Characterization of the 2021 unrest phase of Vulcano Island (Eolie islands, Italy) through numerical modelling: an integrated approach based on Electrical Resistivity Tomography and Magnetotelluric data.

The island of Vulcano is one of the most exposed volcanic edifices of the Aeolian islands. Vulcano has been responsible for many eruptions, including at least three phreatic eruptions. The most recent eruption on the island occurred between 1888 and 1890 AD, and since then, Vulcano’s hydrothermal system has been the site of different volcanic unrest. In 2021, geochemical and geophysical parameters showed changes, suggesting a potential increase in magmatic fluid within the shallow geothermal system. GPS data revealed an expansion of the crater zone, consequently increasing the Vulcano alert level.

The main volcano-tectonic structures and geothermal fluid pathways have been resolved down to a depth of 2.5 km through a magnetotelluric (MT) survey conducted in 2022 (Di Giuseppe et al., 2023). A more detailed definition and mapping of the significant volcano-tectonic structures were subsequently acquired through a high-resolution Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) investigation campaign conducted in 2023. These surveys provided original information regarding the main structure of the caldera and the related subsurface fluid circulation. The overall characterization of the link between the shallow geothermal system and the volcano activity furnished by the electromagnetic and electrical investigations represented the basis for developing thermo-fluid dynamic modeling of Vulcano island using the TOUGH2 numerical code. Adopting a conceptual model of the island mainly derived from the results of the MT and ERT investigations and considering literature and monitoring data collected immediately before and after the 2021 Vulcano unrest phase, a thermodynamic stationary state has been reconstructed, which likely represents a reliable description of the actual condition of the volcano. Subsequently, perturbation analysis of such a thermodynamical state has been performed to characterize the recent unrest phase involving Vulcano during the past two years and to reconstruct its primary mechanisms.