| High-quality broadband seismic data recorded by the USArray and other stations in the southwestern United States provide a unique opportunity to test different models of anisotropy-forming mechanisms in the vicinity of a cratonic edge. Systematic spatial variations of anisotropic characteristics are revealed by 3027 pairs of splitting parameters measured at 547 broadband seismic stations. The western and southern edges of the North American craton show edge-parallel fast directions with larger-than-normal splitting times, and the continental interior is characterized by smaller splitting times spatially consistent fast directions that are mostly parallel to the absolute plate motion direction of North America. Except for a small area in the vicinity of the Llano Uplift in central Texas, no systematic azimuthal variations of the splitting parameters are observed, suggesting that a single layer of anisotropy with horizontal axis of symmetry can adequately explain the observations. Estimation of the depth of the source of the observed anisotropy using spatial coherency of the splitting parameters indicates that the observed anisotropy mostly originate from the upper asthenosphere, through simple shear between the partially coupled lithosphere and asthenosphere. |
