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• A quantitative method resolves the fine structure of the D″ layer. • Small-scale heterogeneities exist at CMB beneath eastern Alaska & northern Pacific. • There is a lack of correlation between D″ topography and shear velocity variation. • The D″ layer may be a mixed boundary layer comprised of thermo-chemical anomalies. • Inferred accumulated slab material suggests different mantle convection styles. The D″ layer, regarded as a thermal boundary layer and a chemically distinct region above the core-mantle boundary (CMB), has been associated with the phase transition from bridgmanite (Bm) to post-perovskite (pPv) in the lowermost mantle. However, the composition of the lowermost mantle and thermal conditions where Bm-pPv phase-transition occurs is still debatable. The methods typically used to study the fine-scale seismic features in the D″ layer has provided important information. However, trial-and-error seismic waveform modelling cannot uniquely quantify D″-layer properties because of subjective model-parameterization choices and inherent non-uniqueness of solutions and the waveform inversion method has a limited resolution of the velocity gradient and depth of the D″ discontinuity. We develop a grid-search scheme to constrain the detailed 1-D shear-wave velocity structure in the lowermost mantle beneath Alaska and the northern Pacific, accompanied with quantitative assessment of the uncertainty of 1D models. Our results show strong lateral variations of the D″ discontinuity from west to east beneath Alaska, along with the existence of smaller-scale heterogeneities in the east. We find a broad velocity increase, as thick as 240 km, at the top of D″ that indicates this region may involve a composite of downwelling thermo-chemical anomalies at the CMB. There are even smaller scale heterogeneities of approximately 120 km × 120 km in size with larger lateral variations in the lowermost mantle beneath northern Pacific. Both the magnitude and gradient of the velocity at the top of the D″ layer vastly change in adjacent regions, with an increase from 2.8% to 4.5% in magnitude and from 0.08% to 1.2% in gradient, but with a relatively consistent depth of the D″ discontinuity at ∼340 km above the CMB. The weak correlation between D″ topography and velocity variations indicate chemical heterogeneities must be present beneath the northern Pacific, which might come from north-westward subducted Pacific oceanic lithosphere. Our characterisation of the spatial pattern of small-scale heterogeneities in the lowermost mantle supports a hybrid thermo-chemical boundary layer (TCBL) model beneath Alaska and northern Pacific. [ABSTRACT FROM AUTHOR]

We present the results of a new global Bayesian tomographic inversionfor a radial anisotropic shear wave velocity model of the Earth. Thedevelopment of radial anisotropic shear wave models are useful forunderstanding mantle dynamics. Our model improves on existing work byproviding robust estimates of uncertainties.A key aspect of this new model is the careful selection of surfacewave observations produced by earthquakes from the Harvard CMTcatalog. We automatically process a large set of seismic signalsrecorded for these events by simultaneously inverting Love andRayleigh waveforms up to the fifth overtone. After initial processing,only events where high quality Love and Rayleigh surface waves areobserved are retained for the tomographic inversion. This careful dataselection ensures that inference of radial anisotropy is not biaseddue to variations in spatial coverage of Love and Rayleigh observations.Our tomographic inversion uses a novel Bayesian Trans-dimensional Treeapproach that dynamically adapts the complexity of the solution toincrease spatial resolution where observations provide additionalconstraint. This leads to a robust estimation of both the shear wavemodel and its uncertainties. Furthermore, we demonstrate how ourapproach can be extended to a joint inversion to include body shearwaves to further constrain radial anisotropy of the Earths mantle. [ABSTRACT FROM AUTHOR]