Probability-based determination of design earthquakes

Controlling earthquakes, which have been developed by the USNRC (1995), are based on a specified risk, namely, the annual exceedance probability of the maximum earthquake ground motion in a probabilistic seismic hazard curve assessed in particular nuclear power plant sites. This idea is currently used to determine Safe Shutdown Earthquake (SSE) not only for future nuclear plants but also for qualification of existing plants in the US. And, this concept has a great impact on seismic design provisions in other earthquake-prone countries like Japan since it is, in general, not an easy task to determine design earthquakes in a rational way. Although the controlling earthquakes are based on the deaggregation of a specified point on a seismic hazard curve, the earthquakes thus determined have not been fully understood in terms of both their character and physical interpretation. Furthermore, in the course of determination process, one or two combinations of earthquake magnitudes and distances are taken in the USNRC procedure, which have maximum conditional probability density corresponding to the specified annual exceedance probability, say, 10(-5). In lie of the structural reliability theory, there exist possible ways to determine design values that are probabilistically selected from a set of random quantities. First-order reliability analyses (FORM) has been developed in 1980s and currently is currently being used for setting design criteria in a rational manner. This analysis method is applied to the controlling earthquake determination issue here. In the paper, it is demonstrated that there are several definitions of the controlling earthquakes based on the reliability concept and their characters are intensively studied. Finally, the paper concludes that the controlling earthquakes based on the FORM have more physical and rational basis for selection of design earthquakes.