Testing Nonlinear Amplification Factors of Ground-Motion Models

Abstract

We explore nonlinear site effects in the new Japanese ground-motion dataset compiled by Bahrampouri et al. (2020). Following the approach of Seyhan and Stewart (2014), we evaluate the decrease of soil amplification according to the increasing and corresponding ground motion on surface rock (VS30=760 m/s). To better predict the rock ground motion associated with each record, we take into account the between-event variability of the ground motion, and to better evaluate the impact of nonlinearity, we correct observed ground motion on soil by the site-specific linear amplification. Instead of grouping the stations by site-response proxy, we focus on individual stations with several strong-motion records. We develop a framework to test recently published nonlinear site amplification models against a linear site amplification model and compare the results with recent building codes that include nonlinearity. The results show that the site response varies greatly from site to site, indicating that conventional site proxies, such as VS30, are not sufficient to characterize nonlinear site response. Out of all of the Kiban–Kyoshin network stations, 20 stations are selected as having recorded sufficient data to be used in the test. Out of these 20 stations, five stations show signs of nonlinearity, that is, the nonlinear models performed better than the linear-amplification model for all periods T. For most sites, however, the linear site amplification models get the best score. This suggest that, for the range of predicted rock motion considered in this study (peak ground acceleration <0.2g), nonlinearity may not have a sufficiently large impact on soil ground motion to justify the use of nonlinear site terms in ground-motion functional forms and seismic building codes for such moderate-level shaking.

An empirical model for the vertical‐to‐horizontal spectral ratios for Italy

“…In particular, the non‐linear site response term in horizontal GMM was neglected because there are insufficient records to robustly constrain these effects. Another strategy, followed in several works (GA2011; AK2014; BC2016) consists in calibrating the non‐linear site scaling on the basis of numerical simulations; however, this approach tends to over‐estimate the effects of the non‐linearity, especially at short periods 44,45 …”

A hybrid approach for deriving horizontal site amplification factors considering both the similarity of HVSRE and the vertical amplification correction function

“…As shown in Figure 14, we supplemented a brief comparison between the HSAFs estimated using GMM performed by Loviknes et al., 44 Hybrid method 2, and GIT at five stations. Please note that the HSAFs by GMM released from them are originally the relative Fourier spectral amplification ratio with respect to the average spectra of all the sites as their GMM is referring to.…”

“…Comparison between the HSAFs estimated using GMM for the Fourier spectra performed by Loviknes et al., 44 Hybrid method 2, and GIT at (A) IBUH01, (B) KGWH03, (C) KGWH04, (D) SBSH09, and (E) TTRH07. At the top of each panel, we labeled the site name selected for Hybrid method 2…”

Empirical nonlinear site effects in Japanese megathrust earthquakes

“…It is not obvious why the nonlinear site-response terms for Japanese sites behave differently from those of other regions. Several studies of individual KiK-net sites have shown that the nonlinear behavior of these sites is quite variable and not necessarily related to the value of V S 30 at the site, which is attributed to various factors, including (1) the averaging of the shear-wave velocity profile over the top 30 m, (2) the strong velocity impedance at shallow depths exhibited by many Japanese KiK-net sites, (3) the relatively shallow soft-soil layers of typical Japanese KiK-net sites, or (4) a combination of these effects (Bonilla et al, 2021; Ji et al, 2021; Loviknes et al, 2021; Zalachoris, 2014). We also note that the small V S 30 , large A 1100 recordings from our database come primarily from two earthquakes and that the number of recordings for the softest sites from these events are limited.…”