STROMBOLIAN EXPLOSIONS .2. ERUPTION DYNAMICS DETERMINED FROM ACOUSTICMEASUREMENTS

Strombolian activity consists of a series of explosions due to the bre aking of a large overpressurized bubble at the surface of the magma co lumn. Acoustic pressure due to sound waves has been measured and analy zed at Stromboli for more than 50 explosions. Three parts can be disti nguished in the acoustic pressure waveform, which are related to the b ehavior of the bubble before, during, and after its bursting. Before t he sharp rise in acoustic pressure, the signal is dominated by waves w ith a frequency of 2 Hz, which develop on the nose of the bubble. They produce sound in air by imposing a rapid motion to the interface, and one could detect a bubble travelling in the uppermost 30 m of the mag ma column. When the bubble reaches the air-magma interface, its strong vibration, driven by a large overpressure inside the gas, generates t he main event with a frequency around 9 Hz. After the bubble has burst , kinematic waves of frequency around 4.5 Hz are the main source of so und. They develop at the surface of the magma left on the conduit side . The three types of motion, although determined independently, give c onsistent results. Furthermore, combining the results obtained for the two types of kinematic waves, the magma viscosity is estimated to be of 300 +/- 65 Pa s, which is in good agreement with petrological const raints and corroborates the validity of our analysis. This suggests th at acoustic measurements constitute a powerful tool in the understandi ng of eruption dynamics.