Strombolian explosions .2. Eruption dynamics determined from acoustic measurements

Strombolian activity consists of a series of explosions due to the breaking of a large overpressurized bubble at the surface of the magma column. Acoustic pressure due to sound waves has been measured and analyzed at Stromboli for more than 50 explosions. Three parts can be distinguished in the acoustic pressure waveform, which are related to the behavior of the bubble before, during, and after its bursting. Before the sharp rise in acoustic pressure, the signal is dominated by waves with 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 magma column. When the bubble reaches the air-magma interface, its strong vibration, driven by a large overpressure inside the gas, generates the 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 sound. They develop at the surface of the magma left on the conduit side. The three types of motion, although determined independently, give consistent 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 constraints and corroborates the validity of our analysis. This suggests that acoustic measurements constitute a powerful tool in the understanding of eruption dynamics.