Insights on the March 1998 eruption at Piton de la Fournaise volcano (La Reunion) from microgravity monitoring

We investigate the temporal gravity changes associated with one of the major recent eruptions at Piton de la Fournaise volcano (March 1998) that occurred after an unusual five-year quiet period and initiated a new eruptive cycle. Repeated microgravity surveys allowed us to measure residual gravity changes up to 100 mu Gal within the Enclos Fouque caldera for four months before the start of the eruption. We first analyzed the temporal gravity changes and the height changes, also measured at the gravity benchmarks, on the basis of an intrusive dyke model previously proposed for this eruption from InSAR and tilt data. This analysis reveals that such simple model (finite rectangular shaped tensile dislocation) cannot fit both gravity and geodetic data and leads us to propose a dual source gravity model for this eruption. The gravity data inversion using a genetic algorithm search method allows us to quantify the mass change produced by the intrusive dyke (3.9 to 8.7 x 10(9) kg) and to suggest an additional mass increase at sea level depth (4.6 to 7.2 x 10(10) kg). We interpret the latter as the effect of magma ascent into a reservoir that occurred within the four months before the eruption. We suggest that high concentrations of gas content in the magma reservoir account for a higher compressibility of the intruded magma and produce significant mass change with moderate surface deformation. The 1996 seismic crisis, interpreted as an upward migration of magmatic fluids into a reservoir located at sea level, might support such a hypothesis.