3-DIMENSIONAL MODELING OF THE 1983-1984 ERUPTION AT PITON DE LA FOURNAISE VOLCANO, REUNION ISLAND

Displacement vectors on 125 points over a 6-km(2) area were calculated [Zlotnicki et al., 1990] for Piton de la Fournaise volcano, Reunion I sland, from photogrammetric surveys conducted in September 1981 and ag ain in September 1984. An eruptive crisis with two episodes occurred d uring the 3 years between the surveys. We modeled the displacements fr om the photogrammetric surveys using a three-dimensional mixed boundar y element method. The model shows that both eruptive episodes were fed by a single dike oriented 165 degrees from north. As the dike approac hed the ground surface, it divided into segments that rotated into an en echelon pattern. The model that best fits all observations suggests that the stress state within the volcano is quasi-isotropic such that horizontal stresses equal the overburden. These high horizontal stres ses are incompatible with an elastic edifice loaded merely by gravitat ional forces. However, they may be attributed to the cumulative effect of repeated dike intrusions.