A three-dimensional shear velocity model for the crust and upper mantle beneath the central United States is presented by inverting Rayleigh wave phase velocities from 20s to 100s periods. These phase velocities were determined using regional and teleseismic earthquakes recorded by the Northern Embayment Lithospheric Experiment stations, the CERI New Madrid Seismic Network, the Earthscope Transportable Array, and the Ozark Illinois INdiana Kentucky Flexible Array. A low Vs anomaly is imaged in the mantle below the Reelfoot Rift, which is the uppermost portion of connected low-velocity zones dipping toward the southwest below the rift and to the northwest below the Illinois Basin. According to the analysis in previous tomographic studies using both Vp and Vs anomalies, the elevation of temperature and the enrichment of iron, water, and orthopyroxene contents are required factors to explain the reduced seismic velocities. These low-velocity zones are produced by silica-rich fluids rising from the stalled Farallon slab. Two weak zones characterized by low Vs are imaged below the Ste. Genevieve and the Wabash Valley seismic zones. The low-velocity, weak areas may be responsible for stress concentration and thus the generation of intraplate seismicity.

Three-dimensional, high resolution crustal and upper mantle P- and S-wave velocity (Vp and Vs) models are presented for the central United States. The study utilizes local and teleseismic data recorded by the Northern Embayment Lithospheric Experiment stations, the New Madrid Seismic Network, the Earthscope Transportable Array, and the Ozark Illinois INdiana Kentucky Flexible Array. The Vp and Vs solutions are very similar and are well resolved in the depth range 40 to 400 km. Two anomalously slow regions are present below the Illinois Basin forming a northwest dipping low velocity zone (LVZ) extending from ~200 to 400 km. Maximum anomaly magnitude in the LVZ reaches about -4 % and -5% for Vp and Vs, respectively. The LVZ appears to connect to a well-documented LVZ located below the northern Mississippi Embayment. As is the case for the northern Mississippi Embayment, the Illinois Basin velocity anomalies cannot be explained by elevated temperature alone and require elevated orthopyroxene content in addition to an increase in iron and water content. The need for additional orthopyroxene suggests that the LVZ is being produced by metasomatism of mantle rocks by hydrous, silica-rich fluids ascending from a slab fragment trapped in or near the transition zone. This supports previous interpretations for the existence of the LVZ below the Embayment. We suggest that the LVZs below the Mississippi Embayment and the Illinois Basin are linked to the presence of the large igneous province Hess plateau currently located below the central United States by inverse convection models.