SURVEYING OF THE MAIN FRACTURE-ZONE ON THE NORTHEAST FLANK OF LA FOURNAISE VOLCANO (REUNION ISLAND) BY AUDIOMAGNETOTELLURIC SOUNDINGS

The fissural vents on Fournaise volcano are located along a major frac ture zone (M.F.Z.) which cuts through the massif from the SE to the NE , passing through the summit. The extension and the root of the M.F.Z. is not clearly understood outside the caldera. In 1994 an audiomagnet otelluric campaign of 37 soundings, in the frequency range 4 Hz - 2300 Hz, was conducted on the NE outer flank. The stacking of recent lava flows hides several structures which take root at more than one kilome tre depth. Two high resistivity axes are evidenced. They coincide with the two cones alignments Sinific-Balman-Bellevue and Trois Tetes-La G lace-Moka-Gros Piton, themselves located along the 1832 and 1977 fissu res, respectively. Low resistive channels are found. They are oriented to the NE to the North and to the ENE to the South. These branches pr obably channel meteoric ground waters flowing downwards. The low resis tivity could indicate a permeation of oceanic water. The different str uctures which compose M.F.Z. could indicate the slow migration of its activity towards the east.