The St. Louis Area Earthquake Hazards Mapping Project

The St. Louis Area Earthquake Hazards Mapping Project is a major urban hazard mapping effort supported by the U.S. Geological Survey (USGS) External Earthquake Hazards Program. The goal of the project is to provide state-of-the-art urban seismic hazard maps for the greater St. Louis area in Missouri and Illinois that can be used in land-use planning, public policy making, and private sector decision making. Urban seismic hazard maps that include the effects of local geology are being prepared. The project includes subsurface geological, geophysical, and geotechnical information to form a three-dimensional soils database. Reference soil profiles were generated from shear-wave velocity (Vs) measurements for the uplands (loess/till) and lowlands (alluvial) portions of the study area. Site amplification ranges (distributions) are then generated by the randomization of the Vs profile, dynamic properties, and appropriate input ground motions and then used to generate probabilistic and scenario ground motion hazard maps. For PGA and 0.2s Sa, the resulting urban hazard maps show increased ground motion hazard in the uplands, which are thinly covered by loess/till, and similar ground motion hazard in the 30–50 m thick alluvium lowlands relative to the 2008 USGS national seismic hazard maps. For 1.0s Sa, the urban seismic hazard maps show a reversed pattern of greater amplification on lowlands soil than upland soils. Liquefaction potential of Quaternary deposits has also been assessed. Holocene alluvial units in river valleys and flood plains are the most susceptible to liquefaction. Because many transportation routes, power and gas transmission lines, population centers, and levee structures exist on the highly susceptible Holocene alluvium, parts of the greater St. Louis area are at significant potential risk from seismically induced liquefaction and related ground deformation.