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Evaluation of Linear Site-Response Methods for Estimating Higher-Frequency (> 2 Hz) Ground Motions in the Lower Wabash River Valley of the Central United States

The horizontal-to-vertical ratio of ambient noise (H/V^*_N) and the time-averaged S-wave velocity of the upper 30 m of soils and rock at a site (V_S30) were evaluated for their suitability as techniques for estimating site effects in the lower Wabash River Valley area of southern Indiana and Illinois. We also calculated the horizontal-to-vertical ratio of the S wave (H/V^*_S) for the southwestern Indiana earthquake of 18 June 2002 and evaluated its effectiveness for estimating site effects. The database for the study consisted of new SH-wave seismic refraction/reflection profiles and ambient noise recordings at several blast monitor sites that recorded the M 4.5, 18 June 2002 earthquake, as well as the S-wave arrivals recorded by the blast monitors during the earthquake. Our results are limited to ground motions > 2 Hz because of the low-cut frequency of the velocity transducers used in the blast monitor systems. The shallow seismic refraction/reflection profiles allowed us to determine the S-wave velocities and soil-bedrock contact geometry, as well as the depth to bedrock, characteristic frequency, and V_S30. These methods were compared with linear one-dimensional site amplification approximations. The results of the study are not encouraging. There is only a weak correlation between the linear 1-D amplification curves and the site effects predicted by the two horizontal-to-vertical ratios or the V_S30 classification of the site. Likewise, there is poor correlation between the two horizontal-to-vertical techniques or between V_S30 and the Modified Mercalli intensities reported for the 18 June 2002 earthquake. The results show that site effects in southern Indiana and southern Illinois are too complex for single-parameter characterization; consequently, a better evaluation likely requires a minimum of site-specific in situ seismic-velocity measurements, geotechnical index tests, and one-dimensional approximation.