Comparison of Site Response Characteristics Inferred from Microtremors and Earthquake Shear Waves

We investigated the validity of seismic site response characteristics es- timated from microtremors by comparing them with those of earthquake motions. For this purpose we observed microtremors as well as earthquake motions using large (5-km diameter) and small (0.5-km diameter) arrays deployed on soft sedi- ments. Specifically, we examined four estimates from microtremors: relative site amplification factors to incident shear waves, site amplification factors by the Naka- mura method, resonance frequency in horizontal-to-vertical spectral ratios, and horizontal-to-vertical spectral ratios. As a result of the comparisons, we obtained the following conclusions. The relative amplification factors can be inferred from horizontal-component ratios of microtremors to a reference site within a small area of several hundred meters. The horizontal-to-vertical spectral ratios inferred by the Nakamura method partly reflect site amplification factors, but do not agree with site amplification factors. A sharp-peak frequency in the horizontal-to-vertical spectral ratios is possibly the resonance frequency. The horizontal-to-vertical spectral ratios of microtremors either agree with those of earthquake motions at some array sites or are slightly smaller at the other sites.

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