Very long-period observations at Piton de la Fournaise volcano, La Réunion

Volcanoes often exhibit very long period (VLP) signals, with periods ranging from 2 to 100 seconds. Due to their very long wavelengths, these waveforms are not strongly affected by volcano structural heterogeneities and provide invaluable insights into dike and magma properties that are not easily accessible though other observations. Historically, only a few VLP events have been recorded at the Piton de la Fournaise, primarily associated with collapses in the summit caldera in 1986, 2002 and 2007. However, since 2010, the monitoring network has evolved significantly and it is now equipped with several broadband stations, allowing a wide range of signals to be recorded. In this study, we show that VLP events are quite common at Piton de la Fournaise. Specifically, we identify swarms of VLP events during eruptions and during magma injections preceding eruptions. Source analysis of VLP events during eruptions indicates the resonance of the dike during sudden decreases in magma flow. Pre-eruptive VLP waveforms exhibit significant differences from those observed during eruptions, pointing to a distinct source process as the dike propagates laterally toward the volcano flank.