classification of seismic source types (Section 4.2) is nonunique, and the categories described in the report are admitted to be arbitrary. Nevertheless, they provide a useful framework for discussion and guidance on methodology.

The practitioner experienced in PSHA will have no trouble understanding SSHAC's Chapter 4. However, the nonpractioner scientist may be confused by the subtleties between differing concepts of a “seismic source ” presented in chapters 4 and 5. Chapter 4 describes a seismic source as a geologic structure or as a domain within which the spatial and temporal occurrences of earthquakes are approximately uniformly distributed.Chapter 5, on ground motion, describes seismic source basically as a dynamic excitation in the earth that causes ground motion at the surface.

Readers of the SSHAC report should be aware that two different terms, upper-bound and maximum magnitude, and two symbols, mu and Mmax, are used Section 2.1 and in Chapter 4 to denote the largest-magnitude earthquake that a particular seismic source is capable of producing. This magnitude is the upper bound of the frequency of occurrence magnitude curve used in the analysis. A value for this parameter must be specified in order to carry out the integration over all relevant magnitudes when calculating seismic hazard. The problems encountered and conventional procedures used in the selection of Mmax (mu) and the specification of the substantial epistemic uncertainty often associated with it are discussed in Sections 4.2.2 and 4.3.2 of the SSHAC report.

If one accepts the basic formalism of uncertainty analysis presented in Section 2.2 of the SSHAC report, the approaches for characterizing uncertainties in SSC (Section 4.3) will seem logically consistent and well established in practice. Similarly, the guidance described in Section 4.4 for the expert elicitation process follows one's acceptance of the decision science methodology laid out in Chapter 3.

A notable gap in Chapter 4 of the SSHAC report is the absence of discussion on and guidance for earthquake catalogs. In Section 4.4 the technical facilitator/integrator (TFI) or the technical integrator (TI) is given responsibility for providing a comprehensive and uniform data base to the experts for use in the PSHA. The only guidance given, under the subheading “Area Sources” in Section 4.2.3, is the recommendation that “seismicity catalogs should be reviewed for uniformity in designation of magnitudes and for completeness as a function of magnitude, location, and



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