UNCERTAINTY AND CONSERVATISM IN UK SEISMIC HAZARD ASSESSMENT

Uncertainty and conservatism are words as common in the vocabulary of engineering seismic hazard assessment as seismology and geology, yet r arely are they defined and explained in a coherent manner. Often, comm ents on uncertainty and conservatism are left as technical footnotes f or the cognoscenti. With the conviction that principles should be open ly expounded if they are to gain broad acceptance, a review is present ed of the role of uncertainty and conservatism in UK seismic hazard as sessment. A rigorous mathematical framework exists for treating uncert ainty, based on logic-tree formalism which requires that alternative s cenarios are enumerated and weighted according to relative likelihood. The way in which some of the most critical individual seismological a nd geological parameters are considered in constructing seismic source models is discussed, and attention is drawn to conservative decisions on weighting. With respect to ground motion parameterization, a brief review is presented of the various empirical procedures which underli e the seismic ground motion criteria which have been produced for a nu mber of British sites, and the merits of alternative approaches are ev aluated from the perspective of uncertainty and conservatism.