Characterizing the Distributed Faulting During the 30 October 2016, Central Italy Earthquake: A Reference for Fault Displacement Hazard Assessment

Moderate to strong earthquakes (i.e., Mw>similar to 6.0) commonly produce a complex network of ground ruptures, which are responsible for significant damage. Distributed faulting can affect wide areas (tenths of square kilometers), and expected displacement can be estimated through a probabilistic approach, considering distance from the primary fault and earthquake magnitude. Other factors may have a role in driving the occurrence of distributed faulting; nevertheless, they are not adequately addressed in the current modeling, due to a sensible lack of information. We study the 30 October 2016, Central Italy earthquake (Mw 6.5), to analyze the spatial pattern and geometric characteristics of distributed faulting. We found that distance from the primary structure, fault geometry, and lithology are key factors controlling the distributed faulting occurrence; the local structural setting (i.e., synthetic versus antithetic normal faults systems and relay zones) drives the spatial distribution of faults and the partitioning of the deformation. We also examine other four events occurred in the Italian Apennines since 1980, confirming that traditional models can underestimate the probability of distributed faulting. We suggest that a purely distance-based probabilistic approach should be integrated using additional parameters derived from earthquake deformation fields or considering the reactivation of preexisting faults. Plain Language Summary Surface faulting caused by moderate to strong earthquakes may occur along the major fault plane (primary fault) or along other structures in its proximity (distributed faulting). Distributed faulting can affect wide areas (tenths of square kilometers) and is responsible for significant damage. We assess the spatial pattern and geometric characteristics of distributed faulting of the 30 October 2016 Central Italy earthquake. We found that distance from the primary structure, fault geometry, and lithology are key factors controlling the distributed faulting occurrence. We also analyze four events that hit the Italian Apennines since 1980 and compare the results with the models currently adopted for estimating the possible amount of surface faulting caused by earthquakes. We found that current models, which are based on traditional field mapping methods, underestimate the probability of distributed faulting. We argue that the Central Italy 2016 event can act as a reference case study for fault displacement hazard assessment and that an increasing number of case histories will improve future mitigation strategies.