S-Wave Site Amplification Factors from Observed Ground Motions in Japan: Validation of Delineated Velocity Structures and Proposal for Empirical Correction

Abstract

We first derived site amplification factors (SAFs) from the observed strong motions by the Japanese nationwide networks, namely, K-NET and KiK-net of National Institute of Earthquake Research and Disaster Resilience and Shindokei (Instrumental Seismic Intensity) Network of Japan Meteorological Agency by using the so-called generalized spectral inversion technique. We can use these SAFs for strong motion prediction at these observation sites, however, we need at least observed weak motion or microtremor data to quantify SAF at an arbitrary site. So we tested the capability of the current velocity models in Japan whether they can reproduce or not the observed SAFs at the nearest grid of every 250 m as the one-dimensional theoretical transfer functions (TTF). We found that at about one-half of the sites the calculated 1D TTFs show more or less acceptable fit to the observed SAFs, however, the TTFs tend to underestimate the observed SAFs in general. Therefore, we propose a simple, empirical method to fill the gap between the observed SAFs and the calculated TTFs. Validation examples show that our proposed method effectively predict better SAFs than the direct substitute of TTFs at sites without observed data.

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

“…14 later. More optimized matching can be obtained by tuning the velocity structure based on the EHVR mentioned above or by applying empirical frequency and amplitude modification functions as proposed by Ito et al (2021a). However, it is important to point out that basically the empirical SAF for the S-wave part from the seismological bedrock to the surface could be modeled by the entire 1D structure from the seismological bedrock to the surface.…”

“…Because we separated the SAFs for the horizontal and vertical components, we have WSRh for the horizontal component and WSRv for the vertical component. At a site without observed data, sHSAF can be calculated using a 1D S-wave theoretical ground response analysis based on the velocity structure proposed, such as a unified model by Senna et al (2014Senna et al ( , 2018Senna et al ( , 2019, and Wakai et al (2019), together with the FMR and AMR corrections (Ito et al 2021a). Then wHSAF could be calculated as:…”

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

“…After this stage, the signal is known as the bedrock motion. Subsequently, site effects, which are associated with the propagation velocities of S and P-wave in media and deposit density structures, 1,2 soil damping, subsurface topographies, 3,4 and nonlinear soil behaviors, [5][6][7] determine the level of amplification on motions from the seismological bedrock to the surface. The horizontal site amplification factor (HSAF), which represents local site effects at a given site, is essential for efficient seismic risk reduction and seismic hazard assessment.…”