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

Abstract The 1994 Hokkaido Toho-oki earthquake of Mw 8.2 and its after-shocks 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 re-claimed land. 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