Anisotropy of the asthenosphere: The higher mode data of the Pacific revisited

Abstract

Surface‐wave data previously used by the authors to constrain upper mantle anisotropy in the Pacific Ocean are re‐examined after an error was found in the calculations of the Love‐wave phase‐velocities. Sub‐lithospheric anisotropy is still required by the corrected data but it extends to shallower depth than we originally proposed. Using different parameterizations of the upper‐mantle, we present strong evidence that the bottom of the anisotropic layer has been reached around 250 km in the Pacific. Beneath North‐America, no revision of our data is required and anisotropy could extend to around 400 km‐depth. Our results are in agreement with an upper mantle anisotropy due to preferred orientation of olivine crystals in an overall horizontal shear strain parallel to the present‐day plate motion. We suggest that this preferred orientation of the crystals mainly occurs within the upper mantle low‐velocity layer.