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Effusive eruptions are explained as the mechanism by which volcanoes restore the equilibrium perturbed by magma rising in a chamber deep in the crust. Seismic, ground deformation and topographic measurements are compared with effusion rate during the 2007 Stromboli eruption, drawing an eruptive scenario that shifts our attention from the interior of the crust to the surface. The eruption is modelled as a gravity-driven drainage of magma stored in the volcanic edifice with a minor contribution of magma supplied at a steady rate from a deep reservoir. Here we show that the discharge rate can be predicted by the contraction of the volcano edifice and that the very-long-period seismicity migrates downwards, tracking the residual volume of magma in the shallow reservoir. Gravity-driven magma discharge dynamics explain the initially high discharge rates observed during eruptive crises and greatly influence our ability to predict the evolution of effusive eruptions.

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Open-conduit volcanic systems are typically characterized by unsealed volcanic conduits feeding permanent or quasi-permanent volcanic activity. This persistent activity limits our ability to read changes in the monitored parameters, making the assessment of possible eruptive crises more difficult. We show how an integrated approach to monitoring can solve this problem, opening a new way to data interpretation. The increasing rate of explosive transients, tremor amplitude, thermal emissions of ejected tephra, and rise of the very-long- period (VLP) seismic source towards the surface are interpreted as indicating an upward migration of the magma column in response to an increased magma input rate. During the 2014 flank eruption of Stromboli, this magma input pre- ceded the effusive eruption by several months. When the new lateral effusive vent opened on the Sciara del Fuoco slope, the effusion was accompanied by a large ground deflation, a deepening of the VLP seismic source, and the cessation of summit explosive activity. Such observations suggest the drainage of a superficial magma reservoir confined between the crater terrace and the effusive vent. We show how this model successfully reproduces the measured rate of effusion, the observed rate of ground deflation, and the deepening of the VLP seismic source. This study also demonstrates the ability of the geophysical network to detect superficial magma recharge within an open-conduit system and to track magma drainage during the effusive crisis, with a great impact on hazard assessment

[1] Stromboli is an open conduit volcano characterized by persistent conduit degassing and explosive dynamics. During the 2003 effusive eruption we monitored this activity with a five-element array deployed at $450 m from the active craters. The array allows us to track in real time changes of explosive activity in terms of source position and excess pressure release. Infrasonic monitoring located the new active vents in the same preeruptive position, revealing that the collapse of the crater terrace has not changed the geometry of the shallow feeding system. Elevation of the infrasonic source is stable at $750 m above sea level, coincident with the elevation of the crater terrace. This suggests a shallow position of the magma column or, if the source is embedded in the conduit, diffraction at the crater rim. At the end of the eruption, back azimuth analysis indicates that explosive activity was progressively reestablished at all of the craters following a NE toward SW direction along the collapsed trench. The array detected significant small-amplitude ($1 Pa) infrasonic activity occurring at a rate of approximately one pulse every 2 s and generally located in only one vent at a time. This persistent activity is not related to Strombolian explosion, but we infer it is reflecting the overpressurized degassing of the magma column. Amplitude distribution of $7 million infrasonic signals shows that this degassing is 1 order of magnitude more energetic than the explosions. Besides, infrasonic amplitude distribution presents a decay with a double slope, indicating that explosions and degassing are driven by conduit dynamics acting at two different rates.