Array Tracking of Infrasonic Sources at Stromboli Volcano

Infrasonic array represents a major tool in monitoring the activity of explosive volcanoes and could easily track the position of the source of the sound produced by volcanic activity. On volcanoes such as Stromboli the possibility to control the evolution of the explosive activity at each single crater, is believed to be crucial for a correct risk assessment. Explosions were recorded at Stromboli by a small aperture (120 m) infrasonic array at ~ 350m from the 3 active craters. A grid searching technique based on semblance allows a fine location of the explosive vents. The location of the sound source lead to define, by stacking the infrasonic signals, the main characteristics of the acoustic wavefield produced by each crater. The NE crater produces short (<1 s) and strong (~ 40 Pa) acoustic waves, while explosions at the SW crater generate a sharp low-pressure (~16 Pa) onset followed by a long (~ 15 s) acoustic coda. These acoustic waves reflect the eruptive styles observed during the experiment. Activity at the NE crater was characterised by short-lived (4-5 s) explosions, highly energetic and with a large amount of scoria. The explosions at the SW crater were long lasting (10-20 s) and rich in cold volcanic ash. We found that, in spite of the longer coda, the acoustic waves of the two craters show very similar onsets indicating that the source process is the same at the two craters. The duration of the explosion is mainly a function of the gas overpressure, which directly controls the gas jet velocity. High gas overpressure will produce large pressure perturbations in the atmosphere, but the duration of the explosion itself will be short. However, low regimes of gas overpressure will generate long living explosions with small acoustic amplitude as in the case of fountain-like explosions. The correlation of the acoustic waveforms with eruptive styles is the evidence that mass discharge rate is controlling the acoustic emission of the infrasonic coda. We suggest that the long acoustic coda can be associated to the mass discharge rate and reflects fluctuations of a sustained pressure release.