ANALYSIS OF SPECTRAL RATIOS FOR ESTIMATING GROUND MOTION IN DEEP BASINS

Use of Nakamura's spectral ratio (horizontal versus vertical component s) is investigated theoretically for deep sedimentary basins by consid ering semicircular and semi-spherical valleys. The ratio is evaluated from the steady-state surface response for different incident waves. B ased on this ratio, both the resonant frequencies and ground motion am plification are determined. The results based on Nakamura's ratio are compared with those based on the sediment-to-bedrock spectral ratios ( Kagami's ratio). The results show that for both two- and three-dimensi onal models, Nakamura's technique predicts well the fundamental resona nt frequency, but it could not determine higher resonant frequencies o f the basins. The error in Nakamura' s estimate of the fundamental res onant frequency increases for stations near the valley center. For all uvial valleys considered in this article, Nakamura's ratio failed to p redict accurately surface ground-motion amplification.