S-wave velocity measurement and the effect of basin geometry on site response, east San Francisco Bay area, California, USA

We measured S-wave velocity profiles at eleven sites in the east San Francisco Bay area using surface wave methods. Data acquisition included multichannel analysis of surface waves using an active source (MASW), a passive surface-wave method using a linear array of geophones (Linear-MAM), and a two-station spatial autocorrelation method (2ST-SPAC) using long-period accelerometers. Maximum distance between stations ranged from several hundred meters to several kilometers, depending on the site. Minimum frequency ranged from 0.2 to 2 Hz, depending on the site, corresponding to maximum wavelengths of 10 to 1 km. Phase velocities obtained from three methods were combined into a single dispersion curve for each site. A nonlinear inversion was used to estimate S-wave velocity profiles to a depth of 200-2000 m, depending on the site. Resultant S-wave velocity profiles show significant differences among the sites. On the west side of the Hayward fault and the east side of the Calaveras fault, there is a low velocity layer at the surface, with S-wave velocity less than 700 m/s, to a depth of approximately 100 m. A thick intermediate velocity layer with S-wave velocity ranging from 700 to 1500 m/s lies beneath the low velocity layer. Bedrock with S-wave velocity greater than 1500 m/s was measured at depths greater than approximately 1700 m. Between the Hayward Fault and the Calaveras Fault, thicknesses of the low velocity layer and the intermediate velocity layer are less than 50 m and 200 m respectively, and depth to bedrock is less than 250 m. To evaluate the effect of a lateral change in bedrock depth on surface ground motion due to an earthquake, a representative S-wave velocity cross section perpendicular to the Hayward fault was constructed and theoretical amplification was calculated using a viscoelastic finite-difference method. Calculation results show that the low frequency (0.5-5 Hz) component of ground motion is locally amplified on the west side of the Hayward fault because of the effect of two-dimensional structure.