High-Resolution P- and S-Wave Velocity Structure of the Post-Paleozoic Sediments in the Upper Mississippi Embayment: Collaborative Research between the University of Kentucky and the University of Memphis

Site response, sedimentary basin geometry, earthquake induced strong ground motion, geometry and characteristic feature of active faults, and reliable earthquake hypocenters are among the most essential elements for seismic hazard assessment in the upper Mississippi Embayment. However, these elements cannot be successfulIy evaluated without reliable P- and S-wave velocity information for the embayment soils/sediments. Under the support of the USGS NEHRP program, many shalIow boreholes were drilled, mostly less than 100 meters and localized in the Memphis area of Shelby County, to provide direct measurement of shear wave velocities, to determine soil profiles as a function of depth, and to construct seismic hazard maps for the Shelby County (Gomberg et aI., 2003; Cramer et aI., 2004). Shear-wave velocity averaged to 30 m depth (Vs30) and the localized soil classification have commonly been applied to predict earthquake ground-motion amplification (e.g. Cramer 2001; Gomberg et aI., 2003; Cramer et aI., 2004), and it is the basis of site hazard classification under National Earthquake Hazard Reduction Program- Uniform Building Code (NEHRP-UBC). Results of a few shalIow seismic profiles in the central NMSZ reveal that a significant velocity contrast exists at a depth around 100 meters, which contribute to a significant amplification. Deep borehole vertical seismic array observations in the Taipei basin show that the peak ground acceleration (PGA) of the almost vertically incident seismic waves from the bottom of a basin (346 m) is doubled at a depth ~60 meters corresponding to a large velocity contrast (Wen et aI., 1995). Therefore, the critical thickness responsible for most of the site amplification in the uppermost sediment seems to vary from region to region. This is particular true for the upper Mississippi embayment where lateral and vertical seismic velocity and lithologic features of the sediments vary significantly. We propose to study site specific amplification around the central NMSZ region by determine high-resolution Vp and Vs structure profiles for the entire thickness of the sediments. We propose to conduct short seismic reflection/refraction lines at selected 80~ 100 sites evenly distributed around the central NMSZ to cover the area north of Blythville, Arkansas and Dyersburg, Tennessee, in the northern Upper Mississippi Embayment. The available MiniVib source at the University of Kentucky will generate P-wave to sample the entire depth of sediments (~l km), and S-wave to sample 300~400 meters of sediments. In the southern study area, the S-waves will probably not able to sample the entire thickness of the sediments. The deeper Swave velocity structure beneath the thick sediments (Le. >400 m) can be determined either by an assumed Vp/Vs ratio for the compact sediments or by a simple linear inversion of the travel time difference between the direct S and the converted Sp waves using the Vp model and shalIow Vs model «400 m) as priori information. For each site, the final Vp and Vs model will be validated with those from the nearby deep welI logs and will be further constrained by the observed travel time differences between the direct and the converted waves from the bottom of the sediments. Thus, a high-resolution Vp and Vs model for the post-Paleozoic sediments throughout the central NMSZ in the northern Upper Mississippi Embayment can be determined via this colIaborative effort between the University of Kentucky and the University of Memphis. Site specific amplification map as welI as the predicted strong ground motion for the northern Upper Mississippi Embayment will be constructed and evaluated from the resultant Vp and Vs model for the sediments.