Relationship between seismo-volcanic activity and outgassing at Mt. Etna (Italy) in the period January 2007-December 2015

Basaltic volcanoes are characterised by persistent periods of eruptive activity, considerable outgassing and a range of geophysical manifestations. By integrating monitoring data in a multi-parametric approach, volcano observatories attempt to predict eruptions and detect eruptive precursors. Among the monitoring data stream, the strength between volcanic seismic signals and SO2 flux plume outgassing cover a main role. Our study focuses on the relationship between the plume bulk SO2 fluxes, low-frequency seismic transients and their link with the eruptive activity at Mt. Etna between January 2007 and December 2015. Both parameters were statistically explored and their association summarised in an index named ‘Pressurization index’ (Pi). Results revealed multi-monthly cyclicality of the Pi, showing rapid fluctuations in relation to changes in the volcano activity. In particular, prolonged periods of high values of the index coincide with the decrease/end of eruptive stages. More specifically, waxing-waning phases with high values of the Pi, were observed during the occurrence of explosive sequences at the summit Central Craters followed by the resumption of eruptive activity. The behaviour of the Pi with respect to volcanic activity might suggest pressurisation/depression episodes caused by changes in the viscosity grade of the uppermost part of the magmatic column. These changes may modulate the degassing efficiency with consequently pressure increase within the shallow feeding system, leading to sudden explosions and more in general eruptive activity. Results achieved in this study might lead to a better understanding of the relationship between eruptive activity and monitored parameters at Mt. Etna and basaltic volcanoes worldwide.