Spectral and Dynamical Evolution of Volcanic Tremor Priori to the 5 April 2003 Explosion at Stromboli Volcano

Stromboli is well known for its continuous strombolian activity. However, from time to time (the latest in 1985/1986) the volcano enters an effusive phase. The volcano is currently (July 2003) in one of these phases, which started on 28 December 2002. However, this effusive phase showed different paroxysmal events which led to considerable hazard for Stromboli inhabitants and tourists. On 30 December 2002 a major sector collapse interested the Sciara del Fuoco slope, leading to the formation of a tsunami. On 5 April 2003 a powerful explosion, which can be compared in size with the latest one of 1930, covered with bombs a good part of the normally tourist-accessible summit area. The explosion was not preceded by any classical precursor, although the island was effectively monitored by a dense deployment of instruments installed by Civil Defense after the 30 December 2002 tsunami. In this paper we investigate the spectral and dynamical evolution of the volcanic tremor recorded prior to the 5 April 2003 event, in order to estimate the timescale of the dynamical phase that lead to the paroxysmal event. This could give information on the existence of possible useful precursors.