The Adriatic Thrust Fault of the 2021 Seismic Sequence Estimated from Accurate Earthquake Locations Using sP Depth Phases

An earthquake sequence occurred in the Central Adriatic region during March–June 2021. This sequence started on 27 March with a mainshock of moment magnitude (Mw) 5.2 occurring at 13:47 coordinated universal time (UTC). No foreshock was observed before this mainshock. The sequence lasted approximately three months, until the end of June 2021. Approximately 200 seismic events were recorded by the regional seismic network during this time, including four M ≥ 4.0 earthquakes. The 27 March 2021 earthquake was one of the strongest instrumentally recorded events in the area bounded approximately by the Ancona–Zadar line to the north and the Gargano–Dubrovnik line to the south. The mainshock originated at a focal depth of 9.9 km. The seismicity spread from the mainshock up-dip and down-dip along a northeast-dipping plane. Here, we investigate the geometry of the fault activated by this seismic sequence by using sP depth phases. We aim to significantly reduce the large uncertainties associated with the hypocentral locations of offshore earthquakes beneath the Adriatic Sea—an area that plays a fundamental role in the geodynamics of the Mediterranean. These refined earthquake locations also allow us to make inferences with regards to the seismotectonic context responsible for the analyzed seismicity, thus identifying a structure (here referred to as the Mid-Adriatic fault) consisting of a northwest–southeast-striking thrust fault with a ∼35° northeast-dipping plane. The use of depth-phase arrival times to constrain off-network event locations is of particular interest in Italy due to both the peculiar shape of the peninsula and the extreme scarcity of seafloor stations, the cost and management of which are very expensive and complex. Here, we present the first attempt to apply this off-network locating technique to the Italian offshore seismicity research with the aim of improving hazard estimations in these hard-to-monitor regions.