Damage, Weakening and Non-Linear Processes in a Pressurized Volcanic Edifice.

Abstract:

When an eruption occurs in a large basaltic volcano as Piton de la Fournaise volcano (La Reunion, France), accelerations of surface displacements and seismicity rate are recorded before magma reaches the surface, with a hour to week time scale. These eruptions are understood as ruptures of pressurized magma reservoirs. When elastic models are used to account for surface deformation, such accelerations are modelled by an accelerating increase of the reservoir pressure. It is reached for magma flow or pressure conditions at the base of the magma feeding system that may be not realistic at this time scale. An alternative solution to explain such accelerations is the weakening of the volcanic edifice under the effect of magma pressure in the reservoir. In this study we have modelled such a weakening by the progressive damage of an initially elastic edifice. We used an incremental damage model, with seismicity as a damage variable with daily increments. Elastic moduli decrease linearly for each damage increment. When this damage model is used in an initially elastic edifice with a simple constant pressure condition at the base of the system (which leads to an equilibrium in a purely elastic model), surface displacement accelerations are well reproduced when damage is sufficient. We link the damage parameter to the crack density and show that process dynamics is controlled by the ratio between the incremental rupture surface and the surface to be ruptured, this later being directly dependent on reservoir depth. In that case the edifice strength decreases relative to the elastic strength, and magma reservoir pressure decreases with elastic moduli. This later characteristics may eventually cause gaz exsolution, which may generate non-linear instabilities during the eruptive process.