Pre-Eruptive Damage, Weakening and Magma-Edifice Coupling at Piton De La Fournaise Volcano

Eruptions in basaltic volcanoes are often preceded by increasing seismicity and surface deformation, which progressively damage and weaken the volcanic edifice. We show how damage and crack interaction produce the inverse Omori-Utsu law for earthquakes during pre-eruptive periods. Rock mass continuity, representing damage, is shown to decrease exponentially with the earthquake number; we interpret it as a general form of the Omori-Utsu law. Pre-eruptive earthquake time series are shown to be controlled by heterogeneity distribution, finite-size effect and crack interaction, and by the feeding system characteristic time. Magma-edifice coupling is described by state variables that depend on the continuity and the feeding system characteristic time. Pre-eruptive seismicity of the 2004-2017 24 summit/proximal eruptions of Piton de la Fournaise volcano was well modeled by an inverse Omori-Utsu law. It allowed identifying two cases: (a) strong crack interaction and earthquake number acceleration, when failure in strong intact rock and finite-size effects dominate the brittle fracture process; in that case the magma-edifice interaction power exhibits a maximum before the eruption; (b) weak crack interaction, generating an almost constant earthquake rate and corresponding to a brittle fracture process at constant strain in a weak, fractured rock mass. In this latter case eruptions occurred when the continuity reached a critical value, close to 0.25. Specific times are identified, from the time variations of the state variables; they define estimators that provide values of the eruption time within 10% of the true value in 60%-75% of the cases studied, from the complete time series. Eruptions in basaltic volcanoes are often preceded by increasing seismicity and surface deformation. In this article we study the processes at work in the volcanic edifice rock mass. A rock mass, deformed and fractured, weakens: this process is called damage. Rock elastic characteristics decrease with damage. The way damage evolves under the effect of magma pressure contributes to determine the time evolution of the pre-eruptive deformation. We show that continuity, a measure of damage, is a decreasing exponential of the cumulative number of earthquakes; this relationship is comparable to the definition of Boltzmann entropy. A model of initially elastic edifice, damaged under the pressure of a viscous, incompressible, magma in a reservoir has been developed. It is shown that, when fractures interact, continuity and magma pressure strongly drops, magma flux increases and the power of the magma-edifice interaction reaches a maximum before the eruption. A relation between the time of this maximum and the eruption time has been established and checked using 24 summit eruptions at Piton de la Fournaise; it provided an estimation within 10% of the eruption time in 60%-75% of the cases. Checking this relation with very numerous eruptions on basaltic volcanoes will allow knowing its reliability. Rock mass continuity decreases exponentially with earthquake number and controls magma pressure and flow Pre-eruptive earthquake time series are controlled by heterogeneity distribution and crack interaction for quasi-static deformation When crack interact, magma-edifice interaction power exhibits a maximum before the eruption, used to estimate the eruption time at PdF