The coupling between very long period seismic events, volcanic tremor, and degassing rates at Mount Etna volcano

From December 2005 to January 2006, an anomalous degassing episode was observed
at Mount Etna, well-correlated with an increase in volcanic tremor, and in the almost
complete absence of eruptive activity. In the same period, more than 10,000 very long
period (VLP) events were detected. Through moment tensor inversion analyses of the VLP
pulses, we obtained quantitative estimates of the volumetric variations associated with these
events. This allowed a quantitative investigation of the relationship between VLP seismic
activity, volcanic tremor, and gas emission rate at Mount Etna. We found a statistically
significant positive correlation between SO2 gas flux and volcanic tremor, suggesting that
tremor amplitude can be used as a first-order proxy for the background degassing activity of
the volcano. VLP volumetric changes and SO2 gas flux are correlated only for the last part of
our observations, following a slight change in the VLP source depth. We calculate that the
gas associated with VLP signal genesis contributed less than 5% of the total gas emission.
The existence of a linear correlation between VLP and degassing activities indicates a
general relationship between these two processes. The effectiveness of such coupling
appears to depend upon the particular location of the VLP source, suggesting that conduit
geometry might play a significant role in the VLP-generating process. These results are the
first report on Mount Etna of a quantitative relationship between the amounts of gas
emissions directly estimated through instrumental flux measurements and the quantities of
gas mass inferred in the VLP source inversion.