Using seismic noise to monitor volcanic activity (Invited)

Volcanic eruptions are preceded by magma-induced pressurization of volcanic edifices. Monitoring this process in real-time can prove useful to forecast volcanic eruptions. Volcano inflation generates ground deformations that can be revealed by geodesy. In some cases, however, volcano dilatation can be localized at depth with no measurable effects at the surface and it has proved difficult to monitor changes in volcanic interiors, despite considerable effort. Here we present an observation of a new type of precursory phenomena by demonstrating that pre-eruptive dilatations of the Piton de la Fournaise (La Reunion island) volcano can be detected as decreases of seismic velocity a few weeks prior to eruptions. We are able to monitor the changes of the volcano interior continuously in time and space by measuring very small relative seismic velocity perturbations (0.05%) by using the reproducibility properties of the ambient seismic noise. This new direct observation of the dilatation of volcanic edifices should improve our ability to forecast eruptions and to a priori assess their intensity (injected magma volume and flux) and environmental impact.