Constraints on magma flux from displacements data at Merapi volcano, Java']Java, Indonesia

The displacement field has been monitored in the vicinity of the crater rim at Mount Merapi (Indonesia) from 1993 to 1997. During this period the volcanic activity has been quasicontinuous with dome growth, explosions, and pyroclastic flows. We measured a nine-point network every year with the Global Positioning System static method. Interpretation of results is conducted with a three-dimensional elastostatic boundary elements code that takes into account topography, fractures, and complex magma source geometry. The inversion technique yields an estimate of the variation with time of the boundary conditions at the magma duct interface together with the probability associated with the best model. The Young's modulus of the equivalent continuum is found to be very low (of the order of 1 GPa), a feature which suggests that a viscoelastic behavior may be more appropriate for this rock mass, given the observed seismic velocities for the domain of interest. A striking compatibility is outlined between observed deformations and the rate of occurrence of multiphase seismic events, once the main fractures of the structure have been taken into consideration. This suggests that the summit elastic (or viscoelastic) deformation field is controlled by the magma flux within the duct rather than by magma pressure variations. In addition, a nonelastic displacement has been identified at the westernmost point of the network. This was considered critical for the stability of the summit structure, a concern whose validity has been verified a posteriori by the July 1998 explosion.