Problems in evaluating low probability ground motion

Current approaches in evaluating low probability seismic risk follow either of two general methods, i.e. probabilistic or limiting seismic risk analysis. The two approaches are discussed separately. The basic quantitative input to probability seismic risk analysis is a probability distribution of magnitudes within the critical crustal volume; this distribution is obtained by fitting a Gumbel 3 distribution to an enlarged set of data while the limiting analysis concentrates on the short term crustal rupture associated with an expected individual earthquake. It is suggested that the two methods of analysis model different aspects of the problem with vastly different time factors and complement each other. A seismic study is briefly described where both approaches were considered, applied to the city of Tehran, Iran.