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Analysis of sustained long-period activity at Etna Volcano, Italy
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Abstract
Following the installation of a broadband network on Mt. Etna, sustained Long- Period (LP) activity was recorded
accompanying a period of total quiescence and the subsequent onset of the 2004–2005 effusive episode. From c. about 56000
events detected by an automatic classification procedure, we analyse a subset of about 3000 signals spanning the December 17th,
2003–September 25th, 2004, time interval. LP spectra are characterised by several, unevenly-spaced narrow peaks spanning the
0.5–10 Hz frequency band. These peaks are common to all the recording sites of the network, and different from those associated
with tremor signals. Throughout the analysed time interval, LP spectra and waveforms maintain significant similarity, thus
indicating the involvement of a non-destructive source process that we interpret in terms of the resonance of a fluid-filled buried
cavity. Polarisation analysis indicates radiation from a non-isotropic source involving large amounts of shear. Concurrently with LP
signals, recordings from the summit station also depict Very-Long-Period (VLP) pulses whose rectilinear motion points to a region
located beneath the summit craters at depths ranging between 800 and 1100 m beneath the surface. Based on a refined repicking of
similar waveforms, we obtain robust locations for a selected subset of the most energetic LP events from probabilistic inversion of
travel-times calculated for a 3D heterogenous structure. LP sources cluster in a narrow volume located beneath the summit craters,
and extending to a maximum depth of ≈800 m beneath the surface. No causal relationships are observed between LP, VLP and
tremor activities and the onset of the 2004–2005 lava effusions, thus indicating that magmatic overpressure played a limited role in
triggering this eruption. These data represent the very first observation of LP and VLP activity at Etna during non-eruptive periods,
and open the way to the quantitative modelling of the geometry and dynamics of the shallow plumbing system