ESTIMATING LINEAR SITE-RESPONSE AMPLIFICATIONS FOR HIGHER-FREQUENCY (>2 Hz) GROUND MOTIONS IN THE LOWER WABASH RIVER VALLEY OF THE CENTRAL UNITED STATES

WOOLERY, E.W., Dept. of Geological Sciences, University of Kentucky, Lexington, KY 40506, and STREET, R.L., Dept. of Science, Black Hills State University, Spearfish, SD 57783, woolery@uky.edu, lbscamper1@yahoo.com.

The objective of this study is to evaluate techniques for quantifying site amplifications of linear, higher-frequency (>2 Hz) ground motions at locations in the lower Wabash River valley where the M5.0 June 18, 2002, southwestern Indiana earthquake was recorded. Recorded peak ground motions for that event are typically six to nine times greater than those expected based on the predictive equations currently being used in the central United States. SH-wave seismic refraction/reflection profiles and ambient noise samples were acquired at 27 of the sites that recorded the earthquake. These data sets, along with the recorded ground motions of the earthquake, were used to determine the time-averaged S-wave velocity of the upper 30 m of soils and rock (V30), the horizontal-to-vertical ratio of the S-wave, and the horizontal-to-vertical ratio of the ambient noise (Nakamura's technique) at the sites. Peak horizontal-to-vertical ratios of the S-wave plotted as a function of V30, indicates a general trend of decreasing amplification with increasing V30, although the scatter is large. At fourteen of the sites, the predominant frequency of the peak horizontal-to-vertical ratio of the S-wave correlates to the resonance frequency inferred from the SH-wave velocity profile developed from the seismic refraction/reflection data. The horizontal-to-vertical ratio of the ambient noise tests failed to detect site amplification or resonance at all but five sites. At these five sites, the ratio of the S-wave velocity of the bedrock to the overlying soil is three or greater, the soil/bedrock contact is relatively smooth and horizontal, and there is only a minimal amount of contrast between the S-wave velocities of the soils. In general, site conditions throughout the study area are highly variable, the S-wave velocities and thickness of the soils are highly variable, the bedrock S-wave velocities are highly variable, and the soil-bedrock boundary is frequently irregular. Consequently, site responses derived solely from one-dimensional analyses in our study area are, in general, unreliable.