Geological and Seismotectonic Analysis of the area struck by the November 2022 Mw 5.5 Offshore Pesaro Earthquake (Northern Adriatic Sea Region, Italy).

On the 9th of November 2022, a Mw 5.5 earthquake occurred near the Northern Adriatic coast, between Ancona and Pesaro (Marche region, Italy), as part of a complex seismic sequence, including six M>4 events, with NW-SE striking thrust fault focal mechanisms. This study is aimed at reconstructing the subsurface geological setting of the area struck by the seismic sequence, focusing on the active thrusts responsible for the observed seismicity. We interpreted previously unpublished 2D seismic reflection profiles integrating with deep wells, covering approximately 1500 km2.

We analysed the stratigraphic and geophysical log data from eight deep wells. This analysis defined the local stratigraphy, comprising a Late Triassic-Paleogene multilayer of evaporites, carbonates and Tertiary marls, overlain by Pliocene-Quaternary syn-tectonic clastic sediments. Wells data were used to calibrate the strong reflections recognized along the 2D seismic profiles. These profiles include six cross-lines (i.e. SW-NE), connected by three strike-lines (i.e. NW-SE).

Five key-horizons were distinguished: Top Pleistocene unconformity, Base of Pliocene -Pleistocene unconformity, Top of Middle Pliocene, Top of Messinian, and Top of Oligocene. The Messinian's prominent reflection, specifically, played a pivotal role in interpreting and identifying these horizons. These stratigraphic markers are cut and displaced at depth by two major thrust-fault segments, affecting the Mesozoic-Cenozoic carbonate succession.  Along-strike geometry of these major, SW gently dipping thrusts has been identified across four seismic reflection profiles, along a distance of about 24 km. A set of more complex, shallower thrusts, affecting the Tertiary marls and the overlying, syn-tectonic clastic sediments, splays out from these major structures. Time to depth-converted structures were derived by establishing a proper velocity model based on both local and regional log data. The location and depth of the seismic events were plotted along the depth-converted seismic profiles, demonstrating a good correlation with the geometry and kinematics of the deep thrust-fault segments. The interpretation of the deformation observed in the overlying strata suggests a strong Pliocene-Pleistocene contractional phase, up to the end of the Early Pleistocene. In recent times, the increased sedimentation rate masks the continuing tectonic activity.

This study contributes to a deeper understanding of the location, geometry, and kinematics of potentially active buried faults, sheding light on the seismotectonic setting of the study area, leading to a better understanding of the geological structure of the active external thrust in the Northern Adriatic region. The study contributes not only to a better knowledge of the seismotectonic setting of the region, but also plays an important role in formulating effective strategies for seismic hazard assessment and regional seismic risk management.