Very long period volcanic tremor at Stromboli volcano, Italy

We analyze a long time–space series of Stromboli volcanic tremors. A very-low-frequency content in the range of 0.02–0.5 Hz has been found by using spectral analysis and independent component analysis. Independent component analysis is an entropy-based technique. We observe the occurrence of a component having a period of 30 sec. Polarization analysis shows that the wave field comes mainly from the crater area, well evidenced by seismometers located around the summit ring, whereas the radiation becomes increasingly scattered at stations located around the base of this volcano. Based on its apparent velocity, the 30-sec component appears to be a slow wave, related to inhomogeneities of the source and/or gas-pressure fluctuations inside the shallow plumbing system. Introduction Stromboli is one of the most active volcanoes in the world, characterized by persistent explosive activity superimposed on a background volcanic tremor. During recent years, Strombolian events, particularly at high frequencies (0.5 Hz), have been studied by different authors (Del Pezzo et al., 1992; Chouet et al., 1997; Saccorotti and Del Pezzo, 2000; Acernese et al., 2003). Many results have been extracted concerning spectral features, polarization analysis, location, and modeling of the source, leading to the general agreement that tremor and explosions seem to be generated by the same dynamic source process; namely, the source of tremor and explosions at high frequency seems to be located at a shallow depth beneath the active craters. A preliminary study carried out by Neuberg et al. (1994) on broadband observations showed that Strombolian eruptions can produce signals with periods of 10 sec or longer. Moreover, very-long-period events have been revealed on other volcanoes in the world (Rowe et al. 1998; Ohminato et al., 1998). Since then, much attention has been devoted to the study of the broadband nature of Strombolian events and, recently, explosion quakes at low frequency (0.02–0.5 Hz) have been analyzed, constraining the geometry and the dynamics of the source (Chouet et al., 1999, 2003); namely, the source associated with explosions (whose time length ranges from 5 to 15 sec) is localized below the same crater area previously recognized for the high-frequency seismograms. On the contrary, the characteristics of tremor at low frequency are still unknown, so the aim of our work is to study a long series of tremors in the range of 0.02–0.5 Hz. This could give new insight into the fluid-dynamic mechanisms involved in