The use of the area deformed by earthquakes (deformed area method) for potential fault capability assessment

• Surface faulting poses a major hazard to NPPs and constitutes and exclusion criterion to be addressed. • The extent of the area permanently deformed by earthquakes scales with the earthquake magnitude. • The deformed area method can be used to solve cases where a direct assessment of fault capability cannot be performed. This paper proposes a conceptual framework to support the assessment of fault capability at nuclear power plant (NPP) sites, with a particular focus on situations where direct geological or geochronological evidence is insufficient or ambiguous. The approach builds on the conditions outlined in IAEA Safety Standards Series No. SSG-9 (Rev. 1), particularly Conditions (b) and (c), which address the presence of structurally linked faults and areas where regional seismotectonic characteristics suggest possible surface faulting despite limited direct evidence. The methodology is based on the assumption that large earthquakes induce permanent deformation in the surrounding rock mass, and that the extent of this deformation correlates with earthquake magnitude, as already discussed in previous publications by the Authors. By applying empirical relationships derived from global datasets, the approach estimates the surface area potentially affected by coseismic deformation. Faults located within this area are considered to have an increased likelihood of activation or reactivation during future seismic events. This framework offers a means to partially resolve uncertainties in assessing Conditions (b) and (c), particularly in data-limited or complex tectonic settings. It provides a rational, conservative basis for identifying faults that may warrant further probabilistic fault displacement hazard analysis (PFDHA). The approach is not intended to replace detailed site investigations or deterministic assessments where feasible, but rather to complement existing methodologies when direct evidence is lacking. The proposed methodology supports decision-making in nuclear siting by offering a structured procedure for addressing gaps in current standards and for managing epistemic uncertainty in fault capability evaluations. This opinion paper aims to contribute to ongoing discussions on fault displacement hazard assessment and has informed recent developments in IAEA guidance and technical benchmarking studies.