Lateral variations in compressional/shear velocities at the base of the mantle

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 (V(p)/V(s) = 1.88, also the value predicted by reference Earth models) to beneath Alaska (V(p)/V(s) = 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.