Seismic and infrasound signals at Mt. Etna: modelling the North-East crater conduit and its relation with the 2008-2009 eruption feeding system

On 13 May 2008, an eruption began at Mt. Etna from an eruptive fissure that opened on the upper eastern flank of the volcano. During 12-13 May, 157 infrasonic events, together with the related seismic transients, were collected. We carried out several analyses to obtain dominant frequencies, pseudospectrograms, peak-to-peak amplitudes, source locations and time lags between infrasonic and seismic events. Spectra of the infrasonic events show two main spectral peaks in the frequency bands ~. 0.4-0.7. Hz and 1.5-2.0. Hz, respectively. Both infrasonic and seismic events were separately located below the North-East crater, where no eruptive activity was observed. Moreover, significant changes in infrasound spectral content, as well as in the infrasonic-seismic lags, were found a few hours before the beginning of the eruption. On the basis of the collected information the infrasound source mechanism was modeled as a superposition of pipe and Helmholtz resonance, also leading to outline the geometry of the shallower portion of the North-East crater plumbing system. The occurrence of these seismo-infrasonic events together with other geological and geophysical evidences, led us to inferring a direct link between North-East crater activity and the eruptive fissure. Further, based on variations over time of both spectral features and seismic-infrasonic time lag, shallowing phenomena of the free magma column inside North-East crater conduit were hypothesized. Such an uprise of magma was likely caused by a pressure increase inside the plumbing system occurring before the beginning of the 2008-2009 eruption.