Lateral Variations in Compressional/Shear Velocities at the Base of the Mantle
Michael E. Wysession,
1*
Amy Langenhorst,
2
Matthew J. Fouch,
3
Karen M. Fischer,
3
Ghassan I. Al-Eqabi,
1
Patrick J. Shore,
1
Timothy J. Clarke
4
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.
1 Department of Earth and Planetary Sciences,
Washington University, St. Louis, MO 63130, USA.
2 Department of Geological Sciences, Northwestern
University, Evanston, IL 60208, USA.
3 Department of
Geological Sciences, Brown University, Providence, RI 02912, USA.
4 Department of Earth and Environmental Science, New
Mexico Institute of Mining and Technology, Socorro, NM 87801, USA.
*
To whom correspondence should be addressed. E-mail:
michael{at}wucore.wustl.edu
