Mean-to-median spectral acceleration ratios in Senior Seismic Hazard Analysis Committee Level 3 probabilistic seismic hazard analyses: An analysis of reported results

In many countries, seismic characterization of the site selected for a critical structure or industrial facility is required in terms of site-specific seismic ground motion hazard. For this purpose, a probabilistic seismic hazard analysis (PSHA), performed under the Senior Seismic Hazard Analysis Committee (SSHAC) protocol, is an extended practice for nuclear facilities. In the past decade, SSHAC Level 3 studies have been performed for sites in North America, Europe, Japan, Taiwan, and South Africa. When analyzing PSHA results, the mean-to-median spectral acceleration ratios given by the hazard curves can be interpreted as a measure of the degree of epistemic uncertainty associated with the results. In this article, results of 33 SSHAC Level 3 studies have been used to determine mean-to-median spectral acceleration ratios and the statistics of these ratios, as a function of spectral frequency and annual frequency of exceedance (AFE). The purpose was to develop a reference for the range of uncertainty that is typically captured in this kind of studies. It has been found that, for a given AFE, ratios corresponding to different sites are within a relatively small interval, especially for the spectral frequency band between 2.5 and 10 Hz, which is the band normally more relevant for the seismic design of nuclear installations. In this band, for 10-4 yr-1 AFE, a mean/median ratio of 1.40 would envelop practically all investigated sites.