GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 Accurate sP Depth Phases Earthquake Locations of the Adriatic Thrust Fault of the 2021 Seismic Sequence R. Di Stefano 1 , M. G. Ciaccio 1 , P. Baccheschi 1 , D. Zhao 2 1 Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy 2 Department of Geophysics, Graduate School of Science, Tohoku University, Japan Introduction We re-located 70 earthquakes belonging to the seismic sequence started on March 27 with a mainshock of Mw 5.2 at 13:47 UTC, in the Central Adriatic region (Fig. 1). We use the on-purpose designed code by Zhao et al. (2007, 2011) to add the sP converted phases to the P and S phases in locating events, strongly constraining depth of off-network earthquakes. The use of depth-phase arrival times to constrain the off-network event locations is of particular interest to Italy due to both the peculiar shape of the peninsula and the extreme scarcity of seafloor stations whose cost and management are very expensive and complex. We present the first attempt to apply this off-network location technique to the Italian off-shore seismicity with the aim of improving the hazard estimation of these hard-to-monitor regions (Di Stefano et al., 2022). The 2021 Seismic Sequence The mainshock of the 2021 seismic sequence occurred about 20 km north of the Palagruza islands, 80 km from the Gargano promontory and about 40 km from the Croatian island of Lastovo. It was felt in many central-southern Italian regions, from Ancona to Foggia, and also in Central Dalmatia. Their epicenters lie in the open sea, about 100km to the SE of the 2003 Jabuka seismic sequence, and about 50 km to the NW of the 1988 Palagruza seismic sequence (Herak et al., 2005). No foreshock was observed before the mainshock. The sequence lasted about three months, from March until the end of June 2021. About 200 seismic events were recorded by the regional seismic network, and 4 earthquakes have M ⋝ 4.0. The March 27, 2021 earthquake is one of the strongest instrumental events that occurred in this area, bounded approximately by the Ancona-Zadar line to the north and the Gargano-Dubrovnik line to the south.