Precursory signal of the last eruption of Mount Etna detected by continuous gravity observations

Mount Etna volcano is characterized by a fractured edifice so that the seismic activity and the ground deformation are often minimal and even insignificant during the pre-eruption phase. In this case, the microgravity represents a good alternative volcano monitoring technique to detect the mass changes in relation with the movement of fresh magma to shallower levels. In 1992, the LaCoste & Romberg gravimeter LCR8 was installed in Serra La Nave station on the Southern flank of the Etna edifice. As the spring gravimeters are strongly influenced by the Earth tides and the meteorological parameters, it was necessary to remove their effects in order to obtain reliable gravity residuals which could be related to the volcanic activity. Before the beginning of the 1995-2001 eruption, the LCR8 recorded gravity variations of about 350 µgal (1µgal = 10nm.s-2), which are clearly correlated with seismic activity and other precursors of the paroxismal activity. These observations are not incompatible with the moderate 40µgal gravity increase at the Etna summit between the September 1994 and October 1995 discrete gravity measurements, due to the epoch of these campaigns