Iacop, F.

On the basis of the results of previous works by our group, this paper aims to investigate the correlation between features of a kind of seismic events recorded at Stromboli (the so called explosion-quakes) and the source of such explosions, i.e. the different craters. The purpose is that of finding parameters in order to try an automatic assignment of new events to their crater of origin. These parameters must be searched for both in time and in frequency domain. Afterwards the stability in time of the discrimination is discussed. An important « side effect» of this analysis is the discovery that the percentage of summital explosions not producing seismic signals recordable by a 2 s seismometer is not negligible.

In this work we investigate the characteristics of the seismicity at Stromboli volcano during more than two years, i.e. from 11 May 1992 to 21 August 1994. The three paroxysmal phases of 1993 mark significant changes in the Strombolian activity; nevertheless, these are not the only ones observed. In fact, 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, accompanied by a change in the spectral content of the tremor. However, equally abrupt changes in the frequency content, not accompanied by evident intensity variations, can be observed some weeks after the end of the crises. The volcano seems therefore to behave like a dynamical system with many «quite stable » states with abrupt transitions between them. An interesting observation is the appearance of an energy concentration in the spectral sectors below 3 Hz before more violent eruptive episodes; although the duration of such a phenomenon is variable, it has to be investigated as a possible precursor of potentially dangerous activity of the volcano. A continuous monitoring of the spectral content of volcanic tremor on Stromboli is confirmed to be an essential tool in order to understand the behaviour of Stromboli volcano and to try to forecast its paroxysmal phases.