"After the 24-hour eruption of 14-15 June, seismicity and deformation had returned to low levels. Sustained seismic activity began suddenly on 9 July. The intrusive seismic crisis, similar to the ones that have preceded all the outbreaks in the central area since 1981, continued until about 2250. The earthquakes were located between 1 and 2.5 km beneath the summit. The intrusion failed to reach the surface. The deformation pattern shown by the summit dry-tilt stations seems to indicate that the intrusion was emplaced E of the summit area.

"After declining for about 1 hour, the seismic activity resumed with a new swarm of earthquakes that were mostly centered 3-6 km WNW of the summit area at depths of 1-4 km. This crisis peaked on 10 July with more than 1,200 events, and progressively decreased during the following days. From 13 July to the end of the month the rate of seismicity decreased from 5-10 events/day to ~1 event/day.

"This second swarm is the first to be observed outside of the central area since the Observatory was established in 1980. It is also by far the largest recorded seismic crisis at Piton de la Fournaise. A large number of epicenters are also found along the 'Grandes Pentes' line which is the inland boundary of the 'Grand Brule' slump structure that is thought to result from the instability of the free flank of the volcano.

"On 5 August at 2250 a new intrusive seismic swarm began at shallow depth beneath the summit area and more than 200 events were recorded in less than 3 hours. An eruption started at 2340 N of the summit at the base of the central cone (E of Cratère Magne). Deformation related to this eruption affected the summit area and the descent to the Plaine des Osmondes, corresponding to a major inflation on both sides of a N-15°E fissure. From the beginning of the eruption until 9 August, 4 x 10⁶ m³ of aphyric basalt have been emitted at a rate of 10-15 m³/s. Temperatures of 1,100-1,120°C have been recorded. The eruption was continuing as of 11 August."

Geological Summary. Piton de la Fournaise is a massive basaltic shield volcano on the French island of Réunion in the western Indian Ocean. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three scarps formed at about 250,000, 65,000, and less than 5,000 years ago by progressive eastward slumping, leaving caldera-sized embayments open to the E and SE. Numerous pyroclastic cones are present on the floor of the scarps and their outer flanks. Most recorded eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest scarp, which is about 9 km wide and about 13 km from the western wall to the ocean on the E side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures outside the scarps.

Information Contacts: J. Lenat, H. Delorme and J. Delarue, OVPDLF; A. Hirn and J. Delattre, IPG, Paris; P. Bachelery, Univ. de la Réunion.