Volcano-Tectonic seismicity during the 2021-2023 unrest phase at Vulcano island (Italy) and its connection with the regional geodynamic context

Abstract

The island of Vulcano, in the South-Eastern Tyrrhenian Sea (Southern Italy), is the southernmost out-of-water part of a larger submerged volcanic edifice of the Aeolian archipelago. Since its last explosive eruption (1888-1890), relevant episodes of volcanic unrest unfollowed by eruptive activity have been documented. These episodes were characterized by notable increase in geochemical parameters, ground deformation, and local seismicity related to fluid dynamics within the shallower part of the hydrothermal system. Volcano-Tectonic (VT) seismicity located on the island did not play a major role during these unrest phases, except for the 1985 and 1988 seismic sequences that preceded and accompanied a significant increase in temperature and gas flux at the fumaroles causing a depletion of the shallow hydrothermal source.  The last phase of unrest occurred from mid-September 2021 to December 2023. A climax in high temperature, CO2 flux, fumarolic gas emissions, ground deformation together with LP and VLP seismicity was achieved in early November 2021. In the following months, after a period of stability of the major anomalies, a gradual decrease was observed. Conversely, VT seismicity showed a moderate increase in the time intervals 30/10/2021-31/12/2021, 31/03/2022-30/04/2022 and 04/12/2022-31/12/2022. In this work, the application of the tomoDDPS algorithm to relocate the seismicity occurred from January 2020 to December 2022, points to the identification of three seismogenic volumes. The space-time distribution and energy release of the relocated seismicity together with waveform correlation analysis enabled us to infer a connection between the unrest process and the activation, at different depth ranges, of a NW-SE trending wrench faults system and associated NE-SW structures.