NUMERICAL MODELLING OF GROUND DEFORMATION ASSOCIATED WITH THE 1998 – 2000 ERUPTIONS AT PITON DE LA FOURNAISE VOLCANO, REUNION ISLAND Yo Fukushima(1), Philippe Durand(2), and Valérie Cayol(1) (1) Laboratoire "Magmas et Volcans", Université Blaise Pascal, CNRS UMR 6524, France (2) Centre National d'Etudes Spatiales, France

InSAR (Radarsat) data (1) Mar 98 Sep 99 Jul cm of range decrease 12 cm of range increase/decrease 10 cm of range decrease

InSAR (Radarsat) data (2) Feb 00 (A) Feb 00 (D) Jun 00 (A) Jun 00 (D) 40 cm of range decrease 12 cm of range increase two sources? 17 cm of range increase + 8 cm of range decrease two sources? 42 cm of range decrease

East (km) North (km) Modelling : 3D Mixed BEM Mesh the structures (topography and dike in this case) and calculate the displacements on every element of the mesh. (Cayol and Cornet, 1997)

Meshing the dike ● The top is located at the ground surface (fixed). ● The dike is modeled by a quadrangle (roughly). ● The bottom is a straight line (the location and length can change). ● The dike meshing procedure is parameterized and automated for the inversion.

Inversion: Neighbourhood Algorithm A Monte-Carlo Search Method which efficiently searches acceptable models (Sambridge, 1999) At each iteration Ns new samples are put into Nr Voronoi cells which are chosen after evaluating the misfit function. We use Ns = Nr = 50, which is explorative enough.

Parameters vs Misfit values These figures give an idea how the 'good-fit model region' is constrained by the parameters.

Geometry for the Best-fit Model (Model A)

Synthetic Data for Model A Misfit = 15.1 %

Geometry of Another Good-fit model (Model B) * Roughly speaking, Model A and Model B are the two extremes of good-fit models.

Synthetic Data for Model B Misfit = 16.6 %

Opening and Volume Model A Av. Opening = 0.59 m Volume = 9.4 x 10 5 m 3 Model B Av. Opening = 0.66 m Volume = 1.3 x 10 6 m 3

What is certain? What is uncertain? ● Bottom ~ 1600 m a.s.l. (= 500 – 800 m below surface) ● Dip 45 – 55 degrees ● Bottom line 3 – 4 times longer than the top line, lying beneath the Dolomieu crater ● Existence of either a very shallow (300 – 500 m below surface) magma chamber, or a well-developped magma path which does not cause any deformation ● Assuming that magma starts to migrate from the southern end point of the bottom line, the depth of the starting point is uncertain between 50 – 500 m. ● Is it really 'twisted' like Model A? --> Need other information (inclinometry, seismicity, etc.) to constrain