Allowing multi-fault earthquakes and relaxing fault segmentation in central Apennines (Italy): hints for fault-based PSHA

Italy is one of the most seismically active countries in Europe. In the last decade, three seismic sequences produced extensive damage in central Italy and caused more than 600 fatalities. The fault-based models developed for this area in the past have shown better performances with respect to the standard approaches; they consider each fault independently as an individual seismogenic source and do not contemplate the occurrence of multi-fault earthquakes but seismological, geological, and paleoseismological studies in central Italy suggest that multi-fault earthquakes can occur in the Apennines. Then, it is necessary to apply a modern fault-based approach that includes the occurrence of multi-fault earthquakes, where earthquakes can rupture multiple faults during the same event going, thus, beyond strict fault segmentation assumptions. I used a public available tool (SUNFiSH) to obtain multifault earthquakes occurrences in central Italy, by defining a fault model and assigning to each subsection a slip rate value through a geologic deformation model. Then, I compared the long-term time-independent earthquake rates of all possible ruptures computed by this approach with the time-independent activity rates obtained by using individual seismogenic sources, and with the observed historical rates. Results highlight the necessity to consider a model that relaxes segmentation and considers the multifaults events similarly to what has been done, for example, in western U.S.A.