Viscoelastic Controls on Volcanic Ground Deformation and Reservoir Failure

Abstract

Volcanic crises pose a significant hazard to communities worldwide. A comprehensive understanding of volcanic unrest is therefore essential for recognising any precursory activity that may indicate the onset of an eruption. Surface deformation patterns contain important information regarding the dynamics and characteristics of a deforming magmatic system, which can be interpreted using geodetic models. The inferences and conclusions of such models are intrinsically linked to the rheological assumptions of the crustal rock surrounding the magmatic system. Viscoelasticity is commonly used to provide a component of inelastic crustal behaviour, accounting for thermal heterogeneity by incorporating temperature-dependence. However, there are very limited studies that explore how the viscoelastic deformation response can vary within geodetic models. This thesis critically evaluates the implementation and mechanics of linear viscoelastic models based on their fundamental stress-strain relationships, demonstrating that pressurisation and volumetric expansion of a magmatic system do not produce the same deformation response, which has implications for the inferred dynamics of the magmatic system. Whilst thermo-viscoelasticity is increasingly used to characterise crustal behaviour in volcanic regions, there are no sensitivity analyses that consider the effect of varied thermal constraints. This thesis demonstrates that reservoir temperature is a key control on spatiotemporal deformation patterns by altering the intrinsic timescales of the deformation response. These timescales also impact the evolution of the tensile stress, affecting the mechanical stability of a magmatic system. Compared to elastic models, magmatic systems within a thermo-viscoelastic crust can maintain greater overpressures prior to failure. Together, these results highlight the importance of parametric studies to thoroughly explore the deformation response and implications for a given crustal rheology. Consequently, this thesis contributes to a greatly improved understanding of the viscoelastic deformation response, specifically for thermo-viscoelasticity. Ultimately, this research can be used to refine future geodetic models, leading to an improved knowledge of the processes that cause volcanic unrest.