Noise-based passive ballistic wave seismic monitoring on an active volcano

Monitoring temporal changes of volcanic interiors is important to understand magma, fluid pressurization and transport leading to eruptions. Noise-based passive seismic monitoring using coda wave interferometry is a powerful tool to detect and monitor very slight changes in the mechanical properties of volcanic edifices. However, the complexity of coda waves limits our ability to properly image localized changes in seismic properties within volcanic edifices. In this work, we apply a novel passive ballistic wave seismic monitoring approach to examine the active Piton de la Fournaise volcano (La Reunion island). Using noise correlations between two distant dense seismic arrays, we find a 2.4 per cent velocity increase and -0.6 per cent velocity decrease of Rayleigh waves at frequency bands of 0.5-1 and 1-3 Hz, respectively. We also observe a -2.2 per cent velocity decrease of refracted P waves at 550m depth at the 6-12 Hz band. We interpret the polarity differences of seismic velocity changes at different frequency bands and for different wave types as being due to strain change complexity at depth associated with subtle pressurization of the shallow magma reservoir. Our results show that velocity changes measured using ballistic waves provide complementary information to interpret temporal changes of the seismic properties within volcanic edifices.