Localized Anisotropic Domains Beneath Eastern North America

Seismic anisotropy beneath eastern North America likely reflects both the remnant lithospheric fabrics and the present‐day deformation of the asthenosphere. We report new observations of splitting in core‐refracted shear phases observed over 3–5 years at 33 sites in New Jersey, New York, and states in the New England region and also include data from eight previously studied locations. Our data set emphasizes back azimuthal coverage necessary to capture the directional variation of splitting parameters expected from vertically varying anisotropy. We report single‐phase splitting parameters as well as station‐averaged values based on splitting intensity technique that incorporates all observed records regardless of whether they showed evidence of splitting or not. Trends of averaged fast shear wave polarizations appear coherent and are approximately aligned with absolute plate motion direction. The general disparity between the fast axes and the trend of surface tectonic features suggests a dominant asthenosphere contribution for the observed seismic anisotropy. Averaged delay values systematically increase from ~0.5 s in New Jersey to ~1.4 s in Maine. Splitting parameters vary at all sites, and neighboring stations often show similar patterns of directional variation. We developed criteria to group stations based on their splitting patterns and identified four domains with distinct anisotropic properties. Splitting patterns of three domains suggest a layered anisotropic structure that is geographically variable, outlining distinct regions in the continental mantle, for example, the Proterozoic lithosphere of the Adirondack Mountains. A domain coincident with the North Appalachian Anomaly displays virtually no splitting, implying that the lithospheric fabric was locally erased.