The collapse of a caldera (a large crater that forms shortly after the emptying of a magma chamber/reservoir in a volcanic eruption) is a rare phenomenon. Although only seven have been reported in the last 100 years, these events can cause significant – or even catastrophic – changes in the landscape and volcanic activity. Researchers in France, Australia and Switzerland have now succeeded in identifying the first deep collapses before the first surface collapse during the formation of the 2007 Dolomieu caldera on La Réunion island.
The Dolomieu du Piton de la Fournaise caldera collapsed in April 2007 during an unusually slow earthquake of magnitude 4.8, three days after the Piton de la Fournaise erupted.
Data from the GEOSCOPE/IPGP network’s permanent RER seismological station installed near the Dolomieu crater show that the caldera formed in April 2007 following 48 successive summits that occurred in nine days. A 340-metre-deep caldera formed days after the first summit collapse.
The very high bandwidth recordings of the RER seismic station allowed the researchers to identify 20-second-duration ultra-long period (ULP) signals (with frequencies in the 0.003–0.01 Hz range) accompanied by very long period (VLP) signals (ranging between 0.02 and 0.50 Hz) of the same length before the onset of surface collapse and as the caldera formed. The fact that these signals were similar in character suggests that the first deep collapse occurred roughly 20 hours before surface failure and the beginning of caldera formation.
“These events could result from repeating piston-like successive collapses occurring through a ring-fault structure surrounding a magma reservoir,” say the researchers. And “the study shows the importance of considering at least a broadband seismic station in volcano monitoring, particularly for those that can be potentially devastating by their eruptions or collapses.”
Read the research paper (free online): Fontaine F.R., Roult G., Hejrani B., Michon L., Ferrazzini V., Barruol G., Tkalčić H., Di Muro A., Peltier A., Reymond D., Staudacher T., and Massin F., Very- and ultra-long-period seismic signals prior to and during caldera formation on La Réunion Island, Scientific Reports 10.1038/s41598-019-44439-1.