Seismic and infrasonic evidences for an impulsive source of the shallow volcanic tremor at Mt. Etna, Italy

During a seismo-acoustic experiment we recorded volcanic tremor around the summit craters of Mt. Etna volcano. Tremor shows amplitude modulation, whic h disappear approximate to 900 m from the crater area. The infrasonic wavef ield is coherent even at distances of approximate to 750 m. Time delay betw een infrasonic transient is stable around 1.3 s and is consistent with the position of the source in the Voragine crater. Amplitude modulation of trem or is well correlated (0.72) with infrasound amplitude with a time lag of 0 .37 s, coherent with a shallow position of the source. Amplitude of volcani c tremor decays over increasing distances according to geometrical spreadin g of body waves. Tremor wavefield shows a linear polarization following the same time occourrence as the infrasonic pulses. Polarization azimuth indic ates that wavefield rectilinearity is mostly due to P-waves. We infer that most of the volcanic tremor we recorded at Mt. Etna is generated by superim position of small impulsive sources acting at 1-2 s rate caused by pressure instability during magma degassing.