Yoon S. Choi | USGS Science

USGS2011

Liquefaction hazard for the region of Evansville, Indiana

We calculated liquefaction potential index for a grid of sites in the Evansville, Indiana area for two scenario earthquakes-a magnitude 7.7 in the New Madrid seismic zone and a M6.8 in the Wabash Valley seismic zone. For the latter event, peak ground accelerations range from 0.13 gravity to 0.81 gravity, sufficiently high to be of concern for liquefaction. Recently acquired cone-penetrometer test data at 58 sites were used to estimate the factor of safety against liquefaction and liquefaction potential index at each site. To extend the estimation of liquefaction hazard to a grid of sites in the area, the soil columns at these grid sites were divided into three categories, and for each category a sufficient number of cone-penetrometer test sites were available to characterize statistically each group's cone-penetrometer test tip resistance and sleeve friction. At each grid site, Monte Carlo sampling was used to generate values for these two parameters at 2-meter intervals for depths down to 20 meters or bedrock. The groundwater table at each grid site was likewise sampled from a mean value and group-dependent standard deviation. For each grid site, 25,000 realizations of the soil profile were generated and a probability distribution of liquefaction potential index values was obtained. Maps of liquefaction hazard for each scenario earthquake present (1) Mean liquefaction potential index at each site, and (2) Probabilities that liquefaction potential index values exceed 5 (threshold for expression of surface liquefaction) and 12 (threshold for lateral spreading). Values for the liquefaction potential index are high in the River alluvium group, where the soil profiles are predominantly sand, while values in the Lacustrine terrace group are lower, owing to the predominance of clay. Liquefaction potential index values in the Outwash terrace group are less consistent because the soil profiles contain highly variable sequences of silty sand, clayey sand, and sandy clay, justifying the use of the Monte Carlo procedure to capture the consequences of this complexity.

Evansville, Indiana↗

USGS2011

Probabilistic seismic hazard assessment including site effects for Evansville, Indiana, and the surrounding region

We provide a probabilistic seismic hazard assessment for the Evansville, Indiana region incorporating information from new surficial geologic mapping efforts on the part of the U.S. Geological Survey (USGS) and the Kentucky and Indiana State Geological Surveys, as well as information on the thickness and properties of near surface soils and their associated uncertainties. The subsurface information has been compiled to determine bedrock elevation and reference depth-dependent shear-wave velocity models for the different soil types. The probabilistic seismic hazard calculation applied here follows the method used for the 2008 U.S. Geological Survey National Seismic Hazard Maps, with modifications to incorporate estimates of local site conditions and their uncertainties, in a completely probabilistic manner. The resulting analysis shows strong local variations of acceleration with 2 percent probability of exceedance in 50 years, particularly for 0.2-second (s) period spectral acceleration (SA), that are clearly correlated with variations in the thickness of unconsolidated soils above bedrock. These values are much greater than the USGS national seismic hazard map values, which assume B/C site conditions. When compared to the national maps with an assumed uniform site D class amplification factor applied, the high-resolution seismic hazard maps have higher amplitudes for peak ground acceleration and 0.2-s SA for most of the map region. However, deamplification relative to the D class national seismic hazard maps appears to play an important role within the limits of the ancient bedrock valley underlying Evansville where soils are thickest. For 1.0-s SA, the new high-resolution seismic hazard maps show levels consistent with D class site response within the limits of this ancient bedrock valley, but levels consistent with B/C site conditions outside.

Open-File Report↗