Analysis of Mantle Heterogeneity through Array Observations of Multipathing and its Expansion to a Global Scale

Many mysteries remain about the inner workings of the Earth from what the origins of lower mantle structures to when did plate tectonics begin. Seismology provides observations of the Earth’s interior by analysing phenomena such as the reflections of waves or anomalous timings of waves arriving at the recording stations. Through analysis of these phenomena, information such as the morphology, location and velocity perturbation of mantle heterogeneity has been constrained. Information about the velocity gradients, how quickly the seismic velocity of material changes with distance, at the boundaries of mantle heterogeneities has not been analysed to the same extent and can aid our understanding of its thermal and chemical properties. When a wave interacts with a sufficiently strong velocity gradient, the wave moves at two different speeds over a short distance leading to multiple arrivals arriving at the recording station. This phenomenon, called multipathing, has been analysed through the waveform complexity it creates and has led to estimations of the velocity gradients at the boundaries of mantle heterogeneity. In addition to the waveform complexity, the multiple arrivals should arrive with different directions and horizontal speeds through diffraction of the wave by the velocity gradient. The direction and horizontal speeds have seldom been used to analyse multipathed arrivals and could give more information about the heterogeneities. In this thesis, I use array seismology methods to analyse multipathing and diffraction of the wavefield by measuring the direction and speeds of the arrivals and expand the analysis to a global scale. First, I analyse multipathing caused by the African Large Low-Velocity Province (LLVP), a continent-sized anomaly at the core-mantle boundary beneath Africa, and show multipathing is frequency dependent and can be caused by relatively weak velocity gradients. Then, I develop a method to automate the identification of multipathing with array methods using cluster analysis which also provides uncertainty estimates of the measurements. Finally, this automated method is used to create a multi-regional map of SKS multipathing which motivates future studies analysing the conditions needed for multipathing and for comparison with other investigations into the Earth’s current state and evolution.