The spatial properties of the site amplifications of S-waves by generalized spectral inversion technique and the correction method of the site amplifications considering the contribution of later arrivals after major S-waves

Abstract

Site amplification is an important component of strong ground motion prediction as it differs among sites, reflecting its specific local subsurface geology. Here, we confirm that site amplifications are similar in a neighborhood area over a long period. However, few studies have investigated the spatial properties in a wide region (i.e., the whole of Japan). In this study, we explored the spatial properties of site amplifications based on the generalized inversion technique (GIT) using Fourier amplitude spectra (FAS) as well as pseudo-velocity response spectra (pSv) as the latter is an important index for engineering purposes and the most similar type of response spectra to FAS. The spatial distributions of S-wave site amplifications (SA-S), especially within large sediment basins (e.g., the Kanto and Osaka Basins in Japan), were found to be relatively similar in proximate areas for a long period ranging from 2 to 8 s. This suggests that we could easily predict the site amplifications using an empirical approach through spatial interpolation based on the properties obtained by the GIT. Furthermore, we propose a prediction procedure for site amplification for the whole duration from the SA-S at an arbitrary site. We used the correction function, which converts the SA-S to the site amplification for the whole duration (SA-W), including an S-wave portion and a subsequent portion. This function is called the whole-duration to S-wave spectral ratio (WSR) and is stable in terms of spatial properties. As we could estimate the SA-S either by theoretical transfer functions or observed microtremors, we can easily predict the SA-W based on the proposed WSR concept. We found that SA-S in pSv is more or less similar to SA-S in FAS, however, SA-W in pSv fails to capture the effects of the long duration of ground motions inside a large basin so that we cannot recommend to use pSv for the prediction of whole duration of ground motion. Graphical Abstract

Unified approach for evaluation of horizontal site amplification factors with special reference to history of studies on the effects of surface geology on seismic motion

“…Figure 15 shows the entire Japanese map of WSRh based on the GIT results of Nakano (2021) and Nakano and Kawase (2023) at two frequencies, 0.5 Hz and 1.0 Hz, the values of which were interpolated by the surface function of GMT (Smith and Wessel 1990). Evidently, the WSRh is larger inside the sedimentary basins and volcanic calderas and close to 1 in mountainous areas.…”

“…( 1) and ( 2) do sHSAF and sVSAF* for the S-wave part at hand, respectively. From these two pairs of SAFs, the whole duration to the S-wave part ratio, the WSR, is determined (Nakano 2021;Nakano and Kawase 2023). Because we separated the SAFs for the horizontal and vertical components, we have WSRh for the horizontal component and WSRv for the vertical component.…”