In soil radon anomalies and volcanic activity on Mt. Etna (Italy)

We present here the first attempt to understand the fast dynamics of an active basaltic volcano, namely Mt. Etna using soil gas radon measured in some sites located in strategic places around the volcano. Data were measured continuously from July 2015 to February 2017 and the raw signals were treated in order to filter out all possible periodic components that are normally due to non-volcanic factors, applying a method that does not require acquisition of other parameters, which are not always available. The residual signals highlighted seven anomalous changes, with radon values reaching levels from 2 to 5 times higher than the normal background. In six out of seven cases, anomalies were almost contemporaneous in all or almost all of the sites, indicating a common source for the observed radon variations. The pattern of anomalies suggests a transient wave-like propagation in the space/time domain, compatible with pressure-induced displacement of the gas. The observed patterns are most probably caused by the rapid upward motion of gas-rich magma into the volcano conduits, as almost all anomalies precede or accompany major volcanic events. In some cases, an alternative explanation could be the strong and sudden strain releases through earthquakes swarms, with consequent variations in the permeability of rocks at a large scale, given the apparent correlation between those anomalies and intense seismicity.