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) Earth cross-section with representative raypaths for direct (P/S) and CMB reflected (PcP/ScS) phases, and any reflections from deep mantle discontinuities (dashed). The D” region structure in the lowermost mantle is the focus of this study. (B) Map indicating the study configuration, involving 75 earthquake epicenters (green stars), seismic stations (red triangles) and surface projections of PcP core-mantle boundary (CMB) reflection points (blue dots). The dashed line shows the surface trace of a cross-section made through the migration image volume in the lower mantle (Fig. 3). The inset map (C) shows both PcP and ScS CMB reflection points and two data bins where these overlap, used in waveform stacking analysis.
<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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