Relationships between seismicity and geological structures at the hanging-wall of a low-angle normal fault (Alto-Tiberina fault system, Northern Apennines of Italy).

One of the most seismically active areas in Central Italy is located within the Pre-Apennine Umbria region, between the Tiber Valley, the town of Gubbio and the main mountain ridge of the Umbria-Marche Apennines. The fault system that characterizes this region is dominated by a 60 km long low-angle normal fault (Alto Tiberina, ATF), active since the Late Pliocene-Early Pleistocene. This area is mainly monitored and studied though the Alto Tiberina Near Fault Observatory (TABOO-NFO), a multidisciplinary monitoring infrastructure composed of dense arrays of seismic, geodetic, strain , geochemical and electromagnetic sensors deployed both at the surface and on boreholes. This infrastructure is fundamental to investigate the principal geophysical and geochemical processes occurring in this complex geological area (https://www.ont.ingv.it/infrastrutture-di-ricerca/sismologia/taboo). Besides the high rate of micro-seismicity nucleating along the ATF (ML<3.0), there is also a considerable number of synthetic and antithetic faults (e.g., Gubbio Fault, GuF) located in the hanging-wall of the ATF that produced historical and most recent earthquakes of moderate magnitude (e.g., MW 5.1 1984 Gubbio earthquake).

Our study focuses on the most recent seismic sequences, occurred in this area between 2010 and 2023, that produced main shocks of magnitude > 3 (Mw= 3.6 - Pietralunga 2010, Mw= 3.6 - Città di Castello 2013, Mw = 3.9 - Gubbio 2021, Mw = 4.5 - Umbertide 2023).

These seismic events have been registered, located and published in the Database of the Central Eastern Italy by the INGV office in Ancona (https://doi.org/10.13127/resiico/eqs). From these data, all the considered sequences are characterised by dominant normal fault kinematics, coherent with the regional SW-NE active extension. Moreover, they occurred at relatively shallow depth (< 7 km), at the hanging-wall of the ATF, and their location cannot be directly referred to any extensional fault, mapped in the studied area. The aim of our work is to investigate the potential relationship between the cited 2010-2023 seismic sequences and the occurrence of still unknown causative minor faults, at the hanging-wall of ATF. To reach the goal, we propose a revised detailed interpretation of a set of 2D-seismic reflection profiles, calibrated by few deep boreholes, acquired in the 80s for hydrocarbon exploration purposes. Previous studies of these data have been focussed on the ATF and on its major antithetic splay, i.e. the SW-dipping Gubbio normal fault. In this study, we want to explore the presence of other, synthetic and/or antithetic splays, visible at the seismic scale and possibly connected with the 2010-2023 seismic sequences. In order to improve the comparison between the geological structure at depth and the seismicity distribution, we decided to relocate the 2010-2023 catalogue of seismicity for the study area, following two innovative strategies: a 3D velocity model created on purpose for the ATF area and a Markov chain Monte Carlo algorithm for events location.

By combining interpretation of active seismic data with innovative strategies of earthquake re-location, our study proposes as a pivotal experience for seismo-tectonic interpretation of low-magnitude seismic sequences.