Finding Realistic Dike Models from InSAR Data: Applications to 1998 - 2000 Eruptions at Piton de la Fournaise

Dike intrusions often cause surface displacements that cannot sufficiently be explained by simple analytical models. We developped a method to find and appraise realistic dike models from InSAR data based on the combination of a three-dimensional mixed boundary element method and a neighbourhood algorithm inversion method. Dikes are represented by a quadrangle-like shape with their top reaching the ground surface. The inversion consists of two stages: searching and appraising. In the searching stage, data correlation is taken into account. The appraising stage involves calculations of marginal probability density functions using the samples collected in the searching stage. Synthetic tests confirmed that the method well retrieves the true model with small uncertainties, and showed that all the tested data-subsampling methods are relevant to our problem. In the years between 1998 and 2000, Piton de la Fournaise volcano (Reunion Island) experienced five eruptions, which had been recorded by the RADARSAT-1 satellite. The data set for the February 2000 eruption on the northern flank, made from two ascending and two descending orbits, showed asymmetric displacements indicating an eastward slip of the east side of the eruptive fissures. The acceptable dike models inverted for this eruption share common characteristics such as a moderate seaward dip, a north-south strike, and a subhorizontal bottom line at a shallow depth of 800 to 1000 m, passing beneath the summit Dolomieu crater. The data set for the September 1999 eruption on the summit and southern flank showed, on the contrary, displacements that are symmetric with respet to the eruptive fissures. Preliminary analysis found a very shallow (less than 500 m for the deepest point) and vertical dike model for this eruption.