Shear wave speeds at the base of the mantle

We inverted 4864 ScS‐S and 1671 S diff ‐ SKS residual travel times for shear wave speed anomalies at the base of the Earth's mantle. We applied ellipticity corrections, accounted for mantle structure outside of the basal layer using mantle tomography models, and used finite size sensitivity kernels. The basal layer thickness was set to 290 km; however, the data allow thicknesses between 200 and 500 km. The residuals were inverted using a spherical harmonic basis set of degree 30 for a model that both is smooth and has a small Euclidean norm, which limited spectral leakage of higher‐order structures into low‐order wavelengths. Hotspots dominantly overlay slow wave speed regions. Nonsightings of ultralow‐velocity zones (ULVZs) most frequently appear in fast regions, suggesting that slow regions at the base of the mantle are associated with ULVZs. However, ULVZ sightings appear in both slow and fast regions. Recently active subduction zones do not correlate with velocity anomalies; however, the locations of subduction zones active prior to 90 Ma correlate extremely well with fast anomalies, implying that slabs descend as fast as 2 cm yr −1 through the lower mantle. The correlation continues through the historical subduction record to 180 Ma, suggesting that slabs remain in the deep mantle at least 90 Myr. Fast anomalies reach +2%, while slow anomalies extend to −5%. If we assume that the anomalies are thermal and anharmonic in origin and apply a wave speed/thermal anomaly conversion, the temperature deviations would be over −500°K (cold) in fastest regions and over +1000°K (hot) in the slowest regions, which would initiate plumes much hotter than those observed at the surface. Alternative explanations for the large anomalies are widespread partial melt or compositional differences in the lowermost mantle.