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

Abstract

During a seismo‐acoustic experiment we recorded volcanic tremor around the summit craters of Mt. Etna volcano. Tremor shows amplitude modulation, which disappear ≈ 900 m from the crater area. The infrasonic wavefield is coherent even at distances of ≈ 750 m. Time delay between infrasonic transient is stable around 1.3 s and is consistent with the position of the source in the Voragine crater. Amplitude modulation of tremor 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 volcanic tremor decays over increasing distances according to geometrical spreading of body waves. Tremor wavefield shows a linear polarization following the same time occourrence as the infrasonic pulses. Polarization azimuth indicates 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 superimposition of small impulsive sources acting at 1–2 s rate caused by pressure instability during magma degassing.