Anti-Correlated Seismic Velocity Anomalies from Post-Perovskite in the Lowermost Mantle

Anti-Correlated Seismic Velocity Anomalies from Post-Perovskite in the Lowermost Mantle (A) P-wave double array stacking results for two 5° latitudinal bins beneath the Cocos Plate. The solid black line shows the data stack amplitude relative to the direct P amplitude. The grey shaded region is the 95% confidence interval of the stack from bootstrap re-sampling, and the dashed line is the number of seismograms contributing to the data stack at a particular depth (right scales). Above 400 km the stack for 5-10°N is contaminated by P coda. The red lines are stacks of synthetic seismograms generated from our preferred models. The synthetics were sampled to match the source-receiver geometry and had the same processing as the data. (B) Elasticity models obtained by modeling the data: ρ is density (gm/cm3), V is S-wave velocity (km/s), Vb is bulk-sound velocity (km/s) and Vp is P-wave velocity (km/s). Percent changes in these parameters are shown at first-order discontinuities at the indicated depths. The depth range likely to contain post-perovskite (pPv) is indicated on the right.<p>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 resolves structures above the core-mantle boundary (CMB). 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 (Vb = [Vp2−4/3Vs2]**1⁄2) 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 negligible effect on the bulk modulus while increasing the shear modulus by ~6%, resulting in local anti-correlation of Vb and Vs anomalies; this explains the data well.
</p><p>References
</p><p>Hutko, A. R., T. Lay, J. Revenaugh, E. J. Garnero (2008). Anti-correlated seismic velocity anomalies from post-perovskite in the lowermost
mantle, Science, 320, 1070-1074.
</p><p>Acknowledgements: This work was supported in part by the U.S. National Science Foundation under grants EAR-0453884, and EAR-0453944.</p>

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