ESTIMATES OF SOIL AMPLIFICATION FOR PROBABILISTIC SEISMIC HAZARD MAPPING IN THE URBAN AREA NEAR EVANSVILLE, INDIANA

HAASE, J.S., CHOI, Y.S., and NOWACK, R.L., Purdue University, West Lafayette, IN, 47907-2051, jhaase@purdue.edu.

The central US has a low rate of seismicity, but because of the occurrence of past earthquakes such as the 1811-1812 New Madrid events as well as prehistoric events in the Wabash Valley Fault Zone, there is a significant seismic hazard. Evansville, Indiana, is one of the closest large urban areas to both the New Madrid and Wabash Valley fault zones. For this reason, it has been targeted as a priority region for urban seismic hazard assessment. The probabilistic seismic hazard methodology that will be used in the Evansville region incorporates information on the near surface. Data have been compiled from past seismic refraction studies, well logs, geotechnical borings, and cone penetrometer test data to characterize the material properties at relatively high resolution. The variations in seismic velocity of the soil materials in the region are often less important than the soil depth in terms of the resulting variations in amplification. A methodology is being developed to convert information on lithology from bore logs to approximate velocities, and the uncertainties in this process have been characterized for units surrounding the Ohio River. Results describing the spatial variability of amplification and the consistency of the calculation in Indiana and Kentucky will be shown. The objective of the preliminary calculations are to demonstrate whether or not, there is a clear correspondence between the lithologic properties of the bedrock valley fill and the amplification to extend the analysis to surrounding units. In particular distinguishing among outwash sediments from Pleistocene glaciers, modern Ohio River fluvial deposits, lacustrine sediments in tributary valleys, older river terraces and loess-covered uplands in their seismic properties will be key to the successful application of probabilistic seismic hazard at these local scales.