IDENTIFICATION OF SITE EFFECTS USING STRONG MOTION DATA

Abstract

Site amplification factors at subsurface layers are often used as one of the parameters for estimation of strong ground motion. The site amplification factors influence strongly the ground motion at subsurface layers. For example, waveforms obtained from observation sites are sometimes clearly different even if the observation sites are situated in the vicinity of each other. Therefore, it is assumed that the consideration for the influence of site amplification factors on the ground motion at subsurface layers is important, and improving the accuracy of the site amplification factors contributes towards the technology of earthquake disaster mitigation. The author investigated the site amplification factors at subsurface layers using strong motion records of Sendai and Fukushima. The site amplification factors estimated by spectral ratio of horizontal component of surface strong motion data to horizontal component of borehole strong motion data. It was found that amplification factor is quite large at low frequencies up to 10-30 Hz and amplification factor is smaller for high frequencies because of low quality factor in a soil layer. Amplitude spectra of spectral ratio consists predominant peaks due to either amplification of surface soils or due to interference.