The use of seismic catalogs enhanced through advanced detection techniques improves the understanding of earthquake processes by illuminating the geometry and mechanics of fault systems. In this study, we performed accurate hypocentral locations, source parameters estimation and stress release modelling from deep catalogs of microseismic sequences nucleating in the complex normal fault system of the Southern Apennines (Italy). The application of advanced location techniques resulted in the relocation of ~ 30% of the earthquakes in the enhanced catalogs, with relocated hypocenters clearly identifying local patches on kilometer-scale structures that feature consistent orientation with the main faults of the area. When mapping the stress change on the fault plane, the inter-event distance compared to the size of the events suggests that the dominant triggering mechanism within the sequences is static stress transfer. The distribution of events is not isotropic but dominantly aligned along the dip direction. These slip-dominated lineations could be associated with striations related to fault roughness and could map the boundary between locked and creeping domains in Apulian platform and basement.

Approximately 23,000 well-located earthquakes from 2009 to 2016 are used as templates to recover seismic activity preceding the 2016 Central Italy seismic sequence. The resulting spatiotemporal pattern is analyzed employing ~91,000 newly detected events. In the 8 years before the sequence onset, microseismicity (Ml minor or equal to 3.7) develops at the hangingwall of the 2016 fault system and along a sub-horizontal shear zone. The events, mainly organized in clusters, represented by foreshock-mainshock and swarm-like sequences, migrate toward the nucleation area of the first Mw 6.0 mainshock of the sequence that occurred on the 24th of August in Amatrice. We propose an unlocking model based on variable temporal clustering of the seismicity, including repeaters, identifying fault portions with different degree of coupling and rheology, responding differently to the tectonic loading, and working to progressively localize the deformation process, increasing rock damage and weakening the nucleation patch of the Amatrice mainshock.

The Alto Tiberina normal fault (ATF) in Central Italy is a 50 km long crustal structure that dips at a low angle (15-20◦). Events on the fault plane are about ten times less frequent than those located in its shallower syn- and antithetic hanging-wall splays. To enhance ATF catalogue and achieve a better understanding of the degree of coupling in the fault system, we apply a template matching technique in the 2010-2014 time window. We augment by a factor 5 the detections and decrease the completeness magnitude to negative values. Contrary to what previously observed on ATF, we highlight intermittent seismic activity and long-lasting clusters interacting with sequences on the shallower splays. One of these episodes of prolonged seismic activity, detected at the end of 2013 on a 30 km long ATF segment, suggest the ATF active role during an aseismic transient unravelled by geodetic data.