MAGNETIC VARIATIONS ON PITON-DE-LA-FOURNAISE VOLCANO - VOLCANOMAGNETIC SIGNALS ASSOCIATED WITH THE NOVEMBER 6 AND 30, 1987, ERUPTIONS

A magnetic network has been installed on Piton de la Fournaise (Reunio n island) during the last months of 1985. Up to seven stations are con tinuously measuring the Earth's magnetic field with a sampling rate of 1 minute. During the period 1986-1990 twenty-one eruptions have occur red: some of them inside the summital Dolomieu crater, and others on t he northern and southern flanks of the cone. In this paper, we present the main characteristics of the observed magnetic variations. Volcano magnetic signals are observed in association with onsets of volcanic a ctivity: seismic crises or eruptions. They can precede an eruption som e days or weeks. The amplitude of these forerunner signals is a few na noteslas (nT). The most important signals (up to tens of nT) are recor ded during the intrusive crises: sequence of quakes, seismic swarms, t remor, opening of fissures, tremor activity. Whatever the seismic cris es and the eruptions are, annual magnetic variations are observed. The y are centered along the main fracture zone which cuts the whole massi f from the southeast to the northeast. This annual wave is maximum on the volcano summit with an amplitude of about 20 nT and decreases alon g the main fractures zone (20% at 2.5 km towards the north, 80% at 2.5 km perpendicularly). On these slow variations are superimposed long-t erm drifts which are well correlated with families of eruptions (north , summit, south). At the northern base of the cone, a decrease of 50 n T is associated with the northern flank eruptions.