Lower mantle lateral heterogeneity beneath the caribbean Sea

Seismic wave reflections from Earth's core recorded at seismic arrays in North America from events in the Caribbean Islands, Venezuela, and the Mid-Atlantic Ridge have observed slownesses more than 64 percent greater than predicted by the IASPEI91 standard Earth model. P waves turning in the lowermost mantle beneath the same region also have anomalous slowness. The slowness anomalies are not accompanied by significant travel time residuals and appear to be caused by lateral inhomogeneities in the velocity structure of the lower mantle.

Lateral variations in Compressional/Shear velocities at the base of the mantle

Observations of core-diffracted P (Pdiff) and SH (SHdiff) 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.