Simple Site-Dependent Ground Motion Parameters for the San Francisco Bay Region

The amplification of earthquake ground motion is one of the most difficult parameters to quantify. The difficulties stem from (1) the lack of sufficient data on local soil parameters, (2) lack of sufficient strong motion data at different soil surface locations, (3) lack of strong ground motion data at various depths from the ground surface at the same location, (4) our inability to quantify the nonlinear characteristics of soils and (5) the use of approximate models to represent the nonlinear behavior of soils when subjected to dynamic forces.

In this report we present soil amplification factors for peak horizontal acceleration and velocity for earthquakes in the San Francisco Bay region. The method of Sugito (1986) was modified to reflect availability of data for this region. Primarily peak ground acceleration and velocity amplification parameters were evaluated. The ground motion amplification relationships are given in the following simple form: As =βaAr for peak ground acceleration and Vs =βvVr for peak ground velocity. The amplification factors β are functions of the depth to bedrock dp, the ratio, St, of surface shear wave velocity and a standardized shear wave velocity, and the respective peak ground motion parameters.

The project was accomplished through the following tasks:

*The strong ground motion data from the October 17, 1989 were used for estimating the . .amplification factors. In particular, initial analysis using data from 24 strong ground motion stations were used to determine the amplification from rock sites to soil sites. Subsequently, the analysis was repeated with strong ground motion for 52 sites. The ground motions were modified for distance from the rupture zone and for azimuthal direction. The corrections for distance was based on the Joyner and Boore (1988) attenuation equation scaled to reflect the higher accelerations recorded during the Loma Prieta earthquake.

* An extensive search was conducted to obtain soil parameter data at the locations of the strong ground motion stations. The parameters sought included variations of shear wave velocity or blow count with depth, depth to bedrock, and variations of density with depth. Initially data was obtained for 16 strong motion recording stations. Two months prior to the end of this project additional soil parameter data were made available by USGS for all the strong ground motion stations. Thus, the soil amplification analysis was repeated with more stations in the data

*A sensitivity analysis was performed to determine the variability of the amplification parameters β to changes in the St and dp parameters. The amplification factors appear to be more sensitive to variations in depth to bedrock than to variations in the St parameter.

Analysis of the data shows nonlinearity in soil amplification. This nonlinearity is reflected in the soil amplification parameters β. Some of the nonlinearity may be attributed to resonance between the input motion and the soil deposits in the San Francisco Bay region. In order to determine the primary sources for the nonlinear behavior observed in this study a more detailed investigation is needed. Such an investigation, however, is beyond the scope of this study.

The parameters developed in this study can be used in development of microzonation maps, for regional damage estimation, for rehabilitation decisions and resource allocation purposes.