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Abstract: Recent results suggest that an iron-rich oxide may have fractionally crystallized from a primordial magma ocean and settled on the core–mantle boundary (CMB). Based on experimental results, the presence of only a few percent of Fe-rich oxide could slow seismic waves down by several percent. This heavy layer can become highly undulating as predicted from dynamic modeling but can remain as a distinct structure with uniform velocity reductions. Here, we use the large USArray seismic network to search for such structures. Strong constraints on D″ are provided by the core-phase SKS where it bifurcates, containing a short segment of P-wave diffractions (P d ) when crossing the CMB, called SKS d . Synthetics from models with moderate velocity drops (less than 10%) involving a layer with variable thickness, perhaps a composite of sharp small structures, with strong variation in thickness can explain both the observed SKS d waveforms and large scatter in differential times between SKKS and SKS. A smooth 3D image is obtained from inverting SKS d waveforms displaying rolling-hills with elongated dome-like structures sitting on the CMB. The most prominent one has an 80-km height, ∼8° length, and ∼4° width, thus adding still more structural complexity to the lower mantle. We suggest that these results can be explained by a dynamically-stabilized material containing small amounts (∼5%) iron-rich (Mg,Fe)O providing a self-consistent physical interpretation. [Copyright &y& Elsevier]

Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling. [ABSTRACT FROM AUTHOR]

A recent study of the lower mantle structure beneath Africa revealed strong lateral changes in S-velocity extending upward from the core–mantle boundary to about 1500 km. SKS travel times observed on the South African Array display jumps of about 6 s when ray paths cross these nearly vertical boundaries. Back projecting these delays onto the core–mantle boundary allows a clear image of the horizontal extent of this structure starting at mid-Africa (15°S, 5°E) where it strikes roughly northwest to beyond the tip of South Africa (45°S, 55°E) where it bends toward the Indian Ocean. Waveform sections of S, ScS, SKS, and SKKS are modeled along two corridors, one along strike and one at right angles to establish its uniformity. The structure is about 1200 km wide and has about a 3% drop in S-velocity although some small-scale features are apparent in the roof structure and midsection. If this structure is stabilized by a localized viscosity condition or a dense core as suggested by some joint inversions, gravity, and free oscillations, it may be isolated from mantle stirring and therefore very old with unique chemistry. [Copyright &y& Elsevier]