We conduct a systematic and quantitative evaluation of effects of a deep sedimentary structure model between the engineering and seismic basements on the theoretical broadband site-amplification spectra, taking sites in the Osaka basin, Japan, as a test site. First, from a parametric study on theoretical evaluation of site amplification, adopting a one-dimensional S-wave propagation theory, we show that spectral peaks in lower frequency rang are controlled by velocity structure of the deep sediments, while a spectral amplification trend in higher frequency range is controlled by attenuation structure of the deep sediments. On the contrary, a shallower sedimentary structure model above the engineering basement mainly controls a shape of site-amplification spectra in higher frequency range. Next, we validate a macroscopic modeling for the deep sedimentary structure, based on an inversion of empirical site-amplification spectra. Finally, we show that the spectral reproduction in low frequency ranges is improved by a change in a macroscopic-model representation of deep sedimentary structure from a stack of three homogeneous velocity layers to a gradual velocity increasing model.
Japan Association for Earthquake Engineering