Volcanic tremor at Mt. Etna: Inferences on magma dynamics during effusive and explosive activity

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.

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