Enhancing the resolution of microseismicity through dense array monitoring in complex extensional settings

Characterizing geometry and mechanics of structures hosting moderate-to-large earthquakes is essential for seismic hazard assessment, yet remains challenging in extensional environments, where fault systems include multiple segments and bends. In this study, we demonstrate how a short-term array deployment can provide critical insights into seismicity patterns and fault geometry in Southern Apennines, Italy.We integrated data recorded by arrays during a one-year experiment with machine learning methodologies, producing a seismic catalog that enhances the manual catalog for the same period by nearly an order of magnitude, lowering completeness magnitude by one unit. Approximately 65% of the detected events can be accurately relocated, with median uncertainties of ~ 100 m, comparable to those of long-term catalogs. Our results reveal consistent seismicity properties down to decametric earthquake size, with hypocenters and b-value mirroring those from the previous decade. We distinguish a shallow, diffuse seismicity, likely influenced by hydrological loading, from deeper clusters, mostly rupturing patches a few-hundred meters across. Beyond asperity-scale complexity, seismicity follows the boundaries of tomographic anomalies, delineating a 50–60 km-long curving fault, featuring a right-stepping jog several kilometers wide. Dynamic simulations suggest that ruptures nucleating on this fault could propagate through these complexities, potentially generating earthquakes up to magnitude