The Role of Salt Tectonics in the 2021 Central Adriatic Seismic Sequence

Abstract

On 27 March 2021 a 3-months lasting seismic sequence struck the Central Adriatic Basin that is part of the Adria plate, 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 M_w 5.2 mainshock and the larger aftershocks. Most events align along a WNW-ESE trending, 30 km long, narrow belt. The depth distribution of events indicates that the mainshock and a few aftershocks occurred within the upper 4 km, while most aftershocks were located below 5 km within the carbonate platform. We propose that the evolution of the 2021 earthquake distribution is primarily ruled by the top ductile salt layer. Moreover, the presence of a salt layer explains the relatively high V_P/V_S 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 by reverse kinematics in response to the regional compressive stress. These results, and the understanding that salt deposits play a key role in focusing deformation and seismogenesis, represent a novel contribution to the long-standing challenge of seismic hazard assessment of the Central Adriatic Basin, where moderate to large events could have devastating impacts along the densely populated coasts.