EPISODES OF PIT-CRATER COLLAPSE DOCUMENTED BY SEISMOLOGY AT PITON-DE-LA-FOURNAISE

Calderas are oval fault-bounded depressions in volcanic regions. The formation of a small analog, a summit pit-crater, has followed a ten-month cycle of eruptive activity at Piton de la Fournaise volcano, Reunion Island, Indian Ocean. In 1985, six short episodes of summit seismic activity, with shocks indicating shear failures, have been followed by eruptions in five cases. The eruptions occurred near the base of the summit cone which is at 2.5 km elevation, inside the Enclos Fouque, an ancient caldera 7 km in diameter. In another instance, the summit seismic crisis did not evolve to an eruption but was followed by a seismic crisis 4 km deeper. In March 1986, low-level activity both at shallow depth beneath the summit and around 4 km depth to the southeast, a simultaneous occurrence not previously observed, culminated by a summital swarm on March 18. Activity resumed its low level and five hours later a small and short-lived eruption occurred at mid-distance between the summit and the rim of Enclos Fouque. One day later a flank eruption began at 1000 m elevation, outside of the Enclos, 9 km southeast of the summit. Four days later, effusion began near sea level 14 km southeast of the summit. The shift of activity from the vent inside to that outside the Enclos occurred whilst both shallow summital and deeper flank seismicity were sustained but without notable shallow activity between one vent and the next. Then the seismic activity, which in all five previous instances stopped when the vents opened, persisted for ten days increasing in energy release rate. Activity only consisted of shallow shocks in the summit area whilst eruption was low on the flank, 9 km, then 14 km away. The focal mechanisms are different from all those previously determined. They correspond neither to shear failure nor tensile fracturing by fluid overpressure. They suggest cracking due to volume increase in a low fluid-flow environment which may be consistent with the simultaneous tapping of magma deeper in the edifice. A final paroxysmal seismic crisis lasting one and a half days led to the collapse of a 200-m diameter, 80-m-diameter, 80-m-deep the collapse of a 200-m diameter, 80-m-deep pit-crater inside the Dolomieu summital caldera on March 29. This crisis released an order of magnitude more energy than previous activity. The long-period signature of these seismic events indicates low stress drop with respect to source dimensions. Sources are shallow. However, rather than directly marking the progressive development of a depression at the surface, they appear to correspond to fractures opening above the retreating magma and progressive stoping of a cavity. Similarities with seismic observations during the last phase of the Fernandina, Galapagos, 8-km2 caldera collapse of June 1968 suggest that the same kind of mechanical behaviour occurred with a scaling of three orders of magnitude in seismic moments and in volumes of collapsed blocks between the two cases.