Gas bubble dynamics model for shallow volcanic tremor at Stromboli

Volcanic tremor at Stromboli (Aeolian islands, Italy) is correlated to smal l infrasonic transients [Ripepe et al., 1996] which repeat almost rythmical ly in time in a range between 0.8 and 1.2 s. We demonstrate that infrasonic transients are associated to small gas bubble (similar to 0.5 m) burstings which produces no transients in the seismic signal. Tremor ground displace ment attenuates with the inverse of the distance from the craters indicatin g that the source is shallow. Short-term energy release shows that infrason ic and seismic signals are linked to the same dynamical process, while at t he long-term scale it is evident that the two signals are controlled by two distinctive mechanisms. We suggest that the possible physical model acts i n 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 lay er 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 ba rrier. We calculate that forced coalescence induces in magma a pressure cha nge (similar to 10(4) Pa) 2 orders of magnitude higher than free coalescenc e, and it explains best the tremor ground displacement (10(-5) m) recorded at Stromboli. Moreover, forced coalescence evidences the role of a structur al 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 depress urization. The spectrum of our source function is controlled by the time du ration of the pressure pulse, which represents the viscoelastic relaxation time of the magma and gas bubble growth rate. The predicted asymptotic deca y of the frequency contents fits the spectral behavior of the vocanic tremo r ground displacement recorded at Stromboli. We show that the same spectral behavior can be found in ground displacement spectra of volcanic tremor re corded on different volcanoes.