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Estimation of the Wave Fields in the Three Geotechnical Zones of Tokyo

¹ Earthquake Research Institute
University of Tokyo
1-1-1, Yayoi, Bunkyo-ku
Tokyo 113-0032, Japan

We estimate the wave fields in the three geotechnical zones (the hill, alluvial, and reclaimed zones) of Tokyo by analyzing the three components of surface and downhole strong-motion accelerograms recorded at six borehole stations. Although the wave field of the reclaimed zone was already estimated in a recent study, it is partly re-estimated by an improved technique. The target-period range is mainly between 0.1 and 2.0 sec. The wave types of observed ground motions are identified by calculating the theoretical vertical amplitude ratios of body waves and surface waves and the cross-correlation functions between the surface and downhole recordings. In the calculation of the theoretical amplitude ratios, the damping for wave propagation and the damping for the eigenfunctions of surface waves are separately considered. Further, the horizontal components of the surface and downhole recordings are separated into S-wave and surface-wave accelerograms. The main conclusions are summarized as follows: (1) the ground motions observed in the alluvial and reclaimed zones are mostly composed of S waves and Love waves; (2) the Love- wave amplitude ratios are larger than the S-wave ratios in a soft surficial deposit around the theoretical predominant period. The large Love-wave amplitude ratios are excited by the surficial deposit and a deep sedimentary basin; and (3) quality factors near 10 are obtained for the soft deposit around the predominant period.