The large-scale structure of the deep Earth as constrained by free oscillation observations

Large earthquakes are capable of exciting the elasto-gravitational free oscillations of the Earth to a detectable level. In this thesis, recordings of such earthquakes from the last 16 years are used to observe the resonance frequencies of the Earth. Given that the deviations from spherical symmetry are small, the determination of the large-scale mechanical structure of the Earth separates into two problems. Observations of the complex multiplet degenerate frequencies allow us to constrain the spherically symmetric elastic and anelastic structure and deviations from spherical symmetry can be constrained by observations of mode splitting.
Chapter two reports observations of the degenerate frequencies of 600 spheroidal and toroidal multiplets with frequencies less than 10 mHz. We pay particular attention to the assessment of bias in these observations caused by the effects of large-scale aspherical structure. Current Earth models do not predict correctly our observations and we present a new model, CORE11, which goes a long way to fitting the new dataset. We are unable to find models which adequately fit the frequencies of modes sensitive to core structure.
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In chapter three, we use two techniques, which explicitly take the effect of large-scale aspherical structure into account to estimate degenerate frequency and axisymmetric degree 2 structure coefficients of 10 core-sensitive multiplets in the frequency band 5-10 mHz. We show that axisymmetric structure in the inner core cannot explain the anomalous splitting of core-sensitive multiplets. Our analysis of the data leads us to suggest that the cause of anomalous splitting is in the outer core. In chapter four we use recordings of the 1989 Macquarie rise earthquake to analyze the lowest order fundamental toroidal modes and we report the first unambiguous observation of the gravest toroidal mode oT2. Chapter five discusses the use of observations of the attenuation of free oscillations to infer the depth dependence of the intrinsic attenuation. We find that models with only five homogeneous layers can explain the data. Observations of PKIKP equivalent modes lead to a shear Q of 110±25% in the inner core. Chapter six critically summarizes the data set constraining spherically symmetric Earth structure.