Rayleigh-wave Phase Velocity Structure and Azimuthal Anisotropy of the Lower Crust Beneath the East-central US

We use the 2-stations method to compute dispersion curves for Rayleigh-wave phase velocities ( C) along more than 70 paths located in the East-central United States, implying 17 seismic stations from different networks (USNSN, GSN, and FLED). The area sampled by these paths extends from 33°N to 39°N, and from -90°E to -82°E. For each path, the dispersion curve can be interpreted as the average Rayleigh-wave phase velocity (< C>) along this path as a function of the period of the Rayleigh-wave T. To map the variations of C in the lower crust and shallow mantle beneath the East-central United States, and to test the presence of azimuthal anisotropy in this region, we invert our collection of < C> for isotropic anomalies of the Rayleigh-wave phase velocity (d C) and anisotropic terms (2-psi and 4-psi), as a function of T. The model is parameterized with a grid of 33 knots and a grid spacing of 150km. The inversion method is a general LSQR, including damping and smoothing. We report a positive gradient in the isotropic anomalies from SE to NW. Between periods of 10s and 70s, azimuthal anisotropy is clearly present, the fast direction being SW to NE. Interestingly, we do not see indication for azimuthal anisotropy at larger periods. These results suggest that the azimuthal anisotropy beneath East-central United States observed by shear-wave splitting methods is, at least partly, located in the lower crust.