Statistical analysis of the volcano seismicity during the 2007 crisis of Stromboli, Italy: a 3-day oscillatory signal as onset of the activity

Abstract

We analyze the volcano seismicity recorded during the 2007 eruption of Stromboli. Data-set is composed of the continuous recordings of a three-component broad-band seismometer and of a strainmeter. Starting from the characterization of the standard activity as a stationary phase of equilibrium, we investigate the non-equilibrium phase of the effusive process. A statistical analysis of the explosions reveals that the occurrence is always driven by a Poisson process as for the standard activity, even approaching the effusion phase, with the only difference in shortening the inter-times just during the effusion. A slightly different process can be advocated for the swarms of the explosions, because a maximum in the distribution of inter-times can be evidenced. Regarding the amplitudes of the explosion-quakes, they have a log-normal distribution until the effusion onset as in the standard Strombolian activity. The actual departure from that stationarity seems to be traced by an early deformative response at very long period. It appears as a transient oscillating signal characterized by a period of about three days that modulates the explosion amplitudes. In a conceptual organ pipe-like model it is related to the chocking of the pipe. The successive activity can be interpreted as the response of volcano to restore the equilibrium condition.