Radially anisotropic shear velocity structure of the upper mantle globally and beneath North America

Abstract

A surface wave dispersion data set of unprecedented size is used to obtain a variable‐resolution model of the radially anisotropic shear wave velocity structure of the upper mantle beneath North America and globally. Love and Rayleigh wave phase velocities for periods in the range 35–150 s constrain a three‐dimensional model of velocity variations on a length scale of a few hundred kilometers within the North American continent and a few thousand kilometers globally. The short‐ and long‐wavelength models are determined simultaneously. Long‐period surface wave phase velocities (200–350 s) are used to help constrain longer‐wavelength and transition zone structure. Laterally varying velocity sensitivity kernels are used to account for the dependence of the velocity sensitivity on lateral variations in crust and mantle velocity structure. The sensitivity kernels are updated in several iterations to avoid nonlinearities associated with the inverse problem for the determination of mantle structure. Variations in isotropic velocity in the uppermost several hundred kilometers of the mantle are found to correlate well with surface tectonic features. Within the North American craton, the locations of strongest radial anisotropy generally correlate with the locations of fastest isotropic velocity. Variations in radial anisotropy show a clear continent‐ocean signature. Strong anisotropy occurs at shallow depths (<100 km) under the continents, with a secondary peak found at a depth of ∼200 km. Maximum anisotropy under the oceans occurs at a depth of ∼125 km, with no secondary maximum. Combined interpretation of isotropic and anisotropic continent‐ocean differences suggests a different role for the low‐velocity zone under continental and oceanic regions.