A model for internal dynamical processes on Mt. Etna

Results of studies concerning seismic activity (earthquakes and tremor) and its relation with eruptive events on Mt. Etna are briefly discussed. Seismic records collected from 1977 up to 1986 have been analysed and special care was given to observation of changes in some seismic parameters, such as the rate of microseismic activity, the amplitude and dominant spectral peaks of the volcanic tremor, which vary significantly in relation to the occurrence of eruptive events. The systematic nature of variations in the seismic parameters considered allows us to propose a preliminary model which puts all the observations made so far into a wide context giving an interpretative hypothesis of the magma uprise mechanisms preceding either summit or flank eruptions. Summit eruptions are modelled in two stages and occur without changes in the seismic parameters considered, except for a sharp increase in tremor amplitude almost coincident in time with the eruption onset. Flank eruptions are modelled in three stages and follow a simultaneous change of all the mentioned parameters with time. the onset of these eruptions is in fact preceded by an increase in both the daily number of shocks and the amplitude of the volcanic tremor as well as a shifting from relatively high values in the dominant peaks of the tremor spectra, which appear in the pre-eruptive stage, towards usual lower frequency values (1.0–2.3 Hz).