A fault-based application to model seismicity rates for seismic hazard assessment in the southern Apennines (Italy)

Although fault-based approaches to seismic hazard assessment have been increasingly adopted worldwide, the official Italian hazard model, on which the national building code is based, still relies on a catalogue-based framework, with well-known limitations in capturing the long-term recurrences of large-magnitude events. In this study, we present a fault-based application to model seismicity rates for the southern Apennines (Italy) that incorporates a multi-fault rupture assumption. This area is of particular interest due to its active seismicity and the presence of large dams, for which robust long-term hazard estimates are essential. We use the SHERIFS code to model seismicity rates at the fault system-level, which allow us to explore epistemic uncertainties of fault and seismicity parameters (rupture scenarios, scaling laws, b-values and background seismicity). Our results highlight the key role of rupture models: scenarios allowing multi-fault ruptures outperform single-fault rupture models in terms of agreement with the regional seismicity and paleoseismic rates. Our findings support the inclusion of multi-fault rupture models in PSHA logic trees for the region and emphasize the need for improved fault behaviour characterization in southern Italy.