Anticorrelated Seismic Velocity Anomalies from Post-Perovskite in the Lowermost Mantle

Earth's lowermost mantle has thermal, chemical, and mineralogical complexities that require precise seismological characterization. Stacking, migration, and modeling of over 10,000 P and S waves that traverse the deep mantle under the Cocos plate resolve structures above the core-mantle boundary. A small -0.07 ± 0.15% decrease of P wave velocity (Vp) is accompanied by a 1.5 ± 0.5% increase in S wave velocity (Vs) near a depth of 2570 km. Bulk-sound velocity [Formula: see text] decreases by -1.0 ± 0.5% at this depth. Transition of the primary lower-mantle mineral, (Mg₁₋x₋y FexAly)(Si,Al)O₃ perovskite, to denser post-perovskite is expected to have a negligible effect on the bulk modulus while increasing the shear modulus by ~6%, resulting in local anticorrelation of Vb and Vs anomalies; this behavior explains the data well.