Summary
We measured shear-wave velocity (Vs) profiles at five sites in the Hayward, East San Francisco Bay Area using surface wave methods. Data acquisition included multichannel analysis of surface waves using an active source, a passive surface-wave method using geophones in a linear array, and a two-station spatial autocorrelation method using long-period accelerometers. A nonlinear inversion was used to estimate Vs profiles from the surface to a depth of 400 to 800 m.. Resultant Vs profiles show significant differences between sites. To evaluate the effect of a lateral change in bedrock depth on surface ground motion due to an earthquake, a representative 2-D shear-wave velocity model oriented perpendicular to the Hayward Fault was constructed and theoretical amplification was calculated using a viscoelastic finite-difference (FD) method. The calculated response shows that the low frequency (~1 Hz) component of ground motion is locally amplified on the west side of the Hayward Fault due to the effect of 2-D structure. A twelve-story building at Hayward was demolished by implosion. The resulting ground motion was recorded to confirm the velocity model and its effect on amplification. Observed ground motion is generally consistent with the amplification predicted by FD simulation using the Vs model constructed in this study.
