Abstract

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 (VS30) 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 VS30. 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 VS30 classification of the site. Likewise, there is poor correlation between the two horizontal-to-vertical techniques or between VS30 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.

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