Deep shear wave velocity profiles in the Mississippi embayment from surface wave measurements

Abstract

Local geology in the Mississippi embayment is a significant source of site effects due to the deep unconsolidated sediments and basin effects near the edge of the embayment. Proper characterization of the velocity structure of the embayment is crucial for site response and seismic hazard studies. In this paper, deep dynamic site characterization was conducted in the Mississippi embayment to develop shear wave velocity profiles down to bedrock at 24 sites using active and passive source surface wave methods. Study sites cover parts of Arkansas, Missouri, Kentucky, and Tennessee, with bedrock depths ranging from 250 to 1115 m. Multimodal joint inversion of surface wave data, which included both Rayleigh and Love wave dispersion and site fundamental frequency was utilized to develop the shear wave velocity profiles (VS profiles). The developed VS profiles provided reasonable site characterization solution and captured the site signature from experimental dispersion data and fundamental frequency. Average shear wave velocity down to bedrock ranges from 462 to 686 m/s and is proportionally related with the site period and depth to bedrock of the site. Average shear wave velocities estimated by previous studies using the resonance period-sediment thickness relationship tend to systematically overestimate the average shear wave velocity in the embayment by 25–40%. Formation velocities calculated for the commonly found geologic units in the embayment, Quaternary, Upper Tertiary, Memphis sand, Paleocene, Cretaceous, and Paleozoic bedrock are approximately 214, 418, 607, 665, 967, and 2211 m/s, respectively. Comparison of the developed VS profiles demonstrated better agreement with related VS profiles generated from direct measurements, but differences up to 30% are observed with the current velocity models of the Embayment.