NONLINEAR SOIL AMPLIFICATION - ITS CORROBORATION IN TAIWAN

Nonlinear ground response at two strong-motion arrays in Taiwan is stu died using the spectral ratio technique. At the SMART1 array, we calcu late the frequency-dependent soil amplification functions as a ratio o f the spectra at alluvium to rock sites, and study their dependence on the excitation level. Horizontal components of shear waves are consid ered. We compare (1) the average spectral ratios on weak and strong mo tions, (2) the ratios for the mainshocks and aftershocks, and (3) the ratios for the strong shear waves and their coda. At the SMART1 array, ''weak motions'' have a peak horizontal acceleration (PHA) less than 30 Gal. ''Strong motions'' are in the range of 100 to 267 Gal. Compari son of the average weak- and strong-motion spectral ratios shows a sig nificant deamplification of strong motion between 2 and 9 Hz, exceedin g the error margin estimated by the standard deviations. The maximum d eamplification occurs at approximately 6.5 Hz where the average weak-m otion amplification is 2.9 versus 0.40 in the strong motion. A similar pattern is exhibited by the ratios calculated for the mainshocks and the aftershocks, as well as for the shear waves and their coda. The sp ectral ratio calculated from a single realization of coda is identical to the average ratio obtained from many small earthquakes. At the SMA RT2, we analyze spectral ratios between the stations on Pleistocene te rrace deposits and recent alluvium, which characterize the relative re sponse at these two types of sediments. Weak motion is PHA less than 1 3 Gal, while strong motion extends from 100 to 295 Gal. Strong-motion spectral ratios between terrace and alluvial sites are consistently re duced in the frequency range from similar to 1 to 10 Hz, compared with the weak motion. This effect is insensitive to the variation in dista nce between stations from 7.9 to 11.4 km, as well as the azimuthal cha nge of up to 80 degrees in the station pair strike. We attribute the o bserved discrepancies between weak- and strong-motion amplifications t o the differential nonlinear response occurring at terrace and alluvia l sites. Our results document a significant nonlinear ground response at both arrays.