A model for seismic risk analysis consistent with existing theories of earthquake mechanism and characteristics is developed. This model is based on the assumption that an earthquake originates as an intermittent series of fault ruptures in the Earth's crust, and that the intensity of motion at a site is mainly contributed by the segment of the ruptured fault that is closest to the site. Since active faults in a region may be well-defined, partially defined, or completely unknown, various idealized source models are introduced in order to permit the modeling of all conceivable seismic sources.
The significance of model parameters on the calculated seismic risk is studied: certain previous conclusions in this regard are critically reexamined. In particular, it is pointed out that previous seismic risk models, which implicitly assume that the energy is radiated from a point (the focus), can seriously underestimate the real risk, especially for high intensity motions.
As an illustration of the model, the seismic risk analysis of a site in downtown San Francisco is presented.