Shear wave splitting changes associated with the 2001 volcanic eruption on Mt Etna

The time delays and polarizations of shear wave splitting above small earthquakes show variations before the 2001 July 17-August 9 2001 flank eruption on Mt Etna, Sicily. Normalized time delays, measured by singular value decomposition, show a systematic increase starting several days before the onset of the eruption. On several occasions before the eruption, the polarization directions of the shear waves at Station MNT, closest to the eruption, show 90 degrees-flips where the faster and slower split shear waves exchange polarizations. The last 90 degrees-flip being 5 days before the onset of the eruption. The time delays also exhibit a sudden decrease shortly before the start of the eruption suggesting the possible occurrence of a 'relaxation' phenomena, due to crack coalescence. This behaviour has many similarities to that observed before a number of earthquakes elsewhere.