Integrated surface and borehole strong-motion, soil-response arrays in San Francisco, California

An integrated set of four borehole arrays and ten surface installations is installed in the city of San Francisco, California to measure the response of soft-soil deposits to strong earthquake ground motions. The borehole arrays extend through thick layers of softwater-saturated soils of Holocene age and older more consolidated soils of Pleistocene age into bedrock at depths up to 100 m. The surface installations are configured in pairs to provide simultaneous comparative surface measurements of soft soils and nearby rock. The rock locations also permit comparative measurements of rock as observed at the surface and in nearby boreholes. Complementary structural response arrays alsoare installed near each array.

The arrays are designed to address a wide variety of scientific and engineering issues, and especially the issue of anelastic and nonlinear soil response at high strain levels. Exact anelastic models have been developed to account for contrasts in anelastic properties at boundaries and the resultant inhomogeneity of propagating wave fields. These models predict that significant amounts of energy may be trapped in soil basins with resultant larger amplifications than can be predicted using conventional homogeneous wave-field models with damping. Results of these models are discussed.