Site Effects on Large Ground Motions at KiK-net Iwase Station IBRH11 during the 2011 Tohoku Earthquake

Abstract

Large horizontal ground motions with peak ground accelerations (PGAs) of more than 800 cm=s 2 and peak ground velocities of nearly 60 cm=s were observed at a KiK-net station, Iwase (IBRH11), during the 2011 M w 9.0 Tohoku, Japan, earthquake. We investigated site effects on the large ground motions by inverting the subsurface structure and simulating ground motions considering soil nonlinearity. The structure from the seismic bedrock to the surface in the linear regime was inverted from (1) array records of microtremors using Rayleigh-wave inversion, (2) surfaceto-borehole spectral ratios of weak motions using S-wave inversion based on the 1D wave propagation theory, and (3) horizontal-to-vertical spectral ratios of weak motions using the inversion based on the diffuse-field theory for plane waves. The main cause of the large ground motions was found to be a strong impedance contrast between soft layers with an S-wave velocity (V S ) of less than 381 m=s and a layer with V S 2371 m=s at a depth of 30 m. The strong motions at the surface during the 2011 Tohoku earthquake simulated by equivalent-linear analysis using borehole records at a depth of 103 m are in reasonable agreement with the observed records, whereas those simulated using linear analysis were approximately twice as large as the observed values. The results showed that the nonlinearity of the surface soils reduced the amplification factors and the PGAs by half compared with the values for the linear regime. The PGAs of the bedrock motions with V S 2371 m=s were estimated, using equivalent-linear analysis, to have reached approximately 500 cm=s 2 .

Analysis of Site Amplification and Nonlinear Response Based on Ground Motion Records at MYGH10 Station in Japan

“…The two spectral ratio methods can reach consistent conclusions. Satoh et al (2014) studied the influence of site on the large motions by inverting the soil layer's structure and simulating ground motions considering the soil nonlinearity, and they found that the nonlinearity of the soil on large motions results in the reduction of the amplification factor by half compared with those in the linear regime; they also studied the impact time of nonlinearity by comparing the PGA of aftershocks and pointed out that the time of soil returning to linear state varies from site to site. This paper will judge the recovery of nonlinearity by comparing the spectral ratio curve of aftershocks records.…”

Causes of Large Ground Motions at K-NET Tsukidate and KiK-net Haga during the Tohoku Earthquake based on the Analysis of Weak and Strong Motion Records

“…そこで、本研究では、弱震動記録も用いて、MYG004とTCGH16で、地震動ではない振動と地盤の非 線形性の影響により、弱震動に比べてどの程度増幅したかについて分析する。地中の記録がない MYG004では、MYG004より27km離れたKiK-net東和(MYGH04)での、地盤の非線形性の影響がないと みなせる地中(GL-100m)のUD成分の記録で基準化することにより、震源・伝播特性の影響を除き検討 を行う。一方、TCGH16より約20km離れたKiK-net岩瀬(IBRH11)では、震度6強相当(計測震度6.1)の記録 が得られており、Satoh et al 9) では、地中の記録から、等価線形解析により地表の強震記録を良く再現 できることを示している。 また、 TCGH16では、 山中・他 6) により、 アレー微動に基づき表層18m(GL-18m) の工学的基盤以浅のS波速度構造が推定されている。そこで、TCGH16の弱震動記録の地中(GL-112m) に対する地表のスペクトル比を用いて震源・伝播特性の影響を除き、地中GL-112mからGL-18mまでの 地下構造の推定を行う。そして、地中の記録から、等価線形解析により、地震動ではない振動がなか った場合のTCGH16の地表での強震動と計測震度の推定を行う。…”

Spectral ratio analysis of the site amplification effect and nonlinear site response at MYGH10 during the 2021 MW 7.1 Fukushima earthquake in Japan

“…Additionally, Wen et al 15 proposed using the HVSR method to identify nonlinear soil responses and validated its reliability in identifying soil nonlinearity using the surface‐to‐borehole spectral ratio (SBSR) method. Satoh et al 21 studied the site response under strong shaking by inverting the soil layer structure and simulating ground motions considering soil nonlinearity. They found that the nonlinearity of the surface soils resulted in a reduction in the amplification factor and PGA by half compared with those in the linear regime.…”