Gravity studies on active Italian volcanoes: a comparison between absolute and relative gravimetry

Gravity studies aimed at detecting volcanic inputs, precursors to a pre- eruptive state are in process on active Italian volcanoes. To achieve this goal high precision absolute and relative gravity measurements are carried out to identify time variations of the gravity field caused by the ascent of magmatic masses. Since 1980, gravity surveys have been carried out at Mt. Etna, the Aeolian Islands, Pantelleria, Ischia, Mt. Vesuvius and the Campi Flegrei. Absolute measurements of gravity acceleration date back to 1986 and are carried out on selected stations of the relative networks. The absolute stations work as a reference for the relative networks. Moreover, by periodically surveying them, they provide the long-term components for the temporal trend of gravity changes. Intercalibrated LaCoste and Romberg, model D, gravity meters are employed for relative gravity measurements. Absolute measurements of gare carried out by means of the ba1listic Absolute Gravimeter (symmetrical rise and fall method) built at the Istituto di Metrologia \\\"G. Colonnetti\\\" of the Italian National Research Council. Moreover, constantly recording gravity stations are also working at Mt. Vesuvius and Mt. Etna. Their aim is to keep continuous records of the changes versus the time of the gravity field and tidal parameters, possibly consequence of the variations in the physical state of the volcanoes monitored. The absolute stations have been surveyed more than once at Mt. Vesuvius, at the Island of Vulcano and on Mt. Etna. The ensuing comparison between the gravity changes observed by relative and absolute measurements indicates that a combination of both measurements is the most complete and reliable way of defining the long- and short-term space-time evolution of the gravity field associated to volcanic dynamics. The main results concern Mt. Vesuvius and Mt. Etna, where the gravity changes obtained through absolute, relative and recording gravimetry have been associated, respectively, to an increase in seismic activity and to an eruptive event.