CARTOGRAPHY AND TEMPORAL OBSERVATION OF S ELF-POTENTIAL (SP) ANOMALIES AT PITON-DE-LA-FOURNAISE

Extensive geophysical surveys have been made during the eighties on Pi ton de la Fournaise allowing a better understanding of the structure a nd behaviour of this active basaltic volcano. Among them, spontaneous polarization (SP) data were collected several times between 1981 and 1 992, along the same profile as well as recorded continuously using a p ermanent network during two periods in 1983 and in 1987-1988. We prese nt here the very first interpretation of this complete data set. The f irst data gathered in 1981 allow us to present a map of SP anomalies c overing a large part of Enclos Fouque, the 8 km-diameter central calde ira. Anomalies of different wavelengths reveal the main structural fea tures of the central area and North-East rift-zone. In the summit zone , the local maxima of SP anomaly are clearly linked to the concentric fractures network. Periodic reiterations of a profile around the summi t craters allow to study the relative SP variations with time. Some of the variations indicate a global disturbance of the central geotherma l system in connection with the main volcanic events that occurred dur ing the studied period. The continuous records of SP have permitted to discriminate between external and internal causes for the SP anomalie s. For the external causes, the seasonal and daily thermal effects are clearly evidenced but the rain effect is not obvious. Changes in the hydrothermal system, in response to tectonic activity induced by inter nal activity, are proposed to explain the SP variations of internal or igin, in addition to thermal inputs.