The level of effort put into an SHA depends on the investment in the facility that might be lost and the consequences to society should it fail. Critical facilities are those that are deemed so important to the functioning of society or whose catastrophic failure will have such disastrous consequences that a maximum (and necessarily costly) effort to assess seismic and all other natural hazards is justified. The SSHAC project was born in the context of SHA for such critical facilities, nuclear power plants in particular. Even though SSHAC broadened its concept of the applicability of its recommended approach to SHA, its report is strongly influenced by this orientation toward very large, costly facilities for which the end goal is to prevent catastrophic failure, even at great expense.

Two general approaches to SHA have been developed and applied. The first approach uses discrete, single-valued events to arrive at scenario-like descriptions of the hazard. Typically, a seismic source location, a maximum earthquake associated with that source, and a ground motion attenuation relationship are specified. The ground motion at the site of interest implied by the chosen inputs is then calculated. The frequency of earthquake occurrence is usually not taken into account, and there is no formal and open way of treating uncertainties. This approach has been labeled deterministic seismic hazard analysis (DSHA) and has been used for many years in the design of power plants, large dams, and other critical facilities.

The other approach is probabilistic seismic hazard analysis (PSHA) and is the subject of the SSHAC effort. PSHA allows the use of multivalued or continuous events and models incorporating the effects and frequencies of all earthquakes that could impact a site. PSHA can easily incorporate model and parameter uncertainties. The results of a PSHA, including the uncertainties, can be represented as a series of curves (mean, median, or selected fractiles), showing the annual frequency of exceeding different levels of the chosen measure of ground motion. The intent of high-level PSHA is to capture and display as much as possible of the knowledge provided by existing data, theory, and computational simulations.

It should be noted that the procedures recommended by SSHAC for the elicitation and aggregation of expert opinion as input to PSHA are equally applicable for compiling the input for DSHA. The only essential difference between DSHA and PSHA is that the latter carries units of time while the former usually does not (Hanks and Cornell, 1994). In the case of a specific design situation, both DSHA and PSHA result in estimates of



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