Frequency-magnitude distribution of earthquakes at Etna volcano unravels critical stress changes along magma pathways

The high seismic productivity of volcanic areas provides the chance to investigate the local
stress conditions with great resolution, by analysing the slope of the frequency-magnitude
distribution of earthquakes, namely the b-value. Here we investigated the seismicity of Mt.
Etna between 2005 and 2019, focusing on one of the largest known episodes of unrest in
December 2018, when most of the intruding magma aborted, rather oddly, its ascent inside
the volcano. We found a possible stress concentration zone along magma pathways, which
may have inhibited the occurrence of a larger eruption. If the origin of such hypothetical
loaded region is related to tectonic forces, one must consider the possibility that geodynamic
processes can locally result in such rapid crustal strain as to perturb the release of magma.
Strong b-value time-variations occurred a few days before the unrest event, suggesting new
possibilities for investigating the volcano state and impending eruptions.