Seismic Anisotropy Tomography and Mantle Dynamics of Central-Eastern USA

Abstract

We present high-resolution 3-D tomographic models of isotropic P-wave velocity (Vp), azimuthal anisotropy, and radial anisotropy down to 1,300 km depth beneath the central and eastern United States (CEUS), which are obtained by inverting a great number of local and teleseismic data and making a whole-mantle correction to teleseismic travel-time data recorded by the USArray. Trade-offs between azimuthal and radial anisotropies occur due to the correlation between the azimuthal and incidence angles of seismic rays, but the use of uniform and crisscrossing rays can reduce the trade-off effect. Our tomographic images reveal the North American Craton with layered anisotropy and strong anisotropies in the lower mantle related to the deeply subducted Farallon slab and passage of the Bermuda hotspot. In the upper mantle, low-velocity anomalies with significant seismic anisotropy are revealed beneath three intraplate seismic zones in New Madrid, East Tennessee, and South Carolina, suggesting that hot and wet mantle upwelling occurs under the seismic zones, and the related fluids affect the earthquake generation. The most important dynamic processes in the mantle beneath the CEUS are the deep subduction of the Farallon plate and the passage of the Bermuda hotspot, which have caused strong structural heterogeneities, seismic anisotropy, as well as thermal anomalies and fluids that contributed to the formation of intraplate seismic zones in the CEUS.