IMPACT OF NON-LINEAR SOIL BEHAVIOR ON SITE RESPONSE AMPLITUDE

We present an extensive analysis of the quantitative impact of the non-linear soil behavior on site response at 174 sites of the Japanese KiK-net network. The nonlinear to linear site response ratio, RSR NL-L is calculated by comparing the surface / downhole Fourier spectral ratio for strong events and for weak events. 3 thresholds of surface PGA are tested to characterize the "strong events": 100, 200 and 300 cm/s 2 , while weak events correspond to surface PGA in the range [0.1 – 25 cm/s 2 ]. This ratio exhibits a "typical shape"; with a low frequency part above 1 and a high frequency part generally below 1, separated by a transition zone around a site-dependent frequency labelled f NL (characterized by RSR NL-L = 1). The average maximum amplitudes of RSRNL-L are 1.4, 1.5 and 1.6, and the minimums 0.6, 0.5 and 0.5 for PGA thresholds 100, 200 and 300 cm/s 2 respectively, showing that non-linear soil behaviour results in significant site response modifications even for moderate PGA values of 100 cm/s 2 .