New data from borehole strainmeters to infer lava fountain sources (Etna 2011-2012)

In January 2011, eruptive activity resumed at Etna producing a new phase with frequent lava fountain episodes until April 2012. In November 2011, the first two borehole strainmeters were installed, which detected negative strain changes (similar to 0.15-0.8 strain) during the paroxysmal events. A Finite Element Model was set up to estimate accurately the tilt and volumetric strain, taking into account the real profile of the volcano and the elastic medium heterogeneity. The numerical computations indicated an elongated depressurizing source located at 0 km b.s.l., which underwent a volume change of similar to 2 x 10(6) m(3) which is the most of the magma volume erupted, while a smaller remaining part is accommodated by the magma compressibility. This shallow source cannot accumulate large magma volumes and, thus, favors short-term periodic eruptive events with a fairly constant balance between the refilling and the erupted magma.