Seismic Risk Studies for San Francisco and for the Greater San Francisco Bay Area

摘要

Three aspects of seismic risk for the greater San Francisco Bay Area are considered here: First, an evaluation of the overall properties of parameters characterizing the seismicity of the San Francisco region; secondly, an evaluation of methods for computing seismic hazard at a site; and finally, an evaluation of seismic risk in terms of population exposure. For the first item, available data concerning (1) geotectonic evolution during the last twenty million years, (2) historical seismicity of the Bay Area, and (3) characterization of earthquake mechanisms, of propagation of seismic waves and of geological features to obtain a four-dimensional space-time-energy-source continuum model, are studied thoroughly. Then, for the second item, a review is given of mathematical modelling proposed by different authors to obtain probability distribution functions for the site parameters. Distributions of peak ground motion parameters, such as acceleration, velocity and displacement as well as duration, are obtained for a point-source and for a line-source model using either an experimental method or an analytical method. Emphasis is given to a two-parameter source model (magnitude and stress drop), to the direct development of seismic hazard in terms of response spectra, and to a joint probability distribution function of duration and one peak ground motion parameter. The influence of some uncertainties on the final probability distributions is analyzed. Finally, overall seismic risk for the Bay Area is briefly characterized by including the interaction between seismic action and the geographic location of population. Spatial correlation with earthquake action is taken into consideration in developing a probability distribution for the number of people affected by a given level of seismic acceleration.