LIQUEFACTION SUSCEPTIBILITY MAPPING, ST. LOUIS, MISSOURI AND ILLINOIS

The purpose of this effort is to prepare detailed, 1:24,000-scale liquefaction susceptibility maps for the densely populated St. Louis area, Missouri and Illinois. The susceptibility maps will be based on recently completed Illinois Geological Survey (IGS) and in-progress Quaternary geologic mapping. The study encompasses five USGS 7.5-minute quadrangles that include parts of downtown St. Louis and surrounding communities, as well as the low-lying floodplain regions. Much of the metropolitan area is underlain either by saturated granular Holocene fluvial deposits of the Mississippi River (e.g. sands), or artificial fill. Both deposits are susceptible to earthquake-induced liquefaction. According to historical accounts, St. Louis suffered considerable damage from the 1811-1812 New Madrid earthquakes, in particular the community of Cahokia, Illinois, located along the Mississippi River floodplain. Earthquake-related paleo-liquefaction features have been documented along major floodplains elsewhere in the region (e.g. Meramec River). Additionally, the St. Louis area has experienced multiple historical earthquakes from local seismic sources (e.g. within 80 to 110 km), some with earthquake magnitudes estimated up to M5, and Modified Mercalli intensities between V-VI. Based on the presence of alluvial and artificial ground deposits, historic paleo-liquefaction features, previous regional moderate- to large-magnitude earthquakes, and high population density underscores the need for detailed liquefaction susceptibility maps. Our goal is to delineate and classify the areas susceptible to earthquake-induced liquefaction. We will accomplish this goal by: (1) incorporating or developing detailed 1:24,000-scale Quaternary geologic maps; (2) compiling and evaluating geologic and geotechnical borehole information; (3) characterizing depth to groundwater data; (4) using GIS and relational databases to assess the liquefaction susceptibility based on data derived from (1), (2), and (3). The result will be the development of detailed GIS-compatible maps and databases that depict the relative liquefaction susceptibility hazard, ranked from very-low to very-high, which could be used by planners, agencies, or individuals to prepare for and better mitigate the effects of liquefaction.