ADVANCES IN INTEGRATED VOLCANIC GEODETIC MODELLING AND APPLICATION TO DEEP-SEATED UNREST AT UTURUNCU VOLCANO, BOLIVIA
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Abstract
Moving on from simple, analytical deformation models, we have developed a suite of numerical models that can account for a range of realistic crustal complexities and are thus more consistent with multi-disciplinary constraints. Using Finite Element techniques through commercial code COMSOL Multiphysics we can incorporate the following additional intricacies over the homogeneous, isotropic, elastic half-space's usually employed: viscoelasticity, elasto-plasticity, crustal heterogeneity, lateral discontinuities, topography, gravity-loading, multiple or irregular shaped sources and temperature-dependent rheology. These developments allow for more advanced and integrated models of deforming volcanoes that better constrain causative sources of unrest. Successful benchmarking against analytical equivalents has been carried out where possible to validate model results.
We apply the models to an on-going period of unrest at Uturuncu volcano in southern Bolivia, focusing on the driving mechanism behind a 70 km wide region of ground uplift. To constrain a viable model we test for first-order parameters that reproduce the observed maximum uplift rate of 1 – 2 cm/yr between 1992 and 2006. We account for heterogeneous and homogeneous subsurface structure in elastic and viscoelastic rheologies. Contrasting crustal heterogeneity and homogeneity highlights the significant effect of a mechanically weak source-depth layer. This alters surface deformation patterns by absorbing more of the subsurface strain than its surroundings, thereby acting as a mechanical buffer.!As elastic models can only account for the spatial component of deformation, their results are used solely to guide the parameters tested in the viscoelastic models. We explore a range of possible source geometries but reject spherical and oblate shapes on the grounds of their depth and likely unsustainable pressurisation given the expected crustal mechanics. Our preferred model suggests that pressurisation of a magma source extending upward is causing the observed uplift and requires a continued increase in this pressure to explain both the spatial and temporal displacement patterns.