Time constraints for modeling source dynamics of volcanic explosions at Stromboli

The explosive source mechanism, responsible for the volcanic activity of Stromboli, is analyzed by seismic and infrasonic measurements in correlation with digitally processed images of the explosive events. Infrasonic recordings associated with short-period seismic signals give evidence that the high-frequency (>5 Hz) wave field is produced by a seismic source coupled to the atmosphere. The time delay between seismic and infrasonic onsets is not stable and the fluctuations could reflect a change in the magma-gas physical properties and/or a migration of the source in the magma conduit. The gas jet velocity, calculated by videocamera images, and the time delay between seismic and infrasonic onset show a weak inverse correlation. The foam collapse model has been checked in laboratory experiments by recording acoustic signals produced during gas bubbles growth, flow, and burst in a liquid. The observed signals present strong similarities to short-period (>1 s) as well as broadband (10-1 s) seismic signals recorded at Stromboli. Low-frequency rarefaction is observed when the foam collapse occurs and the gas bubble starts to rise up along the pipe, then the bubble film breaking at the liquid free surface emits an audible sound. By analogy, we infer that low-frequency seismic decompression is generated by the rapid expansion of gas in the magma conduit, while high-frequency seismic signal and infrasound are generated by the explosion at the magma free-surface. The time differences between seismic, infrasonic and visible onset of the explosion locate the explosive source at 600 m above sea level.