Microgravity studies at Mount Etna (Italy)

Abstract

At Mt Etna a gravity network for discrete measurements has been regularly surveyed since 1987. An increase in gravity before the 1989 eruption has been attributed to the accumulation of magma (1.7x109 m 3 ) in an elongate zone, oriented NNW-SSE, between 2.5 and 6 km below sea level. The 1991-1993 eruption was preceded by shallower gravity increases along the dry fracture developed during the 1989 eruption. These increases were modelled in terms of passive dyke intrusion into the fracture. A new strong gravity increase was detected between June and September 1992 and is consistent with the arrival in the same deep accumulation zone of 2.2x10 9 m 3 of new magma, which allowed continued flank effusion until 1993. Localized gravity variations caused by magma level changes inside the upper levels of the central conduits marked the final stage of the 1991-1993 eruption. Between September 1994 and July 1996 mass redistributions within the volcanic edifice were detected and correlated with the ensuing 1995-96 explosive summit activity. Meanwhile, independent measurements detected some 3x10 8 m 3 of magma accumulating 2-3 km below sea level within the usual NNW-SSE trending elongate zone between October 1995 and November 1996. Three continuously running stations have also been installed at Mt Etna since 1997. High-quality data sequences have been acquired even at sites close to the volcano summit craters. Once the effect of external perturbations has been removed, continuous gravity changes acquired at PDN station are within 10 μGal of gravity changes assessed simultaneously by discrete relative observations at a site very close to the continuous station.