Assessing Nonlinear Behavior of Soils in Seismic Site Response: Statistical Analysis on KiK-net Strong-Motion Data

Abstract

This article investigates the effects of the nonlinear behavior of soils on site response, through various earthquake recordings from the KiK-net database in Japan. This network is composed of more than 688 surface–borehole instruments, from which a characterization of the shear- and compressive-wave velocity profiles down to the borehole depth is available. We selected events with a peak ground acceleration (PGA) at the downhole station of <10 cm=s2 in order to characterize the linear soil behavior by computing the surface to downhole spectral ratios at each site. Modifications of site-response curves computed with strong events (PGA >50 cm=s2) compared to the linear characterization are supposed to be caused by nonlinear soil behavior. To describe the effects of soil nonlinear behavior on site response per event, we propose the percentage of modification (either amplification or deamplification) of the site-response curve compared to the linear evaluation (PNLev, percentage of nonlinearity) and the associated shift frequency (Shev). These parameters are used to estimate the probability that nonlinear site response is significantly different that the linear counterpart. We find that, regardless the site, this probability is important even for low input-motion PGA (values equal to or larger than 30 cm=s2 at downhole sensor). This indicates that nonlinear soil behavior must be taken into account in site-response evaluation for moderate to strong motion. In addition, for 54 sites of the KiK-net database that have recorded at least two strong events (PGA at the downhole station >50 cm=s2), we define four additional parameters that characterize the effects of soil nonlinear behavior on site responses for each site: (1) a PGA threshold (PGAth), defined as the PGA value for which PNLev is higher than 10%; (2) a site-specific PNL for a PGA of 50 cm=s2 (PNLsite); (3) a site-specific shift of the predominant frequency for a PGA of 50 cm=s2 (Shsite); and (4) a frequency from which we observe deamplification between nonlinear and linear site responses (fNL). We observe that nonlinear soil behavior can increase the amplification at frequencies below fNL.We find that fNL lies in between the fundamental and the predominant resonance frequencies of the site response and that sites having VS contrast close to the surface trigger nonlinear behavior at a lower input-motion PGA threshold. These results suggest that nonlinear behavior occurs mostly in the superficial soil layers. Furthermore, by investigating the nonlinear behavior of soils on earthquake horizontal-to-vertical spectral ratios at the surface, we find that they can give satisfactory results (equivalent to the analysis of borehole site responses) for the evaluation of the fNL frequency and shift frequency (Shsite), which indicates that part of the results obtained in this study can be extended to other databases without downhole sensors

Using ambient vibration measurements for risk assessment at an urban scale: from numerical proof of concept to Beirut case study (Lebanon)

“…Besides the similarity of overall curves, one can notice two significant changes: (a) the slight shift in the location of the peak, from f struct /f soil = 1 in the linear site response case to f struct /f soil around 0.6-0.7 in the NL site response case for moderate to strong PGA values (2 and 4 m/ s 2 ), and (b) the decrease of rock-to-soil damage increment at the peak value and in the high-frequency range (f struct /f soil > 1). Both observations are consistent with the NL site response, which shifts the soil frequencies to lower values, and reduce the high-frequency amplification (Régnier et al 2013Almakari et al 2016).…”

“…The scope of this section is to investigate whether the nonlinearity in the site response can significantly weaken the dominant importance of spectral coincidence effects, as it may significantly change the site frequency and the associated amplification. The soil nonlinear behavior is most often characterized by a degradation of its mechanical properties, involving a shear modulus reduction and a damping increase with increasing shear strain; it basically results in deamplifying the ground motion and shifting the frequency response toward lower values (Bonilla et al 2005;Régnier et al 2013Régnier et al , 2016. The realistic soil profiles adopted previously are thus modeled using nonlinear site response analysis, and the response analysis of the comprehensive set of SDOF oscillators is repeated here for the modified loading including the effects of soil nonlinearity.…”

Characterizing the nonlinear interaction of S- and P-waves in a rock sample

“…Field observation of nonlinear responses induced by strong or weak earthquakes is well documented (see Ref. 4 for instance), and wave-speed variations on the order of 0.05% have been measured during earthquakes on the San Andreas fault. 5 Actively induced nonlinear responses have been observed in-situ at the scale of a few meters.…”

V S30, slope, H 800 and f 0: performance of various site-condition proxies in reducing ground-motion aleatory variability and predicting nonlinear site response

“…The KiK-net database used here consists of shallow crustal events recorded on sites for which several site proxies are already available: V S30 and H 800 values can be directly derived from downhole measurements of V S profile (Dawood et al 2014), the slope values have been compiled , and f 0 values are taken from Régnier et al (2013). The KiK-net data offer the unique opportunity to have, for each strong-motion recording, a reliable measurement of the four SCPs, thus allowing a thorough and meaningful comparative assessment of the performance of each of these proxies.…”