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 (V_p) is accompanied by a 1.5 ± 0.5% increase in S wave velocity (V_s) near a depth of 2570 km. Bulk-sound velocity [V_b = (V _p² - 4/3V_s²)^1/2] decreases by -1.0 ± 0.5% at this depth. Transition of the primary lower-mantle mineral, (Mg_1-x-y Fe_xAl_y)(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 V_b and V_s anomalies; this behavior explains the data well.