Potential earthquake-prone faults identified by dense seismic array monitoring in complex extensional settings

Identifying active faults capable of generating moderate-to-large earthquakes is essential for seismic hazard assessment, yet remains challenging in extensional tectonic environments, where fault systems include multiple segments and bends accommodating strain. In this study, we demonstrate how a short-term deployment of densely distributed seismic arrays can provide critical insights into seismicity patterns and fault geometry in the Southern Apennines, Italy.Integrating arrays with advanced machine learning methodologies, we produce an enhanced seismic catalog that increases the content of the manual one by nearly one order of magnitude, achieving, in just one year, a resolution comparable to a decade of conventional monitoring. Our results reveal spatial consistency of seismicity down to the decameter scale, with hypocenter locations and b-value mirroring those from the previous decade. We distinguish a shallow, diffuse seismicity, likely influenced by hydrological loading from karst aquifers, from deeper seismic clusters characterized by greater spatial coherence. The distribution of deep seismicity, when integrated with a 3D tomographic model, delineates a complex, curving fault structure 50–60 km long, featuring a right-stepping jog several kilometers wide. Dynamic rupture simulations suggest that earthquakes nucleating on this fault could propagate through these structural complexities, potentially generating earthquakes up to magnitude 7.0.