VOLCANIC TREMOR WAVE-FIELD DURING QUIESCENT AND ERUPTIVE ACTIVITY AT MT-ETNA (SICILY)

This paper reports the results of an analysis of volcanic tremor at Etna in the years 1987 to 1992. The data utilized were obtained from the Osservatorio Vesuviano's Serra la Nave permanent station and several seismic surveys carried out by means of portable digital seismic stations equipped with three-component geophones. The spatial-temporal patterns of main parameters such as intensity, spectral content, polarization and wave composition of volcanic tremor in different phases of Etna's activity were studied. Eruptive activity appears to be significantly correlated with volcanic tremor. In particular, it is worth pointing out that the magma upwelling in the terminal part of the conduit is characterized by a marked increase in low-frequency components (3-4 Hz) of the spectrum of volcanic tremor. Hence, volcanic tremor at Etna can be considered as a short-period precursor of eruptions. An analysis of polarization and propagation velocity of volcanic tremor shows the presence of Love waves overlapping other seismic phases.