Strong motion records of 13 earthquakes, including the Kushiro-Oki earthquake of 1993, observed at 10 JMA stations in Hokkaido were analysed. If local site amplification for each station may not be so large except for the Kushiro station, the averaged spectra corrected for attenuation will be flat for wide frequency range. First path effects are corrected by appropriate Q values to have averaged hypocentral spectra. Then a local site factor at each site is calculated as a ratio between the observed spectra and the averaged one. It is found that the Kushiro station has strong local amplification whose predominant frequency and amplification factor are 3.6Hz and 10, respectively. During the Kushiro-Oki earthquake predominant frequency decreases to 2.8Hz, which may suggest the non-linear effect of soil-structure system of the JMA Kushiro station.
