Intrusions and anomalous Vp/Vsratios associated with the New Madrid seismic zone

Abstract

DetailedPwave velocity (Vp) andSwave velocity (Vs) models and Vp/Vsratios 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 forPandSwave velocities and hypocentral locations, yielding the most detailed tomographic image of the upper portion of the crust to date. Low Vpand high Vsanomalies resulted in low Vp/Vsratios 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 Vp/Vsvalues can be attributed to the presence of quartz‐rich rocks. Two regions contain anomalous Vpand Vsvalues and Vp/Vsratios 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 Vpvalues 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.