Seismic and infrasound signals at Mt. Etna: Modeling 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.