From the standard Strombolian activity to the volcanic crises at Stromboli (Italy): an overview of the seismic signatures

The properties of the seismicity of Stromboli volcano preceding and during the 2002-2003 and 2007 crises have been reviewed on a statistical basis. Both the crises show the response to the rising magma in the seismic observations but on very different time scales. A statistical analysis on all explosions reveals that the occurrence is always driven by a Poisson process in both crises even when an effusion phase occurs, with the only difference in shortening the inter-times just during the effusion. A slightly different process can be advocated for the swarms which display an unimodal distribution. Regarding the amplitudes of the explosion-quakes, they have a log-normal distribution until the effusion onset as in the standard Strombolian activity (stationary phase). The departure from the stationarity to a non-stationary phase seems to be traced by a precursor evidenced in the tidal regime, which is related with the duration of the crises. It appears as a transient oscillating signal at about 3-days that modulates the explosion amplitudes in 2007 and as a variation in the inter-occurrence among explosions in the longer crisis of 2002-2003. The successive activity can be interpreted as the response of volcano to restore the equilibrium condition. The escape from the equilibrium produces, at first, variations in the usual statistics of the explosions, then it leads to the lava effusion and to a pressure drop in the plumbing system that induces a deep gas slug nucleation and the excitation of low frequencies