Ground deformation induced by geothermal processes: A model for La Fossa Crater (Vulcano Island, Italy)

Since the last eruption in 1888-1890, volcanic activity at Vulcano Island (Aeolian Archipelago, Italy) has been limited to fumarolic degassing, concentrated near the active cone of "La Fossa'' crater in the northern sector of the island. During the period 1990-1996, EDM and leveling data indicate a deflation and contraction focused on the La Fossa Cone. We jointly inverted these data sets using for volume change both a spherical Mogi (1958) source and an ellipsoid as defined by Davis (1986). The best fitting source for the 1990-1996 deflation is a shallow subvertical prolate ellipsoid positioned roughly under the crater. Our model is consistent with increased activity in a shallow hydrothermal system, supported by observations of water evaporation testified by the increase of steam emission and temperature at crater fumaroles. The fluid loss from a shallow geothermal reservoir is the most likely cause of the subsidence recorded at La Fossa Cone during the same period. This clear linkage between the deformation signals and hydrothermal activity, has important implications in the hazard assessment and monitoring of the Vulcano activity.