New insights into banded tremor from the 2008-2009 Mount Etna eruption

We investigated the banded tremor activity occurring at Mt. Etna volcano between August-October 2008 during the 2008-2009 eruption. The banded tremor occurred in episodes lasting 25-30 min with intervals in between the episodes of about 25 min. Seismic signal analyses showed that the banded tremor was characterized by spectral contents, wavefields, and source locations that differed from the "ordinary" volcanic tremor. The infrasound recordings exhibited an intermittent infrasonic tremor alternating with the banded tremor episodes. Finally, nonlinear analyses suggested that a banded tremor system can be considered chaotic, implying (1) sensitive dependence on initial conditions, suggesting not only that a banded tremor system requires particular conditions to generate but also that slight variations of these conditions are able to greatly change the features of the banded tremor or even to stop it; and (2) long-term unpredictability, that is, the impossibility to forecast the long-term evolution of the banded tremor. On the basis of all these results and analogies with geyser models, we suggest a model of banded tremor that invokes alternating recharge-discharge phases. Banded tremor is due to "perturbations" in shallow aquifers, such as fluid movement and bubble growth or collapse due to hydrothermal boiling, triggered by the heat and hot fluid transfer from the underlying magma bodies. This heat-fluid transfer also causes an increasing pressure in the aquifer, leading to fluid discharge. During this process, the seismic radiation decreases, and, if the fluid discharge is well coupled with the atmosphere, acoustic signals are generated.