Analysis of simultaneous gravity and tremor anomalies observed during the 2002-2003 Etna eruption

In this paper we discuss the data collected by a large aperture array of broadband seismometers and a continuously recording gravity station during the 2002-2003 eruption of Etna volcano (Italy). Seismic signals recorded during the eruption are dominated by volcanic tremor whose energy spans the 0.5-5 Hz frequency band. On three different occasions (12 November, 19-20 November and 8-9 December 2002), we observed marked increases of the tremor amplitude (up to a factor of 4), which occurred simultaneously with gravity decreases (up to 30 μGal). The three concurrent gravity/tremor anomalies last 6 to 12 hours and terminate with rapid (up to 2 hours) changes, after which the signals return back to their original levels. Based on volcanological observations encompassing the simultaneous anomalies, we infer that the accumulation of a gas cloud at some level in the conduit plexus feeding a new eruptive vent could have acted as a joint source.This study highlights the potential of joint gravity-seismological analyses to both investigate the internal dynamic of a volcano and to improve the confidence of volcanic hazard assessment.