Detailed P wave velocity (V_p) and S wave velocity (V_s) models and V_p/V_s ratios were determined for a major portion of the New Madrid seismic zone using arrival times recorded by the New Madrid seismic network and Portable Array for Numerical Data Acquisition (PANDA) stations. We performed a simultaneous inversion for P and S wave velocities and hypocentral locations, yielding the most detailed tomographic image of the upper portion of the crust to date. Low V_p and high V_s anomalies resulted in low V_p/V_s ratios that correspond to the major arms of seismicity north of the intersection of the Cottonwood Grove–Blytheville Arch (CG-BA) fault with the Reelfoot fault. The unusual low V_p/V_s values can be attributed to the presence of quartz-rich rocks. Two regions contain anomalous V_p and V_s values and V_p/V_s ratios that cannot be attributed to variations in rock composition and are probably produced by overpressured fluids. One region is located on the hanging wall of the northern portion of the Reelfoot fault and is aseismic. The other region corresponds to a portion of the southern Reelfoot fault that experiences swarm activity. A distinct velocity contrast exists across the CG-BA fault at depths exceeding 4.65 km; basement rocks southeast of the fault have V_p values that are 4%–6% slower than values for rocks located to the northwest. The most logical explanation is that the fault follows a preexisting structural feature or lithologic change in basement rocks.