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Title: Seismological studies of inner core anisotropy
Author: Irving, J. C. E.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2009
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In this dissertation I examine the inner core using two complementary seismic perturbations: seismic body waves and free oscillations of the Earth. I investigate the velocity anisotropy present in the inner core by measuring PKPdf-PKPbc and PKPdf-PKPab differential travel time residuals. A model for an inner core with 3.5% cylindrical anisotropy is consistent with the data. However, the data are better fitted by an inner core which is divided into two ‘hemispheres’, an isotropic eastern hemisphere between 14°E and 151°W and western hemisphere which contains 4.4% anisotropy between 151°W and 14°E. The axis of anisotropy in the inner core is coincident with the Earth’s rotation axis, and an isotropic layer at the top of the western hemisphere of up to 150km in depth is consistent with the data. I find that there is also hemispherical variation in inner core attenuation anisotropy, the western hemisphere displays attenuation anisotropy whilst the eastern hemisphere does not. I examine the possibility of detecting a hemispherical velocity anisotropy structure using free oscillations of the Earth’s normal modes. New theory is derived to allow any existing cylindrical anisotropy model to be confined to only one part of the inner core. When this new theory is used and full coupling between modes is permitted, radial, PKIKP and PKJKP modes all show changes in their frequencies and quality factors. These changes are large enough to be observed in the data, and are dependent on the model of inner core anisotropy which is used. Normal mode data can therefore be used to detect hemispherical structure in inner core anisotropy. I also show that normal mode data can be reconciled with evidence presented by body wave data that the top of the inner core is isotropic.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available