Gas bubble dynamics model for shallow volcanic tremor at Stromboli

Abstract

Abstract. Volcanic tremor at Stromboli (Aeolian islands, Italy) is correlated to small infrasonic transients [Ripepe et al., 1996] which repeat almost rythmically intime in a range between 0.8 and 1.2 s. We demonstrate that infrasonic transients are associated to small gas bubble (-•0.5 m) burstings which produces no transients in the seismic signal. Tremor ground displacement attenuates with the inverse of the distance from the craters indicating that the source is shallow. Short-term energy release shows that infrasonic and seismic signals are linked to the same dynamical process, while at the long-term scale it is evident that the two signals are controlled by two distinctive mechanisms. We suggest that the possible physical model acts in two steps' first, gas coalescence and, then, gas bursting. In our model, the seismic signal is related to the coalescence of a gas bubble from a layer of small bubbles, while the infrasonic signal is linked to the bursting of the bubble when it reaches the magma surface. Gas bubbles could form by free coalescence in magma or could be forced to coalesce by a structural barrier. We calculate that forced coalescence induces in magma a pressure change (-.•104 Pa) 2 orders of magnitude higher than free coalescence, and it explains best the tremor ground displacement (10 -• m) recorded at Stromboli. Moreover, forced coalescence evidences the role of a structural barrier, such as a dike, in volcanic tremor source dynamics. In this gas dynamic process, the delay time of 1-2 s between infrasonic pulses could reflect the gas nucleation interval of basaltic magma [Thomas et al., 1993;Manga, 1996]. We propose that the source function for the shallow volcanic tremor at Stromboli could be the viscoelastic reaction of the magma to the pressure decrease induced by gas bubble growth rate under constant depressurization. The spectrum of our source function is controlled by the time duration of the pressure pulse, which represents the viscoelastic relaxation time of the magma and gas bubble growth rate. The predicted asymptotic decay of the frequency contents fits t;he spectral behavior of the vocanic tremor ground displacement recorded at Stromboli. We show that the same spectral behavior can be found in ground displacement spectra of volcanic tremor recorded on different volcanoes.

Passive vs. active degassing modes at an open-vent volcano (Stromboli, Italy)

“…The role of puffing in the degassing dynamics at Stromboli has been previously investigated only by indirect analysis of thermal image and infrasonic data (Ripepe and Gordeev, 1999). Here we provide the first direct gas measurement based constraints on the total SO 2 mass and flux rate from puffing.…”

“…Gas puffing activity at Stromboli has been characterised over the last two decades by combined analysis of infrasonic transients and thermal imagery (Ripepe and Gordeev, 1999;Harris and Ripepe, 2007). Based on outcomes of geophysical model calculations predicated upon results of the above acoustic and thermal studies, it was concluded that puffing activity may account for a substantial fraction ( $ 45%) of Stromboli's total gas flux Ripepe et al, 2008).…”

Monitoring methane emission of mud volcanoes by seismic tremor measurements: a pilot study

“…In particular, energy bursts commonly observed in the infrasonic range (few hertz to tens of hertz ), both in acoustic and seismic measurements, were associated with bubble dynamics (resonance or breaking) inside the volcano reservoir (e.g. Ripepe and Gordeev, 1999;Vergniolle and Brandeis, 1994;Vergniolle et al, 1996;Vergniolle and Caplan-Aurebach, 2004;.…”

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

“…Assuming the resonant model, frequency content of tremor may depend on the interactions between viscous attenuation of magma and elastic properties of ground [Chouet, 1985]. However, tremor spectral content may be also modeled as the viscoelastic reaction of the magma-gas medium to pressure changes in magma induced by gas expansion [Ripepe and Gordeev, 1999].…”