報告書番号

N07024

タイトル（和文）

表層地質の影響を考慮した露頭岩盤観測点の地震動特性の解明－根室観測点における検討－

タイトル（英文）

Characteristics of ground motions on the rock outcrop with irregular subsurface structure -Case study at Nemuro observation station-

概要 （図表や脚注は「報告書全文」に掲載しております）

電力中央研究所の露頭岩盤上観測点である根室地点の強震動特性を明らかにするために，応答スペクトルによる既往の距離減衰式との比較検討，ならびに2次元地盤モデルに基づくFEM解析を実施した．
観測記録の経験的な予測スペクトルに対する残差スペクトルを詳細に検討した結果，観測点の南東側から到達する地震の記録にのみ，周期0.3秒付近に残差スペクトル振幅の特異なピークが認められた．
また2次元FEM解析の結果，根室地点では，西側斜め入射の場合の地震波地表応答に対して鉛直および東側斜め入射の応答が相対的に大きくなり，また周期0.2秒程度以下の短周期領域では観測強震記録の相対的な応答スペクトル特性を比較的よく説明できることを示した．
さらに常時微動の水平／上下スペクトル比との比較でも，地下構造の推定精度が高い観測点直近の地点では，FEM解析によるS波伝達特性を比較的よく再現できることが明らかになった．

概要 （英文）

We examine the characteristics of observed ground motions at Nemuro site, where strong-motion seismometer is installed on rock outcrop, by comparing their pseudo velocity response spectra with the empirical attenuation relation assumed in the seismic design for the nuclear power plants. Spectral ratios of observed motions to the empirical model show the obvious peak at the period of 0.3 second in the event group whose epicenter distributes in the south-eastern area from the Nemuro site, while no distinct peak is found in the group occurring in the south-western area.
In order to prove the effect of local site structure at Nemuro station to the observed strong motions, we model two-dimensional subsurface velocity structure beneath the station, including irregularity of surface geology, based on the results from geophysical surveys such as PS logging, seismic refraction method, and microtremor observation. The seismic bedrock layer with S-wave velocity higher than 3 km/s is added at the depth of 120 m to obtain the predominant period of 0.3 second through the test run using one-dimensional response analysis, because the geophysical surveys we applied reach the depth of 50 m. Then, the two-dimensional FEM simulation is carried out to see the behavior of strong motions in and around the Nemuro site by considering three cases with different incident angles of input motions to the FEM model. As a result the calculated site response shows roughly good agreement with the observed motions in the period shorter than 0.2 second. On the other hand in the longer period including the main peak the FEM simulation cannot describe the observed motions, because of a lack of information in the deeper subsurface structure.

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