GEOPHYSICAL INVESTIGATIONS OF SUBSURFACE STRUCTURE IN THE SAINT CHARLES-SAINT LOUIS AREA

The Saint Charles-Saint Louis area is located between the Mississippi and Missouri Rivers in the eastern-most portion of the State of Missouri. Seismic ground motion prediction is important for seismic hazard mitigation due to its dense population and close distance to two active tectonic zones: the Wabash Valley Seismic Zone and New Madrid Seismic Zone in the central United States. Depth to the bedrock and velocity structure of the sedimentary layers are critical parameters for accurate prediction of ground motion during future earthquakes, and active-source seismic methods are effective and inexpensive ways in obtaining such parameters. As part of the Saint Louis Area Earthquake Hazards Mapping Project (SLAEHMP) conducted by Missouri Department of Natural Resources (DNR) and the United States Geological Survey (USGS), we conducted seismic refraction surveys using a 24-channel Geometrics Geode seismograph to map the subsurface structure, velocity, nature of the lithology and depth to the bedrock in the area. Three layers were identified with an average velocity of 400, 1654, and 4000 m/s which corresponds to a weathered top soil layer, an alluvial/glacial till layer, and the limestone bedrock, respectively. The average thicknesses of the overlying sediments are between 4 m and 53 m. The bedrock has an average dip of less than 8^o. Results generated from the refraction survey correlate well with independently determined depths and velocities using Cone Penetration Test conducted by the Missouri Department of Transportation.