Space-time gravity variations to look deep into the southern flank of Etna volcano

Abstract

A microgravity 14-year-long data set (October 1994 - September 2007) recorded along a 24-kilometer East-West trending profile of 19 stations was analyzed to detect underground mass redistributions related to the volcanic activity involving the southern flank of Mt Etna volcano (Italy). A multiresolution wavelet analysis was applied to separate the volcano-related signal from the unwanted components due to mainly instrumental, human-made and seasonal effects. The residual space-time image evidenced two complete gravity increase/decrease cycles mainly affecting the central and eastern stations of the profile. The first gravity increase (early-1995 to end-1996) – decrease (end-1996 to late-1998) cycle reached a maximum amplitude of approximately 90 µGal. The second gravity increase (mid-1999 to mid-2000) – decrease (mid-2000 to early-2004) cycle attained an amplitude of about 80 µGal peak-to-peak. After about five years of a persistent negative gravity anomaly, a new semi-cycle started at the end of 2006 and continued during the last survey carried out in September 2007. We modeled the 1994-2007 gravity anomalies using a Quadratic Programming algorithm to infer the position and the evolution of the sources beneath the profile. The computed positive mass variations of about 1.05 x 1011 kg were interpreted as magma accumulation, while negative mass changes of about -1.20 x 1011 kg could reflect either magma migration or opening of new voids by tectonic tensile stresses within a source volume, where tensional earthquakes occurred