Very- and ultra-long-period seismic signals prior to and during caldera formation on La Réunion Island

International audience; early detection of the onset of a caldera collapse can provide crucial information to understand their formation and thus to minimize risks for the nearby population and visitors. Here, we analyse the 2007 caldera collapse of piton de la Fournaise on La Réunion Island recorded by a broadband seismic station. We show that this instrument recorded ultra-long period (ULP) signals with frequencies in the range (0.003-0.01 Hz) accompanied by very-long period (VLP) signals (between 0.02 and 0.50 Hz) prior to and during the caldera formation suggesting it is possible to detect the beginning of the collapse at depth and anticipate its surface formation. Interestingly, VLP wave packets with a similar duration of 20 s are identified prior to and during the caldera formation. We propose that these events could result from repeating piston-like successive collapses occurring through a ring-fault structure surrounding a magma reservoir from the following arguments: the source mechanism from the main collapse, the observations of slow source processes as well as observations from the field and the characteristic ring-fault seismicity. Caldera collapses are rare (only seven events over the last 100 years) and particularly destructive volcanic events that can induce catastrophic changes in the shape of a volcanic edifice and its environment 1. Identifying the occurrence of the first collapse at depth is of major importance in evaluating the triggering factors and anticipating the caldera surface formation. This detection can help to predict or at least indicate early future caldera collapses and subsequent consequences such as explosive eruptions, (e.g. those that followed the major Kīlauea Caldera collapse in 1470-1510 2), or atmospheric impacts 3. In the past two decades only four caldera collapses have been monitored by dense geophysical networks: chronologically, the 2000 Miyake-jima, Japan; the 2007 Piton de la Fournaise, La Réunion/France; the 2014-2015 Bárðarbunga, Iceland; and the 2018 Kīlauea, Hawai'i 4-7. Laboratory experiments and numerical analyses predicted the occurrence of precursory collapses at depth before the onset of the surface subsidence 8-12 but observations obtained from adequate broadband seismometers were lacking. Until the present study, the Miyake-jima caldera formation was the only known case with evidence of such deep collapses before the faults reached the surface. The deep collapses were suggested from observations of VLP seismic pulses of 20 s width 13. However, the timing of the first collapse at depth was not reported. VLP signals observed at volcanoes are generally considered having frequencies between 0.01-0.5 Hz (ref. 14) and until the 2000 Miyake-jima event, they were generally considered to result from inertial forces associated with changes in the flow of magma and gases through conduits 14. The VLP signals detected during the Miyake-jima caldera formation were explained by different physical mechanisms: (i) a buried geyser model 15 , (ii) a piston-like model 4,13,16 , and (iii) ring-faulting mechanisms related to shear failure on curved or cone-shaped fault structures 17,18. 46 step-like tilt changes (TC) were observed during the Miyake-jima caldera formation and among them 39 were accompanied by the VLP seismic signals 4,19,20. The origin of these TC associated with the VLP pulses have been attributed to either a piston model with a vertical rock column intermittently sinking into a magma reservoir 4,21 or to a magma sheet model with a large sill-like