Explosion swarms at Stromboli volcano: A proxy for nonequilibrium conditions in the shallow plumbing system

[1] In this paper we look at changes of the statistics of the stationary explosive process at a basaltic volcano as a proxy for departures from thermodynamic equilibrium conditions in the shallow plumbing system. Specifically, we investigate the explosion process of Stromboli volcano that occurred during 2002–2003, 2006–2007 and 2010–2011. The first two periods were characterized by eruptions with significant lava effusion and strong paroxysmal events, while the last one shows persistent explosive activity accompanied by minor episodes of lava flow. We use three-component seismic data recorded by broadband stations operating on the volcano and, for 2007 and 2010–2011 cases, strainmeter data from a Sacks-Evertson borehole dilatometer. For each time interval we study the explosive process by looking at the inter- occurrence times and at the amplitude distribution. Moreover, we analyze its waveforms, spectral content and polarization properties. In all three cases we find sharp increases of the explosion rate associated with swarms. Swarms are characterized by quasi-monochromatic seismic events with frequency peak close to about 3 Hz, higher amplitude than the usual explosions and variability coefficient of the inter-occurrence times close to 0.5. In correspondence to the swarms, we also observe negative variations in the strain signals, which indicate a depressurization in the shallow plumbing system. This depressurization emerges clearly from the data collected during 2010–2011, whereas it is less sharp for the 2007 episode, and has been estimated in about 105 Pa. From the polarization analysis we infer that this depressurization affects the upper 0.3–0.8 km of the plumbing system.