolution Seismic-Reflection Data

Principal Investigator: Robert Williams, US Geological Survey (USGS)

USGS in collaboration with nees@UTexas will collect 13-km of high-resolution seismic-reflection data in two profiles on the Santa Rosa Plain to image basin structure and stratigraphy in this seismically-active area and provide key constraints for earthquake hazard assessment products. In particular, the seismic-reflection data will constrain the geometry and depth extent of the Trenton Ridge, the northeastern corner of the Cotati Basin, and the position of the Rodgers Creek fault in the urban area of Santa Rosa.

The Trenton Ridge is a completely concealed basement high that may contain an active fault and may partition the hydrologic response of the Santa Rosa Plain basin fill. This feature has been mapped using gravity data, which are excellent at defining the shape, but not the exact depth of the basement ridge. The seismic-reflection data will (1) constrain how close to the ground surface this impermeable basement ridge extends, (2) evaluate whether the basement ridge folds or truncates sedimentary layers (aquifers) that lie above the ridge, and (3) test whether the Trenton thrust fault extends east across the Plain.

Santa Rosa, California, experienced unexpectedly high building damage from the 1906 San Francisco earthquake and the 1969 Santa Rosa earthquake sequence (M5.6 and M5.7). Earthquake simulations of the 1906 earthquake also show higher and extended ground motions for the Santa Rosa area that correlate with reported higher 1906 Mercalli intensities (Boatwright et al., 2006; McPhee et al., 2007). The subsurface imaging research in the Santa Rosa area will lead to a better understanding of the underlying basin structure and the higher ground motions observed there. The imaging conducted in this project will better constrain geologic models and lead to improved ground motion simulations and earthquake hazard estimates. This research complements previous studies of the Santa Rosa area using gravity, magnetic, computer earthquake simulations, portable seismograph deployments, and borehole/velocity studies conducted over the past 3 years. Indeed, the proposed profile location will be constrained by the measurements made in these previous studies.

Supplementary Materials