Temporal variations in gravity at Mt. Etna (Italy) associated with the 1989 and 1991 eruptions

Results are presented from 11 microgravity surveys on Mt. Etna between 1987 and 1993, a period including the major 1989 and 1991-1993 flank eruptions and subordinate 1990 activity. Measurements were made with LaCoste and Romberg D-62 and D-157 gravity meters along a network around the volcano between 1000 and 1900 m a.s.l. and, since 1992, a N-S summit profile. Gravity changes of as much as 200 mu Gal were observed at scales from the size of the summit region to that of the volcano. None was associated with significant changes in ground elevation. The data show an increase in gravity for 2 years before the 1989 eruption. The increase is attributed to the accumulation of magma (0.25-1.7 x 10(9) m(3)) in an elongate zone, oriented NNW-SSE, between 2.5 and 6 km below sea level. Part of this magma was injected into the volcanic pile to supply the 1989 and 1990 eruptions. It also probably fed the start of the 1991-1993 eruption, since this event was not preceded by significant gravity changes. A large gravity increase (up to 140 mu Gal) detected across the volcano between June and September 1992 is consistent with the arrival in the accumulation zone of 0.32-2.2 x 10(9) m(3) of new magma, thus favoring continued flank effusion until 1993. A large gravity decrease (200 mu Gal) in the summit region marked the closing stages of the 1991-1993 event and is associated with magma drainage from the upper levels of Etna's central feeding system.