The influence of regional stresses on the mechanical stability of volcanoes: Stromboli (Italy)

Abstract

Abstract An elastic model was developed to study the mechanical stability of a volcanic system that includes a volcanic edifice and a magmatic feeding system (conduits and reservoirs) with axial symmetry around a vertical axis. Stability depends critically on the horizontal regional stress which comes into the problem as a boundary condition imposed at large horizontal distance. We considered three types of boundary conditions: (1) zero horizontal displacement (NHD); (2) lithostatic regional stress (LITH); (3) non-axisymmetrical regional stress corresponding to a strike-slip regime (NAS). For the third case (NAS), we used a 3-D model. The stress distributions were calculated by a numerical finite element method. The development of tensile tangential stress in the elements adjacent to the walls of conduits and reservoirs is assumed to be a sufficient condition for the instability of the plumbing system. In compression, we have adopted the Navier-Coulomb criterion for failure. We apply this model to Stromboli volcano, which is characterized by a steep volcanic edifice and open conduits filled with magma. There is geochemical evidence for a deep reservoir ( c. 10 km depth) of unknown size and indirect evidence of a shallower reservoir (at the base of the edifice) with volume of the order of 1 km 3 . The persistent explosive activity has been steady for the last 2000 years: this implies that the feeding system is mechanically stable. It is found that the deep reservoir has no influence on the stability of the edifice. With NHD boundary condition, implying that the horizontal regional stress is one-third of the vertical component, Stromboli’s volcanic system is unstable. In general, any volcanic system with an open and magma-filled plumbing system, with NHD boundary conditions, is mechanically unstable. With lithostatic regional stresses (LITH), the magmatic feeding system is mechanically stable and the slopes of the subaerial edifice are unstable, being subject to landslides. Both NHD and LITH conditions imply that the regional horizontal stress is isotropic which is not appropriate for the Southern Tyrrhenian Sea. With the NAS boundary condition, the stability of the plumbing system of Stromboli volcano allows us to place an upper limit on the maximum shear stress Δ 0 = (|σ h | max − |σ h | min )/2 For Δ 0 = 10 MPa, the magmatic feeding system is at the limit of mechanical instability. The subaerial edifice has steep slopes and it is at the limit of gravitative instability independent of the boundary conditions.