Possible mechanisms of magma redistribution under Mt Etna during the 1994-1999 period detected through microgravity measurements.

We analyse a microgravity data set acquired along the Etna network and spanning a 5 year long period (1994-1999). The main feature observed is a complete gravity increase (19941996)—decrease (1997-1999) cycle mainly affecting the southeastern sector of the volcano and reaching a maximum amplitude of approximately 100 µGal peak-to-peak. The gravity changes are principally due not to elevation changes but rather to the direct gravitational effect of magma accumulation and drainage below the volcanic pile. In particular, a complete mass increase-decrease cycle took place within a 2-4 km bsl deep reservoir that is elongated, oriented NNW-SSE, and lies in the southeastern sector of the volcano. The observed cycle accounts for a volume of magma ranging between 3 and 10 × 10[sup 8] m³, which is believed to have passed through the inferred reservoir between 1994 and 1999. Since most of this magma has not been erupted nor accumulated at shallower levels (at the summit Etna stations a compatible gravity increase has not been observed), it could have been recycled by the Etna plumbing system, sinking down to a level deeper than that of the inferred source. [ABSTRACT FROM AUTHOR]