Seismic Site Classifications for the St. Louis Urban Area

Regional National Earthquake Hazards Reduction Program (NEHRP) soil class maps have become important input parameters for seismic site characterization and hazard studies. The broad range of shallow shear-wave velocity (V-S30, the average shear-wave velocity in the upper 30 m) measurements in the St. Louis area results in significant uncertainties between the actual spot values and the averaged values used to assign NEHRP soil classes for regional seismic hazard studies. In the preparation of an NEHRP site classification map of the St. Louis urban area, we analyzed 92 shear-wave velocity (V-S) measurements, supplemented by 1400+ standard penetration test (SPT) profiles in areas bereft of V-S measurements. SPT blow counts correlated to V-S values based on the published correlations. The data were then compiled for respective surficial geologic units and bedrock type. These data suggest that the reciprocal of V-S30 exhibits a fairly linear relationship with depth to bedrock, likely because V-S30 is a function of the thickness of surficial materials exhibiting relatively low V-S values. The V-S30 values were interpolated by summing the regressed V-S30 on the depth to bedrock and kriged values of the regression residuals. The resulting NEHRP site classification maps predict that upland areas of the St. Louis area are spatially classified as soil site classes S-B to S-D, while the low-lying floodplains are consistently classified as S-D to S-F.