Features of volcanic tremor associated with recent eruptive episodes at Mt. Etna volcano (Italy)

The South East crater of Mt. Etna has shown since late summer of 1999 a persistent activity with periodic paroxysmal episodes characterised by strombolian activity and lava fountaining. Each eruptive episode has been marked by a significant increase in the tremor energy, which, as usually, was observed both in the time and frequency domains. Present study aims to show the spectral features of volcanic tremor recorded at the stations of TDF and SEN. located at the top and at intermediate elevation on the volcano, respectively. Each eruptive event was characterised by a gradual increase of the tremor amplitude at its beginning and a more rapid decrement of amplitude in its final stage. The maximum amplitude was usually greater than ten times the usual tremor amplitude. The average duration of each eruptive episode was ranging between 45 and 90 minutes. Spectral features of tremors, sampled both during paroxysmal and quite periods, do not show significant differences except a pronounced increment of the overall spectral amplitude during the eruptive episodes. The recurrence of such short eruptions was investigated, using a statistical approach, in order to look for a possible periodicity in the occurrence rate. Preliminary findings encourage pursuing in the used methodological approach, in order to achieve the recognition of significant precursors of Etnean eruptive activity.