The 2002-2003 submarine gas eruption at Panarea (Aeolian Islands, Italy): study of the seafloor and implications for volcanic hazard assessment.

A spectacular submarine gas eruption has started in November 2002 offshore of Panarea volcanic island (Aeolian Islands, Italy) in coincidence with the parossistic volcanic eruption at Stromboli island and the Mt Etna eruption. Submarine volcanic activity in the Aeolian area has been known since Roman times, as reported by Roman historians that described exhalation episodes and submarine eruptions, also in coincidence with eruptions at Mt Etna and Stromboli. This contemporaneity imposes to carefully consider the volcanological implications of such phenomenon, as Panarea is currently considered an extinct volcano. We used the Marine Digital Terrain Model (MDTM), obtained by multi-beam surveys, to investigate the bathymetry at <0,5 m accuracy and detailed underwater scuba diving surveys, to study the morphological-structural features of the seafloor. Our study evidences the role of N240- and N310-trending fractures, as main pathways for the gas exhalation. The data have been summarised in the geological map of the seafloor. The data suggest that the most likely explanation for the November 2002 event is that it represents the parossistic evolution of a hydrothermal system fed by a quasi-steady deep source of magmatic fluids, likely a cooling magma body, able to build-up pressure at depth which periodically overcomes the tensional strength of the rocks allowing the release of the pressurised gas. We believe that the most hazardous scenario for the short and middle term is related to gas and phreatic eruptions, although geochemical data do not exclude the possibility for new magma inputs.