Regional Vp, Vs, Vp/Vs, and Poisson's ratios across earthquake source zones from Memphis, Tennessee, to St. Louis, Missouri

Models of P- and S-wave velocity, Vp/Vs ratios, Poisson's ratios, and densi ty for the crust and upper mantle are presented along a 400-km-long profile trending from Memphis, Tennessee, to St. Louis, Missouri. The profile cros ses the New Madrid seismic zone and reveals distinct regional variations in the crustal velocity structure north and south of the latitude of New Madr id. In the south near Memphis, the upper few kilometers of the crust are do minated by upper crustal sedimentary basins or graben with P-wave velocitie s less than 5 km/sec and S-wave velocities of about 2 km/sec. P-wave veloci ties of the upper and middle crust range from 6.0 to 6.5 km/sec at depths a bove 25 km, and corresponding S-wave velocities range from 3.5 to 3.7 km/se c. The lower crust consists of a high-velocity layer (Vp = 7.4 km/sec; Vs s imilar to 4.2 km/sec) that is up to 20-km thick at the latitude of New Madr id but thins to about 15 km near Memphis. To the north, beneath the western most Illinois basin, low-velocity (Vp < 5 km/sec; Vs < 2.3 km/sec) sediment ary basins are less than I-km deep. The average velocities (Vp 6.0 km/sec; Vs = 3.5 km/sec) of the underlying, near-surface rocks argue against large thicknesses of unconsolidated or noncarbonate sediments within 50 km of the western edge of the Illinois basin. Most of the crust beneath the Illinois basin is modeled as one layer, with velocities up to 6.8 km/sec (Vs = 3.7 km/sec) at 37-km depth. The thick, high-velocity (Vp = 7.4 km/sec; Vs simil ar to 4.2 km/sec) lower crustal layer thins from about 20 km near New Madri d to about 6 km beneath the western Illinois basin. Refractions from the Mo ho and upper mantle occur as first arrivals over distances as a great as 16 0 km and reveal upper mantle layering to 60 km depth. Upper mantle layers w ith P-wave velocities of 8.2 km/sec (Vs = 4.5 km/sec) and 8.4 km/sec (Vs = 4.7 km/ sec) are modeled at 43 and 60 km depth, respectively. Crustal Vp/Vs ratios range between 1.74 and 1.83, and upper mantle Vp/V s ratios range f rom 1.78 to 1.84. Poisson's ratios range from about 0.26 to 0.33 in the cru st and from about 0.27 to 0.29 in the upper mantle. Modeled average densiti es range from about 2.55 in the sedimentary basins to 3.43 in the upper man tle. Geophysical characteristics of the crust and upper mantle within the N ew Madrid seismic zone are consistent with other continental rifts, but the crustal structure of the Illinois basin is not characteristics of most con tinental rift settings. Seismic and gravity data suggest a buried horst nea r the middle of Reelfoot rife, beneath which is a vertical zone of seismici ty and velocity anomalies. The relative depth of the Reelfoot rift north an d south of the Reelfoot graben suggests chat the rift and its bounding faul ts may extend eastward beneath the city of Memphis.