Spatiotemporal gravity variations to look deep into the southern flank of Etna volcano

A 14-year-long microgravity data set (October 1994 to September 2007) collected along a 24-km 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 (Italy). A multiresolution wavelet analysis was applied to separate the volcano-related anomalies from the unwanted components. The residual image having both spatial extension and temporal duration evidenced two complete gravity increase/decrease cycles mainly affecting the central and eastern stations of the profile. The first gravity increase (early 1995 to the end of 1996) and decrease (end of 1996 to late in 1998) cycle reached a maximum amplitude of approximately 90 mu Gal. The second gravity increase (mid-1999 to mid-2000) and decrease (mid-2000 to early-2004) cycle attained an amplitude of about 80 mu Gal. After about 5 years of a persistent negative gravity anomaly, a new semicycle started at the end of 2006 and continued during the last survey carried out in September 2007. The density changes, modeled over time since 1994 using a Quadratic Programming algorithm, are mainly located at a depth of 2-4 km bsl in a region recognized to be a preferential pathway of magma rising and an intermediate zone of magma storage/withdrawal. The computed positive mass variations of about 105 x 10(9) kg were interpreted as magma accumulation, while negative mass changes of about -120 x 10(9) kg were associated with either magma drainage or opening of new voids by tectonic stresses within a source volume, where tensional earthquakes occurred.