NUMERICAL MODELING OF SURFACE DEFORMATION AND MECHANICAL STABILITY OFVESUVIUS VOLCANO, ITALY

This study was undertaken with the aim of contributing to the risk eva luation of Vesuvius. We calculate the surface displacements due to an increase in pressure in a shallow reservoir and assess the mechanical instability of the volcanic edifice and of the feeding system caused b y pressure on the reservoir's walls and by regional stresses. We consi der axisymmetric models that take into account topography, gravity, ho mogeneous and heterogeneous country rocks, spheroidal magma chambers w ith different aspect ratios and variable depths, and subvertical intru sions from the top of the chamber to various depths. We impose both sy mmetric and asymmetric regional stresses increasing with depth as boun dary conditions. The models are static. The ground deformation and the stress distribution are calculated, in the framework of linear elasti city, by a numerical finite element method. The surface displacements refer to an overpressure of 10 MPa. This value is considered an upper limit for the fracture of surrounding rocks. We find the maximum verti cal displacements to be of a few centimeters in the most favorable cas e and the displacement gradients to be at the lower limit of measurabi lity. To evaluate the mechanical stability, we calculate the stress di stributions of a prolate ellipsoidal reservoir within a heterogeneous medium. We consider a total hydrostatic magmatic pressure starting fro m the value of 50 MPa at the reservoir's top and three regional stress regimes from symmetric tensile to asymmetric tensile to strike-slip. In the last two cases we use three-dimensional models. The criteria ad opted for instability are (1) the development of tensile tangential st ress and (2) the Navier-Coulomb criterion, in compression. In no case have we found instability near or on the wall of the reservoir, wherea s the slope of the volcanic edifice exhibits a shear failure instabili ty, which increases with greater regional stress anisotropy.