EGU2020-17915
https://doi.org/10.5194/egusphere-egu2020-17915
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Crustal viscosity and its control on volcanic ground deformation patterns

Matthew Head1, James Hickey1, Jo Gottsmann2, and Nico Fournier3
Matthew Head et al.
  • 1Camborne School of Mines, University of Exeter, Cornwall, United Kingdom (m.head2@exeter.ac.uk)
  • 2School of Earth Sciences, University of Bristol, Bristol, United Kingdom
  • 3GNS Science, Wairakei Research Centre, Taupō, New Zealand

Episodes of ground deformation, relating to the unrest of a volcanic system, are often readily identifiable within geodetic timeseries (e.g. GPS, InSAR). However, the underlying processes facilitating this deformation are more enigmatic. By modelling the observed deformation signals, the ultimate aim is to infer characteristics of the deforming reservoir; namely the size and time-dependent evolution of the system and, potentially, the fluxes of magma involved. These parameters can be estimated using simple elastic models, but the presence of shallow or long-lived magmatic systems can significantly perturb the local geothermal gradient and invalidate the elastic approximation. Inelastic rheological effects are increasingly utilised to account for these elevated thermal regimes, where a component of viscous (time-dependent) behaviour is expected to characterise the observed deformation field.

Here, our investigations are concentrated on Taupō volcano, New Zealand, the site of several catastrophic caldera-forming eruptions. We use 3D thermomechanical models of the Lake Taupō region, featuring thermal constraints and heterogeneous crustal properties, to compare the commonly-used Maxwell and Standard Linear Solid (SLS) viscoelastic configurations under contrasting deformation mechanisms; a pressure condition (stress-based) and a volume-change (strain-based). By referring to models allocated a single viscosity value, we investigate the influence of a temperature-dependent viscosity distribution on the predicted spatiotemporal deformation patterns. Comparisons of the overpressure models highlights the influence of the crustal viscosity structure on deformation timescales, by enabling the SLS rheology to account for both abrupt and long-term deformation signals. For the Maxwell rheology, we show that the viscosity distribution results in unexpected deformation patterns, both spatially and temporally, and so query the suitability of this rheology in other model setups. Further to this, the deformation patterns in volume-change models are governed by the resulting stress response, and the effect of the viscosity structure on its propagation. Ultimately, we demonstrate that variations in crustal viscosity greatly influence spatiotemporal deformation patterns, more so than heterogeneous mechanical parameters alone, and consequently have a large impact on the inferences of the underlying processes and their time-dependent evolution. The inclusion of a crustal viscosity structure is therefore an important consideration when modelling volcanic deformation signals.

How to cite: Head, M., Hickey, J., Gottsmann, J., and Fournier, N.: Crustal viscosity and its control on volcanic ground deformation patterns, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17915, https://doi.org/10.5194/egusphere-egu2020-17915, 2020

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Presentation version 1 – uploaded on 04 May 2020
  • CC1: Questions and answers from the live chat during EGU2020, Michael Heap, 11 May 2020

    Q: Can you elaborate/speculate on why the deformation responses of the models are different?

    A: The deformation repsonses between the Maxwell and SLS models are different due to the formulation of their creep behaviour (see Head et al., 2019). For a single VE model, this study demonstrates that thermomechanical strain partitioning modifies the deformation field - despite the viscosity structure above the reservoir being the same for each thermomechanical setup, the deformation field is modified by the local viscosity structure around the reservoir (where each thermomechanical model starts to differ)

    Q: This is an interesting work. Did you use penny shaped crack source? And do you have prior information to determine the depth/volume of the source?

    A: The deformation source has an oblate spheroid geometry, which are informed by previous studies (e.g. petrological, structural) studies of the 1.8 ka Taupo eruption

    Q: Nice contribution! You say Standard Linear Solid (SLS) response is not only limited to long-term. What are the key short-term characteristics of the SLS response?

    A: The SLS model displays asymptotic rate-decreasing uplift, dependent on viscosity, and so the low viscosities in the vicinity of the reservoir result in high rates of displacement

    Q: Hi, could it be that the low-visc rheology doesn"t actually expand all the way as deep as you assumed? from geophys. info ?

    A: It could well be that low-viscosity regions do not extend as deep as the model suggest, however in the case of a thermally-primed crust in which the thermal field has become steady-state), we still expect elevated temperatures and low viscosities beneath the reservoir. An interesting study would be to look at the variation of parameters that determine the viscosity - however these aren't well constrained.