Volcanic tremor at Mt Vesuvius associated with low frequency shear failures

Mt Vesuvius has been dormant since the eruption occurred in 1944, after which the conduit closed and the volcano entered a quiescent state. Only a minor seismic activity, characterized by low magnitude volcano-tectonic (VT) earthquakes, testifies that the magmatic system is still active. In this paper we report the fist quantitative analysis of volcanic tremor discovered at Vesuvius through the analysis of array data. A seismic array installed in 2012 improved the monitoring performance of the local network, permitting the identification of low amplitude coherent signals. Many of such coherent signals recorded during the last few years have been classified as volcanic tremor. We selected 22 tremor events based on their amplitude and on the number of available stations, and performed detailed analysis aimed at location and characterization of the source. They are characterized by low frequency, duration of a few minutes, and the strongest episodes are recorded at distance up to 90 km from the volcano. In many cases we could identify P-S wave pairs in the seismograms that allowed a precise location of the source depth, which is in the range between 5 km and 6.5 km below the crater. Waveform features, spectral analysis, and comparison with VT earthquakes located at the same depth indicate that the source mechanism of the Vesuvius non-eruptive tremor is a sequence of low frequency shear failures.