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.