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

Observations of core-diffracted P (P-diff) and SH (SHdiff) waves recorded b y the Missouri-to-Massachusetts (MOMA) seismic array show that the ratio of compressional (P) seismic velocities to horizontal shear (SH) velocities a t 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 arri val at the core of ancient lithosphere from the northern Pacific paleotrenc h.