Statistical analysis of the volcano seismicity during the 2007 crisis of Stromboli, Italy.

We analyze the volcano seismicity recorded during the pre- and co-eruptive regimes of the 2007 effusive crisis at Stromboli volcano (Italy). Data-set is composed of the continuous recordings of a three-component broad-band seismometer and of a Sacks-Evertson strainmeter. Starting from the characterization of the non effusive phase as a stationary state of equilibrium, we investigate the effusive phase as a non-equilibrium state. A statistical analysis reveals that the explosion occurrence is always driven by a nearly Poissonian process, as for the standard activity, even during the effusive phase, with the only difference in shortening the inter-times. Explosion-quake amplitudes are lognormally distributed until the effusive phase, becoming then broader. This indicates that many scales are involved. A slightly different process can be advocated for the swarms of the explosions occurring during the effusive phase. This suggests that the dynamics of the exsolution and/or aggregation of the gas slugs should differ from the nucleation mechanism responsible of the standard Strombolian activity. The pre-eruptive regime is characterized by a very long deformative signal that appears as a transient oscillating signal with a period of about three days that modulates the explosion amplitudes. In a conceptual vibrating cavities model, it is related to a chocking phenomenon induced by magma injection, which in turn leads to the effusion.