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- Title
Exploring the Impact of Thermally Controlled Crustal Viscosity on Volcanic Ground Deformation.
- Authors
Head, Matthew; Hickey, James; Gottsmann, Joachim; Fournier, Nicolas
- Abstract
Volcanoes undergoing unrest often produce displacements at the ground surface, providing an important window to interpret the dynamics of the underlying magmatic system. The thermomechanical properties of the surrounding host rock are expected to be highly heterogeneous, with key physical parameters having a strong dependence on temperature. Deformation models that incorporate nonelastic rheological behaviors are therefore heavily reliant on the assumed thermal conditions, and so it is critical to understand how the thermomechanical crustal structure affects the observed deformation field. Here, we use a series of thermo‐viscoelastic Finite Element models to explore how variations in thermal constraints (i.e., reservoir temperature and background geothermal gradient) affect surface displacement patterns when using the Maxwell and Standard Linear Solid (SLS) viscoelastic configurations. Our results demonstrate a strong variability in the viscoelastic deformation response when changing the imposed thermal constraints, caused by the partitioning of deformation and the dissipation of induced stresses. When using the SLS rheology, we identify that cumulative long‐term displacements can vary by over 20%, relative to a reference model with a reservoir temperature of 900°C and background geothermal gradient of 30 K km−1. The relative change increases to a maximum of 35% when thermal weakening of the Young's modulus is also considered. Contrastingly, the deformation patterns of the Maxwell rheology are governed by unbounded displacements and complete stress relaxation. Ultimately, we outline that uncertainties in the thermal constraints can have a significant impact on best‐fit source parameters (e.g., size and depth) and overpressure/volume‐change loading histories inferred from thermo‐viscoelastic models. Plain Language Summary: Deformation of the ground surface sometimes occurs prior to a volcanic eruption due to the movement of magma within the Earth's crust. Shallow or long‐term storage of magma can significantly increase the temperature of the surrounding rock, enabling ductile behavior in response to induced stresses and strains. As a result, the crust surrounding a deforming magmatic system may be best represented by nonelastic materials, which account for time‐ and temperature‐dependent rheological effects. Recent studies use a viscoelastic rheology to represent the crust, which combines components of instantaneous (elastic) and time‐dependent (viscous) behavior and is strongly dependent on temperature. In this study, we explore the impact of assumed thermal conditions on the resultant displacements at the ground surface. We identify that the ground displacements are predominantly affected by the temperature of the magma, whilst the background temperature profile plays a minor role. Furthermore, our results demonstrate the intrinsic fluid behavior of the commonly used Maxwell viscoelastic rheology, presenting implications for its representation of solid rock. Ultimately, this study highlights that source parameters (e.g., size and depth) and the underlying mechanisms, inferred from a deformation episode, can be significantly impacted by variations in thermal conditions when using viscoelastic models. Key Points: Deformation source parameters inferred from thermo‐viscoelastic models are strongly influenced by the imposed thermal constraintsSurface displacements are primarily controlled by the reservoir temperature, affecting the rate and magnitude of deformationBackground geothermal gradient determines the ambient viscosity around the reservoir, having a secondary effect on surface displacements
- Subjects
VOLCANOES; THERMOMECHANICS of magnetic fluids; STANDARD linear solid model; MATHEMATICAL models of viscoelasticity; VISCOSITY
- Publication
Journal of Geophysical Research. Solid Earth, 2021, Vol 126, Issue 8, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2020JB020724