Very- and ultra-long-period seismic signals prior to and during caldera formation on La R\xe9union Island

F R Fontaine,D Reymond,L Michon,V Ferrazzini,G Barruol,F Massin,H Tkalčić,T Staudacher,G Roult,A Peltier,A Di Muro,B Hejrani

doi:10.1038/s41598-019-44439-1

Abstract

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.

Highlights

Caldera collapses are rare and destructive volcanic events that can induce catastrophic changes in the shape of a volcanic edifice and its environment[1]
The very-long period (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 structures17,18. 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]
We report for the first-time, ultra-long period (ULP) seismic signals accompanied by VLP seismic signals associated to precursory collapse at depth

Summary

Introduction

Caldera collapses are rare (only seven events over the last 100 years) and destructive volcanic events that can induce catastrophic changes in the shape of a volcanic edifice and its environment[1]. 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 www.nature.com/scientificreports/. The C-CLVD earthquakes were associated with fluid motion from vertical fluid-filled cracks to a magmatic reservoir[22] These events were not coincident with ULP signals accompanied by VLP signals as reported in this study for later precursory collapse events (E2 to E5) occurring on April 5 (Supplementary Table 1). The total volume of lava emitted and intruded during the whole April eruption was estimated as 2.4 × 108 m3 (ref.28) and the collapse episode led to the development of a caldera volume of 9 × 107 m3 (ref.[27])

Methods

Results

Conclusion