Time variation of spectral and wavefield features of volcanic tremor at Mt. Etna (January–June 1999)

We have studied the volcanic tremor recorded at Mt. Etna during January–June 1999, by using three-component seismic stations. This time period was characterised by both explosive and effusive eruptions occurring at the summit craters.We found significant time variations in the trend of the overall spectral amplitude of tremor, as well as in the dominant spectral peaks. Moreover, the tremor wavefield features (polarization, particle motion and ratios between the amplitude of the three components of the ground velocity) have been studied too, confirming significant time variations. This leads us to suggest the existence of at least two tremor sources: a shallow one, mainly characterised by relatively high frequencies (3.5–7 Hz), is linked to the upper portions of the active conduits, and directly related to the observed eruptive activity. The latter deeper source was active for only a few weeks, and characterised by frequencies lower than about 2.5 Hz. It had no relationship with the observed eruptive activity, and was speculatively related to the gas exsolution from fresh magma refilling a small batch.