Intensity of Strong Ground Motion on Pre-Quaternary Stratum and Surface Soil Amplifications during the 1995 Hyogo-ken Nanbu Earthquake, Japan

Abstract

We obtained an attenuation relation of response spectra to magnitude, source-to-site distance and soil classification by performing regression analysis of strong-motion data recorded in California. We applied the attenuation relation to the 1995 Hyogo-ken Nanbu earthquake and found that the estimates at pre-Quaternary sites agree well with the observed values. Peak ground accelerations on pre-Quaternary stratum in Kobe are consistent with the source models, and they are estimated to be about 350 cm/s², while the estimates for Awaji Island vary with the source models from 600 to 1, 200 cm/s². We calculated the amplification factors of Quaternary stratum during the Hyogo-ken Nanbu earthquake by taking ratios between the observed records at Quaternary sites and estimates on pre-Quaternary stratum. As the amplifications in the heavily damaged area are large near the natural period of wooden houses (04-0.5 s), these short-period amplifications might be one of the reasons Kobe suffered extensive damage. The estimated amplification factors generally agree with those estimated from other earthquake records as well as with one-dimensional wave propagation theory, while the amplifications at alluvial sites were larger at periods of more than 1 s and might be affected by surface waves.