Spectral analysis of volcanic tremor associated with the 1993 paroxysmal events at Stromboli

Abstract

Abstract The year 1993 was characterized by three paroxysmal phases at Stromboli: two (February and October) consisted of a series of violent explosions, the third (May) of strong Strombolian activity, accompanied by a small lava flow. Here we investigate the characteristics of the tremor before, during and after these paroxysmal phases. One evident feature is that the three phases analysed are significant expressions of the Strombolian activity. In fact, they mark a considerable change in the dynamics of the volcano. The energy content, both in terms of volcanic tremor and of number of events drops to very low values after the periods of intense activity. Concerning the evolution of the spectral content of the tremor before the paroxysmal phases, there seems to be a concentration of energy in the low frequency range (below 5 Hz for the crisis in May; below 3 Hz for the February and October events), with clearly identifiable peaks in some cases, while after the crisis the spectrum becomes again much more scattered. This transition, especially in the case of the strong explosions of February and October, happens abruptly. A continuous monitoring of the spectral content of volcanic tremor on Stromboli can therefore be a useful tool to identify the beginning of an instability which may lead to a paroxysmal phase.