Characterizing the shallow hydrothermal system of Vulcano Island (Aeolian Islands, Italy) using geoelectrical survey: implications for the eruptive scenario

Vulcano island is situated in the southern part of the Salina-Lipari-Vulcano ridge and, together with these two islands, forms the central sector of the Aeolian archipelago. Vulcano is an active volcanic system whose last eruption occurred between 1888 and 1890. Since then, intense fumarolic activity has accompanied various phases of unrest of the associated hydrothermal system, making Vulcano the subject of numerous geophysical, geochemical and hydrogeological studies. The aim of this study was to investigate the shallow hydrothermal system of the ‘La Fossa’ caldera, and particularly the northern sector of Vulcano island, which hosts a degassing area very active also during the last period of unrest, started in 2021. The survey was carried out using geoelectrical methods due to their proven effectiveness in studying active geothermal systems. This approach enables to identify changes in subsurface structural features and to assess the presence of discontinuities, such as faults, which play a fundamental role in fluid circulation. Our survey consisted of 2D electrical resistivity and induced polarization tomographies, and self-potential measurements along four profiles located at the base of the La Fossa cone in the northern part of the island, which allowed an investigation depth of about 200 m below the ground level. The reconstruction of the spatial distribution of the electrical properties, in terms of resistivity, chargeability and natural polarization, facilitated the identification of various structures associated with the activity manifestations of the shallow hydrothermal system of Vulcano. Specifically, the observed anomalies revealed a discontinuous layer between 40 m and 140 m below sea level composed of lavas overlying a saturated hyaloclastite layer that is supposed to be the main reservoir of high-temperature hydrothermal fluids. The results obtained, integrated with those of previous studies carried out on the island, have highlighted areas characterized by the presence of hydrothermal fluids in the liquid state (low resistivity and low chargeability anomalies) and in the gaseous state (high resistivity and low chargeability anomalies), upwelling along structural discontinuities, as testified by the fumarolic manifestations observed on the surface. Identifying the island’s main fault system and understanding its effect on fluid circulation in the areas adjacent to the La Fossa caldera represent fundamental information for assessing the establishment of conditions favourable to fluid overpressure or decompression, which can lead to phreatic/hydrothermal-type explosive events.