St. Louis Area Earthquake Hazards Mapping Project; a progress report; November 2008

Draft probabilistic and deterministic seismic hazard maps for the St. Louis metropolitan area, which include the effects of surficial geology on site response, have been prepared for three pilot USGS 7.5-minute quadrangles of the 29-quadrangle study area. The computer codes used in this study, which are similar to those used in the generation of the USGS national seismic hazard maps (Frankel, 2002), were modified for use in this study by Cramer (2003). These codes, which were implemented by Karadeniz (2007) in his Ph.D. thesis on St. Louis area seismic hazards, account for the fully-probabilistic approach in developing the maps and apply the median of site amplification estimates to the hard-rock ground motion attenuation relations in the deterministic maps. All of the seismic hazards were calculated based on a grid of 0.005 degrees , or about every 500 m, the same spacing employed in the amplification distribution calculations. To account for some of the uncertainty found in St. Louis area shear-wave velocity measurements, shear modulus proxies, depth to bedrock calculations, earthquake time histories, and so on, a Monte Carlo randomization procedure was used to generate site-amplification distributions and provide an estimate of the uncertainty, in terms of mean, median, and standard deviation. These distributions were assumed to be lognormal in form. In this study one-dimensional equivalent-linear response analysis was used to evaluate site-amplifications because of the following reasons: 1) high strain levels are not expected; 2) high excess water pressure development is not expected, and 3) the bedrock structure and overlying soft-sediment layering is near-horizontal in the St. Louis area. When compared to the USGS National Maps, the new probabilistic hazard levels calculated in the pilot study area for upland sites (loess covered residuum, drift, till) show zero to 300 percent greater ground motion levels for peak ground acceleration (PGA), 200 to 250 percent greater ground motion levels for 0.2-s spectral acceleration (SA), and 0 to 175 percent greater ground motion levels for 1.0-s SA. Probabilistic hazard levels for lowland (alluvial) sites, generally the modern Mississippi and Missouri River floodplains, exhibit zero to 200 percent greater ground motion levels for PGA, between 20 percent smaller to 150 percent greater ground motion levels for 0.2-s SA, and 100 to 260 percent greater ground motion levels for 1.0-s SA, when compared to the National Hazard Map.