Catching the time-variable gravity at Mt. Somma-Vesuvius volcano (Southern Italy) by means of discrete and continuous relative gravity measurements

We report the results of about 20 years of relative gravity measurements acquired on Mt. Somma-Vesuvius INGV monitoring network, together with about 9 months of continuous gravimetric recordings collected with the new generation relative gravimeter gPhoneX#116, specifically designed for continuous gravity recording. We also present the outcomes of an intercomparison experiment of the gPhone#116 conducted at the J9 gravity observatory in Strasbourg (France). In this intercomparison, we were able to check the meter scale factor with a high degree of precision by comparing them with 2 superconducting gravimeters and a FG5-type absolute ballistic gravimeter. It was also possible to carry out a detailed study of instrumental drift, a crucial topic for reliable monitoring of the long-term gravity variations in active volcanic areas. In fact, a challenge in time lapse gravimetry is the proper separation of the instrumental variations from real gravity changes eventually attributable to recharge or drainage processes of magma or fluids in the feeding systems of active volcanoes.

Since 1980s the relative gravity network of Mt- Somma-Vesuvius has evolved over time becoming progressively larger and denser. We discuss the results of the time-lapse monitoring since 2003, when the INGV network reached an almost stable configuration. The retrieved field of time gravity change shows a pattern essentially related to the ground deformation detected by the permanent GNSS network. Vesuvius is currently experiencing subsidence at a variable rate. A clear topographic effect emerges with a strong correlation with altitude, whereby higher stations subside at a greater rate, up to 7 mm/year, than those at lower altitudes. Most of the observed gravity changes can be explained by this dynamics; only a residual positive gravity is detected in the western sector of the volcano, which could be likely due to hydrological effects. A reliable tidal gravity model was derived from the analysis of the gravity records. We believe that this result should help improve the accuracy of the volcano monitoring as it will be useful for the correct reduction of tidal effects for all relative and absolute gravity measurements acquired in the area.