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Yo Fukushima(1), Philippe Durand(2), and Valérie Cayol(1)

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

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Yo Fukushima(1), Philippe Durand(2), and Valérie Cayol(1)

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  1. 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 yofukushima@opgc.univ-bpclermont.fr

  2. InSAR (Radarsat) data (1) 30.04.97-30.07.98 Mar 98 01.04.98 - 11.09.99 Jul 99 10 cm of range decrease 48 cm of range decrease 11.09.99 - 29.10.99 Sep 99 12 cm of range increase/decrease

  3. InSAR (Radarsat) data (2) Feb 00 (A) 22.10.99 - 25.05.00 Feb 00 (D) 14.12.99 - 19.03.00 40 cm of range decrease 12 cm of range increase 08.05.00 - 25.06.00 Jun 00 (D) 13.05.00 - 30.06.00 Jun 00 (A) two sources? 17 cm of range increase + 8 cm of range decrease two sources? 42 cm of range decrease

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

  5. 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.

  6. Inversion: Neighbourhood Algorithm (Sambridge, 1999) A Monte-Carlo Search Method which efficiently searches acceptable models 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.

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

  8. Geometry for the Best-fit Model (Model A)

  9. Synthetic Data for Model A Misfit = 15.1 %

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

  11. Synthetic Data for Model B Misfit = 16.6 %

  12. Opening and Volume Model A Av. Opening = 0.59 m Volume = 9.4 x 105 m3 Model B Av. Opening = 0.66 m Volume = 1.3 x 106 m3

  13. What is certain? • 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 What is uncertain? • 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

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