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

A P- and S-wave velocity model of the post-Paleozoic sediments has been developed across the Upper Mississippi Embayment between the latitudes of 35°N and 35°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.