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.
Wysession
ME
Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA. Department of Geological Sciences, Northwestern University, Evanston, IL 60208, USA. Department of Geological Sciences, Brown University, Provide.
Langenhorst
A
Fouch
MJ
Fischer
KM
Al-Eqabi
GI
Shore
PJ
Clarke
TJ