Shear-wave refraction profiles were collected to delineate valley-fill geometry. Data was also compiled from a Masters thesis completed in the study area. Shear wave values from profile layers were related to stratigraphic units, and the velocities for map units were then statistically summarized.
Using mapped thickness and extent of geologic map units, and shear-wave characteristics, we calculated standard NEHRP soil classification (Vs30), total soil thickness (t), total average soil shear-wave velocity (Vs), and site frequency (f = Vs / 4*t) for the Owensboro area. Average (mean) velocity for each map unit, as well as +/- one standard deviation, were used to constrain velocity sensitivities in the calculations. Preliminary ground truth was begun using historic accounts from the 1895 M6.2 Charleston MO earthquake.
The 18 April 2008 M5.2 earthquake in southeastern Illinois provided another opportunity for evaluation of the maps. We integrated detailed reports from USGS “Did-You-Feel-It” website and local surveys. The site frequency map showed good correlation with felt reports and strong-motion frequency spectra documented locally. The Vs30 map did not provide a good match to felt reports for this small/moderate earthquake; seismic energy experienced at Owensboro was insufficient to evaluate the Vs30 map with this event. These results support the application of detailed 3-D surficial geology integrated with shear-wave refraction data to produce community-scale ground-condition maps for seismic hazard planning.