Mapping Lowermost Mantle Seismic Structures With Core‐Diffracted Waves

Abstract

Pdiff and SHdiff waves, diffracted along the core‐mantle boundary (CMB), are highly sensitive to the structures of the lowermost mantle (LMM). Using an automated procedure, we measure the absolute traveltime residuals (, ) from cross‐correlation ( cc ) between observations and synthetics for 2.02 million Pdiff + P and 1.96 million SHdiff + SH waves in multiple period bands. The gridded mean and , and , exhibit a degree two pattern consistent with the Large Low Shear‐Velocity Provinces (LLSVPs) observed in global seismic tomography models beneath Africa and the Pacific. Furthermore, the are 2–3 times stronger than , and under the African LLSVP are stronger than under the Pacific LLSVP by a factor of 2–3. The distribution of the ratio shows a broad, dispersive pattern, with the majority of the population between −2 and +5, in contrast to purely thermal effects, which predicted a ratio in the range of 1.7–2.6. Finally, the distribution is skewed to low velocity, while the distribution is Gaussian. Collectively, these observations point to thermochemical LLSVPs.