The IAEA exercise on probabilistic fault displacement hazard assessment

International Atomic Energy Agency (IAEA) Specific Safety Guide SSG-9 (Rev.1) recommends the use of probabilistic fault displacement hazard assessment (PFDHA) for faults that may affect the foundations of safety-related structures of nuclear installations in the specific case of existing nuclear installation sites. To provide practical guidance on performing PFDHA, IAEA initiated an exercise that includes alternative fault displacement predictive models and three case studies representing strike-slip, normal, and reverse earthquakes. This article presents and discusses the findings of the IAEA PFDHA exercise for selected principal and distributed fault displacement scenarios. Analysis of the results underlines that the primary fault displacement estimations by different modelers are in good agreement at the return periods dominated by surface rupture models for exercise cases that involve moderate-to-large magnitude events. At longer return periods, significant differences are observed in the slopes of the hazard curves due to model uncertainties particularly aleatory variability and truncation of the standard deviation. Distributed displacement hazard curves by different modelers have factors of 10–50 differences, mainly caused by different assumptions in the implementation of the problem, particularly in the conditional probability of distributed fault rupturing. The findings of this study highlighted the need for developing a consensus in surface rupture probabilities, especially when multiple fault displacement predictive models are used to capture the epistemic uncertainty in PFDHA estimations.