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 ( Vs) near a depth of 2570 km. Bulk- sound velocity [ V-b = ( V-p(2) - 4/3V(s)(2))(1/2)] decreases by - 1.0 +/- 0.5% at this depth. Transition of the primary lower- mantle mineral, ( Mg1-x-y FexAly)(Si,Al)O-3 perovskite, to denser post- perovskite is expected to have a negligible effect on the bulk modulus while increasing the shear modulus by similar to 6%, resulting in local anticorrelation of V-b and V-s anomalies; this behavior explains the data well.