A long-term charge/discharge cycle at Mt. Etna volcano revealed through absolute gravity and GPS measurements

We present results of repeated absolute gravity and GPS measurements, carried out at Mt. Etna volcano between 2009 and 2018. Absolute gravity measurements are rarely performed along arrays of stations on active volcanoes and, through our unprecedented dataset, we highlight the possibilities of this method to track underground mass changes over long time-scales. Analysis of the residual absolute gravity data and ground deformation reveals a cycle of gravity increase and uplift during 2009 to 2011, followed by gravity decrease and subsidence during 2011 to 2014. Data inversion points to a common mass and pressure source, lying beneath the summit area of the volcano, at depth of ~ 5 km b.s.l. The bulk volume change inferred by the inversion of the deformation data can account for only a small portion of the mass change needed to explain the correspondent gravity variations. We propose that the observed relationship between gravity and vertical deformation was mostly due to the compressibility of the magma in the inferred reservoir, which, in turn, was enhanced by the presence of exsolved gas. Overall, the gravity and deformation data we present reveal a cycle of magma recharge (2009–2011) and discharge (2011–2014) to/from the inferred storage zone. During the recharge phase only degassing occurred from the summit craters of Mt. Etna. During the following phase of discharge, the magma lost from the reservoir at ~ 5 km b.s.l. fed the exceptional phase of volcanic activity during 2011–2014, when tens of lava fountaining episodes took place.