Infrasound reveals transition to oscillatory discharge regime during lava fountaining: Implication for early warning

We present the analysis of similar to 4 million infrasonic signals which include 39 episodes of lava fountains recorded at 5.5km from the active vents. We show that each eruptive episode is characterized by a distinctive trend in the amplitude, waveform, and frequency content of the acoustic signals, reflecting different explosive levels. Lava fountain starts with an similar to 93min long violent phase of acoustic transients at similar to 1.25Hz repeating every 2-5s. Infrasound suddenly evolves into a persistent low-frequency quasi-monochromatic pressure oscillation at similar to 0.4Hz. We interpret this shift as induced by the transition from the slug (discrete Strombolian) to churn flow (sustained lava fountain) regime that is reflecting an increase in the gas discharge rate. We calculate that infrasonic transition can occur at a gas superficial velocity of 76m/s and it can be used to define infrasonic-based thresholds for an efficient early warning system.