Investigating the relationships between recent seismicity and geological structures at the hanging-wall of the Alto Tiberina Fault (northem Appennines of Italy)

The PhD project described in this Thesis is designed to investigate seven recent seismic sequences through three integrated lines of investigation bridging short- and long-term processes. The seismological approach quantifies the impact of velocity models on earthquake location precision by comparing homogeneous, 1D and ad-hoc 3D models using advanced Non-Linear Location algorithms. The seismotectonic approach investigates the association between these sequences and minor faults imaged in seismic reflection profiles. The fluid-related approach explores long-term fluid circulation through fluid inclusion analyses in calcite veins within the active fault zone, where part of the recent seismicity occurred. The study area is located in Umbria, in the northern sector of the Apennines and is dominated by two major geological structures: the Alto Tiberina Fault (ATF) and the Gubbio Fault (GuF). The region is characterized by diffuse microseismicity (ML<3) and by shallow low-magnitude sequences (ML< 4.5) not associated with surface- exposed faults, widely felt by the population and locally damaging. The choice of this area is motivated by the occurrence of diffuse seismicity and seismic sequences, the availability of high-resolution multidisciplinary datasets and the documented presence of overpressured fluids. Results show that the 3D velocity model reduces location uncertainties by up to 90%, especially in depth, providing a robust basis for structural interpretation. The sequences previously located with a 1D model, are relocated using the 3D model and successfully associated with minor normal faults at the hanging wall of the ATF imaged in seismic profiles. Fluid inclusion data reveal compartmentalization of the fault system, deep meteoric fluid circulation and the role of major and minor faults in controlling fluid ascent. Integrating seismological, geophysical, structural, and geochemical data, this study links short-term seismic processes with long-term fault evolution and highlights the seismogenic potential of minor faults not exposed at the surface, with direct implications for seismic hazard assessment in the Apennines.