of objective evaluators and integrators. The extent to which this goal can reasonably be pursued in a particular case should depend on the scope and importance of the project and the resources available to support the study.

The second principle is important because it assigns to an identified entity, the “owner,” clear intellectual or scientific responsibility for the conduct and results of a PSHA. This does not necessarily mean that the “owner” agrees with every particular input or result but that the owner feels confident that the PSHA has fulfilled the purpose of representing the larger technical community and can be defended in scientific and regulatory arenas, as necessary. These principles underlie the primary recommendations of the SSHAC report that deal with the PSHA process.

SSHAC recognizes that a PSHA can be carried out at different levels of effort and emphasizes that the effort expended should match the importance of the facility, the degree of controversy, uncertainty, and complexity associated with the relevant scientific issues, and external decision factors, such as regulatory concerns and the resources available. This is shown in Table 2.1, taken from Chapter 3 of the SSHAC report.

Four levels of study are defined, the first three of which rely on a single entity called the technical integrator (TI), who is responsible for all aspects of the PSHA, including specifying the input. Although experts may be involved on a consulting basis, there is no formal elicitation of their views. The highest level of study (level 4) makes use of formally elicited expert judgment. As such, a new entity called the technical facilitator/integrator (TFI) is needed. The role of the TFI is discussed below. A large part of the SSHAC report is devoted to defining what is necessary to carry out a level 4 study and explaining the function of the TFI because the ideas are new, not because this level of effort is required for every seismic hazard assessment. It would be inappropriate to infer that all PSHAs require the considerable resources needed to carry out the level 4 PSHA described by SSHAC. ²