To achieve a balance between uncertainty and\ud
efficiency in gravity measurements, we have investigated\ud
the applicability of combined measurements of absolute and\ud
relative gravity as a hybrid method for volcano monitoring.\ud
Between 2007 and 2009, three hybrid gravity surveys were\ud
conducted at Mt Etna volcano, in June 2007, July 2008, and\ud
July 2009. Absolute gravity data were collected with two\ud
absolute gravimeters, which represent the state of the art in\ud
recent advances in ballistic gravimeter technology: (1) the\ud
commercial instrument FG5#238 and (2) the prototype instrument\ud
IMGC-02. We carried out several field surveys and\ud
confirmed that both the absolute gravimeters can still\ud
achieve a 10 μGal or better uncertainty even when they\ud
are operated in severe environmental conditions. The use\ud
of absolute gravimeters in a field survey of the summit area\ud
of Mt Etna is unprecedented. The annual changes of the\ud
gravity measured over 2007–2008 and 2008–2009 provide\ud
unequivocal evidence that during the 2007–2009 period,\ud
two main phenomena of subsurface mass redistribution occurred\ud
in distinct sectors of the volcano, accompanying\ud
different eruptive episodes. From 2007 to 2008, a gravity\ud
change of −60 μGal was concentrated around the North-\ud
East Rift. This coincided with a zone affected by strong\ud
extensional tectonics, and hence might have been related\ud
to the opening of new voids. Between 2008 and 2009, a\ud
North-South elongate feature with a maximum gravity\ud
change of +80 μGal was identified in the summit craters\ud
area. This is interpreted to indicate recharge of a deepintermediate\ud
magma storage zone, which could have occurred\ud
when the 2008–2009 eruption was still ongoing
During the onset of 2008 volcanic crisis at Mt Etna, the near‐real time magnetic data provided a continuous updating of the volcano activity state on the northern flank. On the morning of 13th May 2008, significant local magnetic field changes marked the resumption of the eruptive activity characterized by the opening of a fracture field on the northern flank, and an eruptive fissure in the Valle del Bove. In agreement with the northward propagation of seismic events, magnetic signals at 5 stations in the summit area revealed a nearly NNW‐SSE oriented magmatic intrusion, which started at about 9:00 GMT, propagated northward for about 2 km, and stopped at 14:00 GMT before reaching the North‐East Rift. Magnetic variations, with amplitude ranging between 1.8 nT and −6.5 nT, are consistent with those calculated from piezomagnetic models, where stress‐induced changes in rock magnetization are produced by the magmatic intrusion.
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