Characterization of the shallow hydrothermal system of Vulcano Island (Eolie islands, Italy) through Electrical Resistivity Tomography (ERT) and Induces Polarization (IP) tomography.

The island of Vulcano is situated in the southern part of the Salina-Lipari-Vulcano ridge and, together with these two islands, constitutes the central sector of the Aeolian archipelago. It is a complex active volcanic system whose last eruption occurred between 1888 and 1890. Since then, intense fumarolic activity accompanied the various phases of unrest of the associated hydrothermal system, which made Vulcano the object of various geophysical, geochemical, and hydrogeological studies.

The aim of the study here presented was the investigation of the superficial hydrothermal system of Vulcano, specifically the northern sector of the island, where the ‘La Fossa’ caldera is located, which has been the main degassing area since the beginning of the last period of unrest. The survey has been conducted using geoelectrical methods because of the efficiency that these methods revealed in the previous studies on active volcanic systems, especially on Vulcano. The distribution of resistivity in the subsoil, indeed, is influenced by parameters such as temperature, permeability, porosity, presence of fluids and their salinity, which allow the reconstruction of the changes in the lithological and structural features with depth and the evaluation of the presence of discontinuity, such as faults, that have a fundamental role in the circulation of fluids.

During the survey, electric resistivity, chargeability, and self-potential data were jointly acquired along five profiles with lengths between 700 m and 2500 m and an investigation depth of approximately 200 m. The tomographies resulting from the data processing provide information on the evolution of the hydrothermal system, allowing us to identify the prominent resistivity anomalies and the pattern of hydrothermal fluid circulation. By integrating the results obtained with the ones emerging from previous studies conducted on the island, it is possible to implement a more exhaustive evaluation of the hydrothermal mechanism in the areas adjacent to the La Fossa caldera and how the flow is facilitated or hindered by the main fault system of Vulcano, that is necessary to analyze in order to previously verify the actualization of conditions favorable to the overpressure or decompression of the fluids, which could trigger an explosive eruptive event.