Summary Phase velocity and azimuthal anisotropy maps for fundamental mode Rayleigh waves are determined for a portion of the central United States including the seismically active Reelfoot Rift and the enigmatic Illinois Basin. Dense seismic array installations of the Northern Embayment Lithosphere Experiment, the EarthScope Transportable Array, and the Ozarks Illinois Indiana Kentucky array allow a detailed investigation of phase velocity and anisotropy in a broad period range (20 – 100s). We obtain more than 12,000 well-constrained, unique two-station paths from teleseismic events. The two-station method is used to determine dispersion curves and these are inverted for isotropic phase velocity maps and azimuthal anisotropy maps for each period. The presence of fast phase velocities at lower crustal and uppermost mantle depths is found below the Reelfoot Rift, and Ste. Genevieve and Wabash Valley fault zones. At periods of 30s and higher, the Reelfoot Rift is underlain by slow phase velocities and is flanked to the NW and SE by regions of fast velocity. Fast phase velocities are present below the center of the Illinois Basin in the period range 75-100s. Anisotropy fast axis orientations display complex patterns for each period and do not trend parallel to the direction of absolute plate motion. Anisotropy fast directions are consistently parallel to the trend of the Reelfoot Rift from 50s to higher periods, suggesting the presence of either frozen-in anisotropic fabric or fabric related to material transport from a recently discovered, pronounced low velocity zone below the Mississippi Embayment.

Detailed P wave velocity and anisotropy structure of the uppermost mantle below the central United States is presented based on a tomographic inversion of Pn traveltimes for earthquakes in the range 2 to 14°. Dense raypath coverage throughout the northern Mississippi Embayment is obtained using the Northern Embayment Lithosphere Experiment and U.S. Transportable Array data sets. A detailed analysis of the trade-off between velocity and anisotropy variations demonstrates that both are well resolved over most of the study area. Anomalously fast Pn velocities are identified below the northern Mississippi Embayment, centered on the New Madrid seismic zone. A prominent region of low velocity coincides with the southwestern margin of the Illinois basin. Pn anisotropy displays complex patterns and differs from absolute plate motion directions and SKS splitting directions. A circular pattern of fast anisotropy directions is centered on the New Madrid seismic zone and may be related to the presence of the mafic "rift pillow."

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