Lateral Variations in Compressional/Shear Velocities at the Base of the Mantle

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Science  02 Apr 1999:
Vol. 284, Issue 5411, pp. 120-125
DOI: 10.1126/science.284.5411.120

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Observations of core-diffracted P(P diff) and SH(SH diff) waves recorded by the Missouri-to-Massachusetts (MOMA) seismic array show that the ratio of compressional (P) seismic velocities to horizontal shear (SH) velocities at the base of the mantle changes abruptly from beneath the mid-Pacific (VP /VS = 1.88, also the value predicted by reference Earth models) to beneath Alaska (VP /VS = 1.83). This change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin. A textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern Pacific paleotrench.

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