Publication Date:
2016-07-26
Description:
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 (RSR NL–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 RSR NL–L =1). The average maximum amplitudes of RSR NL–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 S 30 (travel-time averaged shear-wave velocity over the top 30 m) or f 0 (site fundamental frequency): f NL decreases when either V S 30 or f 0 decreases. In addition, the amount of the low-frequency amplification increase depends on V S 30 and reaches a maximum of 1.6 for high V S 30 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 S 30 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 S 30 sites. Online Material: Figures of site-response ratio (RSR) curves and tables of site characteristics.
Print ISSN:
0037-1106
Electronic ISSN:
1943-3573
Topics:
Geosciences
,
Physics
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