potential users should be aware. Only certain key points are highlighted here in the summary; the rest are included in later chapters.

MAIN FEATURES OF THE SSHAC REPORT

As stated above, the SSHAC report focuses on procedures for using experts in probabilistic seismic hazard analysis and for determining uncertainties at key stages of the analysis process. In its treatment of the use of expert opinion, SSHAC outlines four possible levels of effort and complexity. But the SSHAC report is strongly flavored by emphasis on hazard analysis for nuclear and other critical facilities, and SSHAC therefore discusses at great length its highest-level (level 4) procedure for evaluating expert opinion. And although SSHAC includes proper disclaimers the unwary reader could gain the incorrect impression that the high-level (level 4) PSHA procedure is needed for every hazard analysis.

The panel agrees that all PSHA projects should share the same basic principles and goals, but that the elaborate level 4 methodology is not required for every PSHA study. SSHAC does indeed recognize that alternate simpler methods are probably adequate for less critical facilities, but the simpler methods are not discussed in detail and the reader is not fully advised about other sources of information. Adequate disclaimers in the SSHAC report should protect the analyst who chooses to use procedures other than those recommended by SSHAC from the need to defend that decision in a regulatory setting.

THE SSHAC METHODOLOGY

SSHAC's contributions to PSHA methodology include the testing and full explication of the technical facilitator/integrator (TFI) entity, which is the essential ingredient in implementing SSHAC's high-level (level 4) analysis.1 The TFI approach was found to be very effective in two workshops on ground motion estimation and led to an unexpected degree of agreement among the experts consulted, who began with many diverse viewpoints. The panel notes that TFI elicitation procedure is not

1  

For a description of the TFI entity, see Chapter 2.



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