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

摘要

[1] In this paper we look at changes of the statistics of the stationary explosive process at a basaltic volcanoas 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 lavaeffusion and strong paroxysmal events, while the last one shows persistent explosive activity accompaniedby minor episodes of lava flow. We use three-component seismic data recorded by broadband stationsoperating on the volcano and, for 2007 and 2010–2011 cases, strainmeter data from a Sacks-Evertsonborehole 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 contentand polarization properties. In all three cases we find sharp increases of the explosion rate associated withswarms. Swarms are characterized by quasi-monochromatic seismic events with frequency peak close toabout 3 Hz, higher amplitude than the usual explosions and variability coefficient of the inter-occurrencetimes close to 0.5. In correspondence to the swarms, we also observe negative variations in the strainsignals, which indicate a depressurization in the shallow plumbing system. This depressurization emergesclearly from the data collected during 2010–2011, whereas it is less sharp for the 2007 episode, and hasbeen estimated in about 105 Pa. From the polarization analysis we infer that this depressurization affectsthe upper 0.3–0.8 km of the plumbing system.