The role of salt tectonics in the occurrence of the 2021 Central Adriatic seismic sequence

On 27 March 2021 a three-months lasting seismic sequence struck the Central Adriatic Basin, part of Adria that is considered a relatively undeformed plate since recent times. Analyzing the waveform data acquired by the Italian and Croatian seismic networks, we computed the location parameters of 160 earthquakes and the focal mechanisms of the Mw5.2 mainshock and larger aftershocks. Most events align along a WNW-ESE, 30 km long, narrow belt. They form two clusters between 0-3 km and 4-14 km of depth, separated by a 1-2 km thick aseismic zone. Based on literature data, we suggest that such a seismic gap corresponds to a ductile salt layer, which constitutes the primary control factor for the evolution of the 2021 earthquake distribution. Moreover, the presence of a salt layer explains well the relatively high Vp/Vs ratio of 1.83 in the sediment rocks surrounding the salt bodies, as also observed in similar tectonic settings. We suggest that the seismogenic fault likely responsible for the 2021 events is an inherited SW-dipping normal fault, reactivated with prevalent reverse kinematics in response to the regional compressive stress. These results, and the recognition of a specific role of salt deposits in focusing deformation and seismogenesis represent a novel contribution to the long-standing problem of seismic hazard assessment of the Central Adriatic Basin, where moderate to large events could have devastating impacts along the highly populated coasts.