LIQUEFACTION SUSCEPTIBILITY MAPPING IN THE EVANSVILLE, IN

Evansville, Indiana, is a current target for urban seismic hazard mapping in the Central US because of its proximity to the New Madrid seismic zone, where a sequence of three Earthquakes with magnitude greater than 7 occurred in 1811-1812. This study deals with creating liquefaction susceptibility maps for the Evansville area, as a complement to recent work on the probabilistic seismic hazard analysis. Cone penetrometer test (CPT) data have been collected that sample the principal surficial geologic units, and the USGS is making a preliminary analysis of the liquefaction characteristics of the units based on this data. Firstly, the factor of safety for each CPT location is calculated using a standard method based on the cyclic resistance ratio and cyclic stress ratio and the properties of the soil column. The Liquefaction Potential Index (LPI) is calculated to evaluate the variability of the liquefaction potential.

Since there are high uncertainties involved in the calculation of liquefaction susceptibility because of the spatial variability of the soil column, a probabilistic method is adapted for the second phase. The deterministic method tells us simply whether or not there would be liquefaction if the soil profile is known exactly at a given site. However, the probabilistic method will gives us the probability of the occurrence of liquefaction based on the statistics of the material properties, so we can determine which general regions are susceptible to liquefaction.

The resulting maps quantify the high liquefaction potential in the river alluvium where the soil profile contains more sand, and describe the contrasting region of lacustrine deposits that predominantly include clayey soil with lower liquefaction susceptibility.

The soil profile within the deposits of the terrace alluvium region is more complex as it contains highly variable sequences of silty sand, clayey sand, and sandy clay. There is no consistency of the liquefaction potential index even though the soil profiles belong to same surficial geologic unit, but there is some indication of a weak correlation with depositional history. Therefore, the probabilistic method is more appropriate for liquefaction susceptibility mapping of the high variation soil profiles in this region. This procedure is used to provide a comprehensive map for liquefaction hazard in and around Evansville.