Gravity studies at Etna volcano: a comparison between relative and absolute gravity measurements

The INGV has been operating at Mt Etna a discrete gravity network since 1986 and three continuous gravity stations since 1998. The combined use of discrete and continuous gravity measurements has provided, through the detection of phenomena with a wide range of evolution rates (periods ranging from minutes to years), both substantial improvements in the knowledge of the dynamics of the shallow plumbing system at Etna and the identification of any gravity transient before and during the last volcanic eruptions. Recently, with the aim of compare relative microgravity measurements routinely acquired on Etna volcano using spring gravimeters with absolute gravity observations, we performed two surveys in June 2007 and July 2008 by using the new IMGC-02 transportable absolute gravimeter. The IMGC-02 transportable instrument, developed by INRiM - Torino, adopts the absolute ballistic method, which was recognized at international level (Comité International des Poids et Mesures - CIPM) as primary method of measurement of the acceleration due to gravity. Taking into account the logistic situation of Etna, four absolute gravity stations were settled in 2007, while a fifth station was installed in 2008. Four of them were located very close to the active craters at: (i) the Serra la Nave Astrophysical Observatory (1740 m a.s.l.); (ii) the Montagnola (2500 m a.s.l.); (iii) the Pizzi Deneri Volcanological Observatory (2810 m a.s.l.); and the newest (iv) the Caserma Donnavita (1250 m a.s.l.). One absolute station was installed out of the volcanic area, inside the gravity laboratory of INGV - Catania, to be adopted as reference. We present the results obtained by comparing relative and absolute gravity measurements and their implications on the latest Etna eruption started on 13th May 2008.