Magnetic measurements on La Soufriere Volcano, Guadeloupe (Lesser Antilles), 1976–1984: A re-examination of the volcanomagnetic effects observed during the volcanic crisis of 1976–1977

During the seismovolcanic crisis of 1976–1977 at La Soufrière on Guadeloupe, a magnetic network of 12 reference markers was set up. Measurements of the intensity of the earth's magnetic field, carried out up to once a day at each marker, showed volcano-magnetic variations of several nanoteslas (nT). The variations, at certain markers, were more or less concealed by transient magnetic variations due to anomalies in conductivity. As early as 1978, measurements were resumed and a telemetering network was coupled with the network of reference markers, to which 4 new markers were added. A detailed study of conductivity anomalies was carried out on the entire volcano. Contrasts in conductivity linked to the existence of a superficial conducting surface, on a SSW/NNE axis, located south of the volcano, caused a great lack of homogeneity in the field variations measured at the surface. Variations greater than about 10 nT appeared in the difference in intensity of the earth's magnetic field between two stations.No long-term magnetic variation was observed between 1978 and 1984. On the network of markers, the accuracy of measurements of volcanic effects was at best 2 nT. Measurements carried out on the telemetering network during the night refined these results, since their accuracy was 1 nT. The only significant volcanic crisis between 1978 and 1984 (5–7 January 1981) seems to be observed by telemetering stations. All the measurements carried out in periods of volcanic inactivity make it possible to re-examine the crisis of 1976–1977. Though volcanomagnetic effects over short periods cannot be accurately determined, variations with a time constant of several weeks were present over the entire volcano. These variations were as high as 7–8 nT in remote stations and they can be linked to the three major phases of eruptive activity at La Soufrière during the crisis of 1976–1977.