What Can We Learn on Damage Process from the Study of Pre-eruptive Periods on Volcanoes? Examples from Grimsvötn (2004-2011) and Piton de la Fournaise (2004-2018) – Implications for Rupture Understanding and Prediction

Abstract:

Pre-eruptive deformation processes in basaltic volcanoes are closely related to the progressive rupture of the volcanic edifice prior to the eruption. Earthquake number and surface deformation often progressively increase around pressurized magma reservoirs when approaching their rupture and the subsequent eruption. These processes are often well recorded by volcanological observatory networks. In basaltic volcanoes, magma reservoir pressurization may be described by simple models, and surface deformation data may be explained by pressurization, damage and edifice weakening around magma reservoirs, using the pre-eruptive earthquake time series. Decrease of elastic properties induced by damage due to pre-eruptive earthquakes allows fitting the pre-eruptive surface displacements.

In some cases the pre-eruptive rupture process corresponds to the progressive rupture of a limited, decreasing, area of cohesive material. It is well described by the Kachanov’s damage theory in mechanics and Failure Forecast Method in volcanology: Kachanov’s effective stress and earthquake rate increase as a power law of time, and rupture may be predicted.

In other cases, earthquake rate does not increase as a power law: a part of the deformation occurs in pre-fractured volumes in which plastic, frictional, deformation occurs at constant earthquake rate, and rupture becomes difficult to predict.

Our approach proposes to estimate variables characterizing the state of the deforming volume directly from its deformation and seismicity, without seismicity model. Earthquake number data constrain damage laws that can be used to fit GPS surface deformation and compute Kachanov’s continuity and effective stress, magma reservoir overpressure variation and magma flow. We show that using these parameters greatly helps to infer how the volcano is close to an eruption, even when seismicity and displacement data do not allow to conclude. The various pre-eruptive earthquake patterns are related to the physics of the rock deformation.