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A permanent network of four to six telemetered magnetic stations has been installed on Piton de la Fournaise Volcano in October 1985 and 1986. Eight eruptive sequences have occurred since then, some of which affected only the summit whereas others affected the whole massif. Volcanomagnetic variations are observed with each of these volcanic events. The geomagnetic field intensity is measured simultaneously every minute at each station, with a precision of 0.25 nT (nanotesla). The main results are the following: Volcanomagnetic variations observed with eruptions have amplitudes of several nanoteslas to more than 10 nT with characteristic time durations ranging from 1 min or less to several months. A good correlation between the magnetic variations recorded at the different stations is observed. Spatial and temporal properties of the volcanomagnetic field appear to be controlled by the features of the eruptions. Several days before the beginning of the December 1985; March, July, November--December 1986; and January 1987 eruptions, changes in the intensity differences are observed with amplitudes of a few nanoteslas (up to 5 nT) at stations located close to the active part of the rift zone. Variations are also observed in the most remote stations for larger eruptions in March 1986, November--December 1986, and January 1987. Correlations between magnetic variations, ground deformations, and seismic activity have been observed for the December 1985, March 1986, November--December 1986, and January 1987 events. Three main mechanisms could account for the observed volcanomagnetic variations: piezomagnetic effects, electrokinetic effects, and electrical resistivity changes since variations in the stress field in the surroundings can modify the induced and natural magnetizations of magnetized rocks, the circulation of water through the interconnected pore network (microcracks, faults aquifers), and also the electrical resistivity of geological structures through the modification of the properties of the pore network. ¿ American Geophysical Union 1988 |
