Features of seismic events and volcanic tremor during the preliminary stages of the 1991-1993 eruption of Mt. Etna

The study of the spectral features of volcanic tremor and low frequency events (l.f.e.) recorded before and during the preliminary phases of the powerful 1991-1993 eruption of Mt. Etna is briefly described. Significant modifications were observed in the spectral signature of l.f.e. before the onset of the eruptive event, as well as in the temporal distribution of the volcanic tremor dominant frequencies. We interpret both l.f.e. and tremor changes in terms of a spatial modification of the source, as the paroxysmal eruptive activity is approaching. Such findings also appear quite interesting for the identification of markers of the modifications which some seismic events of the volcano undergo in the early stages heading the occurrence of an eruption