Frequency-dependent Shear Wave Splitting and Layered Anisotropy Beneath the Eastern Margin of North America

The mantle fabric beneath eastern North America records past deformational events associated with its complex tectonic history, including the assembly and subsequent rifting of Pangaea, in the lithosphere as well as current mantle dynamics in the asthenosphere. Observations of seismic anisotropy offer insights into these past tectonic events as well as into modern-day mantle flow. We measure shear wave splitting of core-refracted, teleseismic phases at EarthScope Transportable Array, Eastern North American Margin Community Seismic Experiment, and national seismic network stations spanning from central Georgia to southern Virginia along the passive margin in three frequency bands: low (0.02-0.1 Hz), mid (0.1-0.2 Hz), and high (0.2-1 Hz), to constrain the relative contributions from lithospheric and asthenospheric anisotropy to the observed anisotropic signals. These stations provide lateral coverage from the Appalachian Mountains to ~400 km offshore spanning the full passive margin. We observe clear evidence of frequency-dependence with predominantly null measurements in the low frequency in contrast to strong splitting in higher frequencies, suggesting the presence of vertically varying anisotropic structure. At the mid and high frequencies, we see a change in fast splitting directions from ~NE-SW or E-W in the continental interior to nearly N-S (margin parallel) along the coast, although some variations across frequencies exist. For stations with >10 years of data coverage, we model multi-layered anisotropy beneath the margin using all 3 frequency bands.