Combined gravimetry in the observation of volcanic processes in Southern Italy

High precision measurements of the gravity field are currently carried out on active volcanoes of Southern Italy to single out the ascent of magmatic masses which could trigger a pre-eruptive state. Different aspects of the gravity field are considered to improve the understanding of the volcano dynamics. The volcanoes considered in this paper are Mt. Vesuvius, in the Neapolitan areal and the isle of Vulcano (Aeolian Islands, Sicily). Both are affected by significant gravity changes in time and space occurring with different patterns. At Mt. Vesuvius, a recording gravity station is also operating. It is aimed to monitor the time variations of the gravity field and tidal parameters, possibly reflecting changes of the physical state of the volcano. The migrating fluids may belong in part to an active geothermal system and in part to hydrological system which control the local level of the water table. At the isle of Vulcano, vertical gravity gradient measurements are also periodically carried out on six stations of the gravity network to detect changes of the mass occurring in shallower layers. The gravity field of the isle of Vulcano is characterized by short- and long-term gravity changes, partly confirmed by the results of repeated absolute gravity measurements. They might be attributed to the migration of fluids through shallow levels of the crust, which seems to be also supported by the repeated measurements of the vertical gravity gradient. At Mt. Vesuvius, the field of the gravity changes appears to be very disturbed on the short-term scale, while a quite clear geometry results on the long-term, evidencing an area with decreasing gravity values in the western and upper Darts of the volcano and an area with increasing values at the southern-eastern base. The gravity changes are believed to result from the migration of fluids probably occurring through pre-existing fissures, A decrease of up to 60 mu Gal has been observed from 1985 to 1994 as also consistently detected by absolute measurements. Continuous monitoring of the gravity indicates an increase of the amplitudes of the main tidal waves from 1987/1990 to 1994, which could be due to a change of the elastic properties of the medium. (C) 2000 Elsevier Science Ltd. All rights reserved.