Surveying the infrasonic noise on a basaltic volcano to understand the eruptive dynamics

Abstract

The frequent eruptions of Piton de la Fournaise volcano (Reunion island) release an important quantity of magmatic gas into the atmosphere and generates infrasonic airwaves. The series of volcanic noise recorded, in the near field, on a microbarometer between 1992 and 2008 bring new constraints on the functioning of the eruptions. The detection and the modelling of the waveforms associated to the overpressurized explosions of gas bubbles leads to conceive the volcanic eruption as a puzzle game. The elementary pieces, the eruptive regimes, are characterized and interpreted in the framework of a two-phase flow. The eruptive gas flow is also quantified. The main flow regime is the Strombolian activity where the infrasound signature come from the slug flow bursting. The tracking of the main source of noise, during the eruptions, shows that the size of the gas pockets which are maximum in the starting stage of the eruption, what corresponds to the Lava Fountain regime, constantly decrease until to disappear with the eruption end: the gas volume fraction constantly decrease in the volcanic conduit during an basaltic eruption. The quantitative analysis of the noise produced by the gas flow allows not only to understand a natural system as complex as a volcano but allows also to better monitor it.