Effects of Nonlinear Soil Amplification on Strong Ground Motions during the Niigataken Chuetsu-Oki Earthquake in 2007

Abstract eng:
Strong ground motions were recorded at a downhole array by the service hall in the Kashiwazaki-Kariwa nuclear power station during the Niigataken Chuetsu-oki Earthquake in 2007. The peak period of spectral amplification ratios between the near-surface ground motion and the deepest underground motion recorded during the main shock was similar to those of weak motions during the aftershocks. The spectral amplification ratios in the short period range less than one second of the main shock are less than those of the aftershocks. It seems that the nonlinear amplification due to surface soils might have had a strong effect on the strong motion during the main shock. A field investigation and its spectral analysis based on the surface wave method were conducted to evaluate the near-surface soil profile at the station. In order to simulate the observed strong motions during the main shock, one-dimensional equivalent linear analyses were conducted using the estimated soil profiles. The Fourier spectra of the computed motions agree reasonably well with those of the observed motions. The analysis simulates well the amplification of strong ground motions between the near-surface and the deep underground during different levels of shaking and this suggests that the nonlinear properties of the surface soil might have had strong effects on the strong motion during the main shock.

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