Two decades of linear site-effect observation (>2 Hz) in the Wabash Valley, Central United States - Context for the 2008 Illinois earthquake sequence

Much of the ground-motion prediction effort in the central United States has been focusedondeep (>>30 m) alluvial sites, such as those found in the Mississippi embayment. During the past 20 years we have collected over 200 high-resolution, free-field digital velocity records at sites in the Wabash and Ohio River valleys which are more typical of the areas outside the embayment. These ground-motion records are from small to moderate-size earthquakes located in the same area. The magnitudes range between M 3 and M 5.2, but the bulk of the recordings is associated with the 1987 M 4.96 and 2008 M 5.2 southeastern Illinois earthquakes, and the 2002 M 4.5 southwestern Indiana earthquake. We summarize the velocity recordings and site investigations for the 2008 southeastern Illinois earthquake sequence, put the findings into context with the previous observations, and quantify the reduction in ground-motion variability that can be achieved with conventional site-effect characterization techniques. Results show ground-motion characteristics for the 2008 earthquake sequence are consistent with previous observations in the area, beginning with those associated with the 1987 southwestern Illinois earthquake. In addition, site-effect corrections reduced the range of spectral amplitude for frequencies greater than 2 Hz between 40% and 70%, as well as the spectral variation by a factor of approximately four. The data also show that a peak ground velocity of 1:2 cm=s delineates a clear boundary separating Modified Mercalli intensities IV and V. This observation can be useful in scaling ground motions of historical seismicity, as well as predicting the effects of future events. We speculate these quantitative characteristics are likely representative of ground motions throughout most of the central United States with the exception of those in the infrequent thick-sediment-filled areas within major river valleys.