Empirical Quantification of the Impact of Nonlinear Soil Behavior on Site Response

We present an extensive analysis of the quantitative impact of the nonlinear soil behavior on site response at 174 sites of the Japanese Kiban-Kyoshin (KiK-net) network. The nonlinear to linear site-response ratio (RSRNL-L) is calculated by comparing the surface/downhole Fourier spectral ratio for strong events and for weak events. Three thresholds of surface peak ground acceleration (PGA) are tested to characterize the strong events: 100, 200, and 300 cm/s(2), whereas weak events correspond to surface PGA in the 0: 1-25 cm/s(2) range. 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 labeled f(NL) (characterized by RSRNL-L = 1). The average maximum amplitudes of RSRNL-L are 1.4, 1.5, and 1.6, and the minimums are 0.6, 0.5, and 0.5 for PGA thresholds 100, 200, and 300 cm/s(2), respectively, showing that nonlinear soil behavior results in significant site-response modifications even for moderate PGA values of 100 cm/s(2). The f(NL) value exhibits a satisfactory correlation with site classifications based on either V-S30 (travel-time averaged shear-wave velocity over the top 30 m) or f(0) (site fundamental frequency): f(NL) decreases when either V-S30 or f(0) decreases. In addition, the amount of the low-frequency amplification increase depends on V-S30 and reaches a maximum of 1.6 for high V-S30 soil classes associated with shallow thin soft-soil layer underlain by stiff substratum. The average high-frequency decrease is about 0.5 for all soil classes defined from either V-S30 or f(0); for a few sites, however, this decrease is replaced by an increase as reported in previous studies, in relation with water contents and pore-pressure issues. The increase of amplification below f(NL) is found to be a quasi-systematic consequence of nonlinear soil behavior, which should be emphasized, because it can reach up to 1.6 for high V-S30 sites.