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Abstract: 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. [Copyright &y& Elsevier]

The pattern of volcanic tremor accompanying the 1989 September eruption at the south-east summit crater of Mount Etna is studied. In specific, sixteen episodes of lava fountaining, which occurred in the first phase of the eruption, are analysed. Their periodic behaviour, also evidenced by autocorrelation, allows us to define the related tremor amplitude increases as intermittent volcanic tremor episodes. Focusing on the regular intermittent behaviour found for both lava fountains and intermittent volcanic tremors, we tried an a posteriori forecast using simple statistical methods based on linear regression and the Student’ t-test. We performed the retrospective statistical forecast, and found that several eruptions would have been successfully forecast. In order to focus on the source mechanism of tremor linked to lava fountains, we investigated the relationship between volcanic and seismic parameters. A mechanism based on a shallow magma batch ‘regularly’ refilled from depth is suggested. [Copyright &y& Elsevier]

Abstract: During 1999, the volcanic activity at Mt. Etna was both explosive and effusive at the summit craters: Strombolian activity, lava fountains and lava flows affected different areas of the volcano, involving three of the four summit craters. Results from analysis of the 1999 volcanic tremor features are shown at two different time scales. First, the long-term time variation of the features of the volcanic tremor (including spectral and polarization parameters), during the entire year, was compared with the evolution of the eruptive activity. This approach demonstrated the good agreement between tremor data and observed eruptive activity; the activation of different tremor sources was suggested. Then, a more refined analysis of the volcanic tremor, recorded during 14 lava fountain eruptions, was performed. In particular, a shift of the dominant frequencies towards lower values was noted which corresponds with increasing explosive activity. Similar behaviour in the frequency content has already been observed in other explosive eruptions at Mt. Etna as well as on other volcanoes. This behaviour has been explained in terms of either an increase in the tremor source dimension or a decrease in the sound speed in the magma within the conduit. These results confirm that the volcanic tremor is a powerful tool for better understanding the physical processes controlling explosive eruptions at Mt. Etna volcano. [Copyright &y& Elsevier]