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Waveform modelling of global seismic anisotropy and geodynamical implications

Kendall, Elodie; (2020) Waveform modelling of global seismic anisotropy and geodynamical implications. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

This thesis uses advanced seismic forward and inverse modelling to test and refine existing isotropic and anisotropic 3D Earth mantle models in two geodynamically relevant regions: (i) East Africa; and, (ii) the Pacific. Direct S waveform comparisons indicate that a deep, negative shear-velocity anomaly <-1% in the uppermost lower mantle beneath Afar is required. This in turn supports the hypothesis that there are at least two distinct mantle plumes beneath Afar and Kenya. Secondly, we perform synthetic inversion tests to investigate whether current global seismic datasets can resolve a hypothetical depth-age dependent radial anisotropy beneath the Pacific. We find good recovery of the input depth-age dependent trend in radial anisotropy. This prompts us to further investigate discrepancies at lithosphere-asthenosphere depths in existing radial anisotropy models through independent forward modelling. Our results show that the inclusion of laterally varying anisotropy in the modelling improves the data fit of T~40s and T~60s Love waves on average by ~2s. We then quantify the adjustments required in existing radially anisotropic models to further improve surface wave data fits. We find that positive perturbations in radial anisotropy at ~100km depth are required near the west coast of South America and near Hawaii (4-6%) and stronger anomalies are needed near the East Pacific Rise (6-8%). Finally, we perform geodynamical simulations to explain the mechanisms behind the observed radial anisotropy in the Pacific. 2D ridge flow models along with mantle fabric calculations confirm that the observed weak age-dependence of radial anisotropy may simply result from shearing in the asthenosphere. These initial results correlate with current radially anisotropic tomographic models. Future work will use a larger spatial domain that replicates the Pacific well and will investigate the role of the Hawaiian plume on the observed anisotropy.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Waveform modelling of global seismic anisotropy and geodynamical implications
Event: University College London
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2020. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10094069
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