Abstract. The Merapi Volcano, on Java island, Indonesia, is a young island arc strato-volcano. It shows almost continuous activity with growth of an andesitic lava dome, which collapses in glowing avalanches, explosions and pyroclastic flows. Five GPS (Global Positionning System) campaigns were carried out between 1993 and 1997 on a 14-point network. Four continuous multi-parameter stations (tiltmeters, extensometers) were installed between 1993 and 1995, on the summit and the flank of the volcano. Signals are corrected for meteorological effects, then combined to reduce the uncertainties. The data are interpreted using a 3D elastic model based on the Mixed Boundary Elements Method (MBEM). Topography and the main discontinuities of the edifice are taken into account. The data are then used in an inversion process which allows the model probability and uncertainties of the computed parameters to be estimated. The far-field deformation modelling shows that data are not consistent with the hypothetical shallow magma chamber suggested by seismic activity analysis. The near-field deformation (1993 to 1997) is modelled assuming a magma conduit under pressure and viscous shear stresses. Three main fractures are included in the upper part of the volcanic structure because of geological observations. Amplitudes of observed displacements vectors cannot be explained by the change in lava dome weight. They are consistent however with the drag of the viscous magma flow. For realistic stress values in the duct, the Young’s modulus of the medium has to be less than 1 GPa. Computed variations in wall shear stress with time are consistent with observed variations in amount of « multiphase » seismic events. These events are taken to be characteristic of the magma flow velocity. These both independant observations are internally consistent. Finally, we localise an area near the crater rim that presents a potential rock slope problem.

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