Source characterisation of low frequency events at Stromboli and Vulcano Islands (Isole Eolie Italy)

The discussion about the source of low frequency events generally wrecorded on active volcanoes is still open and needs deeper understanding of the ph enomena involved in their generation. Most of the models view such a phenom enon as a source effect (oscillation of volcanic fluids in conduits or crac ks), although a different explanation as a path or site effect exists. In t he present paper we analysed 26 seismic signals recorded at Vulcano and 60 at Stromboli in order to put some constraints on the functional shape of th e recorded signals. They evidence all the characteristics of the low freque ncy events. The spectral analysis reveals sharp peaks in the range 0.5-4 Hz , while the cepstra suggest that the signals are composed by a two-sine kin d function. This suggestion is confirmed by the two-dimensional projections of the reconstructed phase space. The correlation dimension of the attract or is very close to 3 for both the volcanoes confirming the existence of a toroidal structure in the phase space. This implies that we can suggest as a model for the low frequency events source a physical mechanism very simil ar to a Duffing oscillator or any other quasiperiodic one. In particular in the cases of the analysed volcanoes we recognise two independent oscillati ons.