Shear-wave Velocities of the Post-Paleozoic Sediments across the Upper Mississippi Embayment

Abstract

A P - and S -wave velocity model of the post-Paleozoic sediments has been developed across the Upper Mississippi Embayment between the latitudes of 35¼°N and 35½°N. The model was constructed by P -wave soundings and reversed SH -wave refraction/reflection profiles acquired at 5-km intervals along the corridor. The results from these data were integrated with previously acquired P - and SH -wave velocity estimates, P -wave CDP reflection profiles, P -wave sonic logs, travel-time differences between earthquake-generated S and Sp waves, and top-of-bedrock elevation from nearby drillholes. A three-layered S -wave velocity model is proposed from this data set. The uppermost layer, which is not discussed in this paper (see Street et al. , 2001), varies from a few tens of meters thick near the edges of the embayment to as much as 190 m thick near the center of the study area; the S -wave velocities of these unlithified to poorly lithified sediments are highly variable (typically ranging between 150 and 600 m/s) and site-dependent. The second layer in the S -wave velocity model extends from the base of the near-surface layer to the acoustical top of the Cretaceous sediments, which is, ∼650 m below sea level near the center of the study area. The lateral S -wave velocity variance of this layer is defined in three segments: Near the western edge of the study area in northeastern Arkansas the velocity varies between 650 and 700 m/s; the central study area ranges between 795 and 840 m/s; and near the eastern edge of the study area in western Tennessee it ranges between 500 and 550 m/s. The S -wave velocities of the third layer, the Cretaceous section, vary between 725 and 775 m/s at the edges of the study area but between 1,010 and 1,060 m/s near the center.