Continent-scale observations of seismic phenomena have provided multi-scale constraints of the Earth’s interior. Of those analysed, array-based observations of slowness vector properties (backazimuth and horizontal slowness) and multipathing have yet to be made on a continental scale. Slowness vector measurements give inferences on mantle heterogeneity properties such as velocity perturbation, velocity gradient strength and quantify their effect on the wavefield. Multipathing is a consequence of waves interacting with strong velocity gradients resulting in two arrivals that arrive with different slowness vector properties and times. The mantle structure beneath the contiguous Unites States has been thoroughly analysed by seismic studies and is data-rich, making it an excellent testing ground to analyse mantle structure with our approach and compare with other techniques. We apply an automated array-analysis technique to an SKS dataset to create the first continent-scale dataset of multipathing and slowness vector measurements. We analyse the divergence of the slowness vector deviation field to highlight seismically slow and fast regions in our data. Our results resolve several slow mantle anomalies beneath Yellowstone, the Appalachian mountains and fast anomalies throughout the mantle. Many of the anomalies cause multipathing in frequency bands 0.15–0.30 and 0.20–0.40 Hz which suggests velocity transitions over at most 500 km exist. Comparing our observations to synthetics created from tomography models, we find model NA13 \citeA{bedle_continental_2021} fits our data best but differences still remain. We therefore suggest slowness vector measurements should be used as an additional constraint in tomographic inversions and will lead to better-resolved models of the mantle.

Perhaps the least ambiguous signal that the mantle is convecting comes from observations of seismic anisotropy---the variation of wave speed with direction---which must arise due to the ordering of material as deformation occurs. Therefore significant effort has been made over many years to infer the direction and nature of mantle flow from these data. Observations have focussed on the boundary layers of the mantle, where deformation is expected to be strongest and where anisotropy is usually present. While prospects for mapping flow seem good, the lack of knowledge of several key issues currently holds progress back. These include the cause of anisotropy in the lowermost mantle, the causative material's response to shear, and the single-crystal or -phase seismic properties of the causative materials. In this chapter we review recent observations of lowermost mantle anisotropy, constraints on mineral elasticity and deformation mechanisms, and challenges in linking geodynamic modelling with seismic observations.

Large Low Velocity Provinces (LLVPs) are hypothesised to be purely thermal features or possess some chemical heterogeneity, but exactly which remains ambiguous. Regional seismology studies typically use travel time residuals and multipathing identification in the waveforms to infer properties of LLVPs. These studies have not fully analysed all available information such as measuring the direction and inclination of the arrivals. These measurements would provide more constraints of LLVP properties such as the boundary velocity gradient and help determine their nature. Here, we use array seismology to measure backazimuth (direction) and horizontal slowness (inclination) of arriving waves to identify structures causing multipathing and wavefield perturbation. Following this, we use full-wavefield forward modelling to estimate the gradients required to produce the observed multipathing. We use SKS and SKKS data from 83 events sampling the African LLVP, which has been extensively studied providing a good comparison to our observations. We find evidence for structures at heights of up to 600 km above the core-mantle boundary causing multipathing and wavefield perturbation. Forward modelling shows gradients of up to 0.7% δVs per 100 km (0.0005 km /s km) are required to produce multipathing with similar backazimuth and horizontal slowness to our observations. This is an order of magnitude lower than the previous strongest estimates of -3% δV per 50 km (0.0044 km /s km). As this is lower than found for both thermal and thermochemical structures, gradients capable of producing multipathing is not necessarily evidence for a thermochemical nature.

Random and small-scale subsurface heterogeneities in velocity and/or density scatter the seismic wavefield when they have scale lengths on the order of the seismic wavelength. Seismic scattering is considered the origin of coda waves. Such inhomogeneities have an important effect on propagating waves, as they generate traveltime and amplitude fluctuations and may be the cause of attenuation or excitation of secondary waves. Understanding the effect of small-scale heterogeneities on the seismic wavefield is important for the characterization of the seismic source (e.g. source parameters of underground nuclear explosions) and to improve our knowledge of the Earth’s structure along the raypath. Several approaches and methods have been suggested to study the scattering of seismic waves and characterise subsurface heterogeneities. Here, we apply a combination of the analysis of the incoherent wavefield component and the coda decay with time to a dataset of over 350 teleseismic events (over 20000 traces) recorded at three seismic arrays (Warramunga, Alice Springs and Pilbara) in Australia. This combination allow us to obtain a series of parameters (correlation length, RMS velocity fluctuations of the heterogeneities and thickness of the scattering layer) that give us a measure of the spatial scale and the magnitude of the heterogeneities present in the lithosphere beneath the arrays. This is the first time such a large dataset is used for a study of these characteristics. Our new results show similar structures and scattering strength for Alice Springs and Warramunga, while revealing a different tectonic signature and stronger scattering in the case of Pilbara, possibly caused by the different thicknesses of crust and lithosphere between these regions and its different tectonic history. These stochastic models of the lithosphere are the first step in the development of a technique analogous to adaptive optics which, in this case, aims at removing the effect of the small-scale, near receiver structure from recorded wavefields, thus enabling us to improve our source characterization and to more clearly image the Earth’s interior.