Time constraints for modeling source dynamics of volcanic explosions at Stromboli

The explosive source mechanism, responsible for the volcanic activity of St romboli, is analyzed by seismic and infrasonic measurements in correlation with digitally processed images of the explosive events. Infrasonic recordi ngs associated with short-period seismic signals give evidence that the hig h-frequency (>5 Hz) wave field is produced by a seismic source coupled to t he atmosphere. The time delay between seismic and infrasonic onsets is not stable and the fluctuations could reflect a change in the magma-gas physica l 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 col lapse model has been checked in laboratory experiments by recording acousti c 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-frequ ency rarefaction is observed when the foam collapse occurs and the gas bubb le starts to rise up along the pipe, then the bubble film breaking at the l iquid free surface emits an audible sound. By analogy, we infer that low-fr equency 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 e xplosive source at 600 m above sea level.