Insights into the internal dynamics of Etna volcano through discrete and continuous microgravity observations

Abstract

Discrete and continuous microgravity observations have been routinely performed at Mt. Etna since 1986 and 1998, respectively. Besides furnishing a full view of how gravity measurements from Etna are accomplished and reduced, this paper is a collection of case studies aimed at demonstrating the potential of microgravity studies to investigate the plumbing system of an active volcano and detect forerunners to paroxysmal volcanic events. As for discrete measurements, case studies relative to the 1994-96 and 2001 periods are reported. During the first period, the observed gravity changes are interpreted within the framework of the strombolian activity which occurred from the summit craters. Gravity changes observed during the first 9 months of 2001 are directly related to the underground mass redistributions who preceded, accompanied and followed the July-August 2001 flank eruption of Etna. As for continuous measurements, a three-year (1998-2000) sequence and a 48-hour (26-28 October 2002) sequence, both from PDN station, are presented and discussed. The first one is maybe the longest continuous gravity sequence ever acquired at a station very close to the summit zone of an active volcano. It allows to discover the cyclic character of a source whose geometrical characteristics are retrieved through data from discrete measurements. The second sequence is also likely to represent an unique item: a gravity sequence encompassing the onset of an eruption and coming from a station only 1.5 km from the eruptive fissures. It allows some constraints to be set on the characteristics of the intrusive mechanism leading to the eruption.