New Perspectives on Explosive Paroxysmal Phenomena at Stromboli Volcano (Italy)

Continuous seismic monitoring and periodic, visual observations of volcanic activity for surveillance purposes started on Stromboli volcano, Italy, in the 1980s. From 1985 on, two eruptions occurred in December 1985 - April 1986 and May 1993, along with two small overflows in 1990 and 1994, respectively. Apart from these few episodes of lava effusion, the persistent Strombolian activity of the volcano throughout these years had several fluctuations, with some episodic climax which yielded powerful explosions. According to seismic records, typical paroxysmal phases consisted of a variable number of explosion quakes in fast succession (i.e., from tens of seconds to a few minutes), associated with an outstanding increment in the amplitude of volcanic tremor. Throughout these phases - which are called explosive sequences (Falsaperla et al., 1989) - the sudden ejection of lapilli, fragments of old rock, and bombs of variable dimensions can easily affect an area larger than the crater terrace where the active craters are located. This can yield serious consequences for tourists who climb up to the top of the volcano. In this note, we study the explosive sequences of Stromboli between 1985 and 1999, providing a characterization in energy and duration. In the time span aforementioned, the occurrence frequency of similar phases was two per year on average. We propose analyses of their frequency content, comparing records obtained from two three-component, short-period (1 s) and broadband (10 - 0.01 s) seismic stations. We also investigate the time span preceding some sequences to highlight possible common features from the seismic and volcanic viewpoints. Petrological analysis indicates a different composition of the volcanic products ejected during these explosive sequences with respect to the ejecta associated with the usual Strombolian activity (Bertagnini et al., 1999). Such a finding is in agreement with seismic data, which lead us to surmise that these paroxysmal phases are the result of partial obstruction of the volcanic conduit during periods of descending magma column. The onset of the sequence, causing the sudden removal of the material which forms the obstruction, would trigger the sudden depressurization of the conduit and the fast rising of magma from depth.