Journal
BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA
Volume 106, Issue 4, Pages 1710-1719Publisher
SEISMOLOGICAL SOC AMER
DOI: 10.1785/0120150199
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Funding
- National Research Agency [ANR-11-RSNR-0022]
- Agence Nationale de la Recherche (ANR) [ANR-11-RSNR-0022] Funding Source: Agence Nationale de la Recherche (ANR)
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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.
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