NONLINEAR SOIL AMPLIFICATION INFERRED FROM DOWNHOLE STRONG SEISMIC MOTION DATA

Nonlinear seismic response of soil is studied by comparison of the fre quency-dependent soil amplification functions calculated on weak and s trong motion. Amplifications are obtained by dividing Fourier amplitud e spectra of acceleration at the ground surface by the spectra at the depths of 11 and 47 m in a borehole. Observed weak and strong motion s pectral ratios are compared with those theoretically deduced from the one of the nonlinear soil models adopted in geotechnical engineering. Significant deamplification of the strong motion having PGA (peak grou nd acceleration) greater than 0.16 g, relative to the weak motion, is exhibited by the experimental ratios in the frequency range that is co nsistent with the model prediction. Existence of the different frequen cy bands, in which specific features of the nonlinear soil response ar e recognized in the theory, is also confirmed. These results give evid ence of that nonlinear response can be observed at soft sedimentary si tes from the real strong motion data.