Azimuthal anisotropy of Rayleigh-wave phase velocities in the east-central United States

We explore the Rayleigh-wave phase velocity structure of the east-central US in a broad period range (10-200 s). Using a recent implementation of the two-stations method, we first measure interstation dispersion curves of Rayleigh-wave phase velocities along 60 paths. We then invert our collection of dispersion curves for isotropic and azimuthally anisotropic (2 Psi and 4 Psi) phase-velocity maps. The inversion is performed by a damped, smoothed LSQR, and the output model is parametrized on a triangular grid of knots with a 140 km grid spacing. Using the isotropic component of the phase velocity maps to constrain regional variations in shear velocity and Moho-depth, we observe that over the upper-middle crust depth range (z < 30 km) shear wave velocities are lower beneath the Grenville and Appalachian orogenic provinces than beneath the Central Plains to the west. The amplitude of (2 Psi) anisotropy and the azimuth of the fast-propagation direction at periods between 20 and 34 s vary laterally. Beneath the Grenville and Appalachian provinces, the amplitude of anisotropy reaches 1 per cent of the average phase velocity, and the azimuth of the fast-propagation direction is uniform and equal to 32 degrees. West of the Grenville front, the average amplitude falls to 0.5 per cent, and the azimuth of the fast-propagation direction is less uniform. In the period range 45-60 s, anisotropy is smaller in amplitude (similar to 0.5 per cent) and with a regionally uniform azimuth of the fast-propagation direction of around 165 degrees. Around 140 s, the amplitude of 2 Psi anisotropy is larger again (>1 per cent), and the azimuth of the fast-propagation direction is uniform over the entire region and equal to 54 degrees. Our results suggest that azimuthal anisotropy beneath the east-central US is vertically distributed in three distinct layers, with a different geodynamic origin for each of them.