The structure of the core‐mantle boundary from diffracted waves

Diffracted P and S waves (Pd, Sd) traveling around the core-mantle boundary (CMB) of the Earth give us information about the velocity structure and therefore the thermochemistry of D″, the base of the Earth's mantle. By examining Pd and Sdarrivals we determined the apparent ray parameter for different regions at the base of the mantle. By comparing the data slownesses to those found from reflectivity synthetic seismograms we were able to quantify D″ average velocities. Using these averaged velocities with a thermochemical modeling of lower mantle minerals using a Birch-Mumaghan equation of state, we have been able to make chemical and physical inferences as to the causes of lateral variations at the CMB. Examinations found significant lateral heterogeneity at the base of the mantle, amounting to ≈ 4% for both P and S velocities. These velocities did not always vary in parallel, and the Poisson ratio varied regionally by almost 6%. The most unusual region of the CMB found was under Indonesia, where velocities 3% slower than the preliminary reference Earth models were found adjacent to a region of faster than average velocities. These regions currently correspond to areas of core up welling and down welling (respectively) found by Voorhies (1986), which if mostly held in place by core-mantle coupling might cause a flux of heat and iron into the mantle, making the anomaly both thermally and chemically derived. At the CMB under the northern Pacific rim the fastest shear velocities were found, but the same region yielded slower than average P velocities. While the presence of fast shear velocities here would support the idea that we are seeing the cold dregs of mantle convection, perhaps continuing down from the North Pacific subduction zones, the presence of slow P velocities suggests additional complications. Our thermochemical modeling suggests that the D″ Poisson ratio is very sensitive to variations in the silicate/oxide ratio and that a decrease in the amount of perovskite relative to magnesiowustite may play an important role in this region.

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