Nonlinear site response in Kushiro during the 1994 Hokkaido Toho-oki earthquake

The 1994 Hokkaido Toho-oki earthquake of M-w = 8.2 and its aftershocks were used to estimate nonlinear site response in Kushiro, Hokkaido, Japan. The records from the mainshock and 10 aftershocks observed at 14 sediment sites in Kushiro and AKS (a reference rock site) were used in this study. The sediment sites were located in a limited area within a radius of 3 km, and some of them were located in the areas where liquefaction damage was observed at backfilled or reclaimed rand. Amplification factors of peak ground acceleration and spectral ratios between sediment and rock sites for strong motion were compared with those for weak motions to investigate nonlinear site response. At several sites, the sediment-to-rock ratios of peak ground acceleration for strong motion differed by more than 50% from those for weak motions. The fundamental peak frequencies of the spectral ratios became 10 to 20 percent lower during the mainshock. At TBS and KKP sites on the reclaimed ground, the strong-to-weak spectral amplification ratios at the high-frequency range 2 to 20 Hz significantly decreased to 0.4 during the mainshock. However, those on the natural ground were 0.6 to 0.9. Using a moving time window, we observed the drop in the strong-to-weak amplification ratios during the S-wave main shaking. The ratios tended to rise gradually in the later part of the records. Both the extent of the falling and the time for recovery in spectral amplification ratios depended on the degree of nonlinearity. Our analyses showed that nonlinearity was observed at the sites in Kushiro during the mainshock, and that the degree of nonlinearity varied greatly from site to site even in a limited area. This variation indicated that the soil response at each site was greatly affected by the surface geology in the Kushiro area.