New geophysical insight into the dynamics of Stromboli volcano (Italy)

A 56-hour gravity sequence, recorded in the crater area of Stromboli volcano, is presented. Data were acquired during a period of “normal” Strombolian activity. High-frequency gravity anomalies (up to 20 μGal; T = tens of seconds) are observed. Comparison with independent data reveals that these changes reflect the response of the gravimeter excited by seismic waves during Strombolian explosions.Correlated changes in the energy distribution over time appear in the continuous wavelet transforms of gravity and RMS-tremor, but over different scales, corresponding to periods centered at about 50 and 25 min, for gravity and tremor, respectively. We infer that the rate of fresh magma supply to the shallow feeding system controls the energy distribution over time of the coupled components in gravity and RMS-tremor signals. In particular, the gravity signal (with an average amplitude of 1–2 μGal) could be induced by temporary accumulations, at shallow depth, of the volatiles discharged by quiescent degassing.Changes in the rate of explosions from the summit craters correlate with changes in the amplitude of the coupled gravity and volcanic tremor oscillations, implying that, even though the slug-genesis process behind the explosions occurs at deeper levels, it is also controlled by the rate of gas-rich magma supply from below.Negative gravity anomalies of about 20 μGal, over intervals of some hours, are also observed, separated by intervals of about 24 h. They could be induced by increases in magma vesicularity in the uppermost part of the conduit plexus, a view supported by (i) changes in the time/space distribution of tremor amplitudes and (ii) increases in spattering activity from a summit vent, both occurring during the development of the negative gravity anomalies.The processes behind mild Strombolian explosions do not trigger measurable gravity changes. Nevertheless, the slug ascent before a major explosion could induce a precursory gravity signal.

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► Phases of fresh magma supply control coupled changes in gravity and volcanic tremor.
► Shallow build-ups of volatiles, emitted by quiescent degassing, induce gravity changes.
► Slug ascent, before a major explosion, could induce precursory gravity changes.