We analyze specific features of multiple-site (MS) probabilistic seismichazard assessment (PSHA), that is, the annual rate of ground-motion level exceedance in at least one of several sites of interest located within a defined area or along a linear extended object. The relation between MS hazard estimates and strong ground motion data obtained during large earthquakes is discussed in the cases of the 1999 Chi-Chi M-w 7.6 and the 2008 Wenchuan M-w 7.9 earthquakes. The strong-motion records obtained in the epicentral zones may be considered as examples of the ground motion exceeding the design level estimated using the conventional pointwise (PW) PSHA. We show that the MS-PSHA, when being performed for the 475 yr standard return period, provides reasonable estimations of the intensity level that may occur during the earthquakes, parameters of which are close to the parameters of events with maximum possible magnitude accepted in PSHA for the regions. The MS-PSHA efficiency is discussed with respect to (1) evaluation of the performance of probabilistic seismichazard maps and (2) application of the MS hazard estimates as a basis for design loads. Based on the results of this work, we propose a multilevel approach to PSHA considering the fixed reference probability of exceedance (e.g., 10% in 50 yrs): (1) a standard PW-PSHA, to be performed in a seismic-prone region (first level), and (2) this analysis should be supplemented by an MS-PSHA for urban and industrial areas or for zones of a particular economic and social importance (second level).
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