The historical record of earthquakes is a crucial data resource for seismic hazard analysis. In every region, the largest events are rare and difficult to parameterize. In cases for which such events are associated with the ruptures of mapped faults, defining the extent of possible fault ruptures is an important challenge that is often guided by historical precedent in which geologic, particularly paleoseismic, studies are limited. Counterfactual risk analysis recognizes that a historical event is just one realization of many possible alternatives: a fault rupture that happened in the past is just one of numerous ways in which seismic energy might have been dynamically released. Stochastic modeling of past earthquakes can provide additional insight into the complex geometry of multifault rupture. This counterfactual approach that is advocated here extends the effective time window of observation of the fault rupture process, well beyond the time span of earthquake catalogs. The basic concepts of counterfactual risk analysis are explained, followed by a seismological discussion of runaway earthquakes, illustrated in the case of the North Anatolian fault (NAF) in Turkey. This notable example demonstrates the practical utility of the counterfactual approach as a new supplementary tool for addressing one of the most difficult problems in probabilistic seismic hazard and risk assessment: selecting the multifault combinations to model as explicit seismic sources.

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