報告書番号

N09013

タイトル（和文）

微動・地震観測に基づく深部地盤のモデル化－柏崎刈羽原子力発電所を対象とした微動アレイ観測の適用性－

タイトル（英文）

Deep subsurface structure modeling based on microtremor and earthquake observation -Applicability of microtremor array measurements at the Kashiwazaki-Kariwa Nuclear Power Station -

概要 （図表や脚注は「報告書全文」に掲載しております）

本研究では，原子力発電所における深部地盤モデルを構築するための探査手法として注目を集めている微動アレイ観測の適用性について，(1)地震基盤に至る深部地盤構造探査への適用性，(2)正三角形ではない観測点配置でのSPAC法の適用性，(3)空間的な構造の変化の把握および(4)速度コントラストが小さい地盤への適用性に着目した検討を，東京電力(株)柏崎刈羽原子力発電所での観測を対象に行った．その結果，反射法探査では困難であった地震基盤深度の推定，またこれまでほとんど明らかとなっていない正三角形ではない観測点配置でのSPAC法の適用性，さらに柏崎刈羽原子力発電所の地震動の空間変動の要因となったサイト南北における深さ1km程度の構造の差異の検出が可能であることを明らかにした．また，サイト北側のPS検層結果にみられる速度コントラストが明瞭でない漸増構造に対する位相速度の逆解析結果について，地質構造境界や速度の漸増成分の中心といったPS検層結果の特徴を捉えた構造を推定していることを明らかにするとともに，漸増構造を仮定した逆解析により，PS検層にみられる深さ方向に大きくなる傾向を概ね模擬した構造を推定できることを示した．

概要 （英文）

During the 2007 Niigataken Chuetsu-oki earthquake, strong ground motion with the peak acceleration of 680cm/s/s which was larger than that of the empirical prediction was recorded at the base mat of the No.1 reactor building of Kashiwazaki-Kariwa Nuclear Power Station (NPS). Furthermore, in the Kashiwazaki-Kariwa NPS, over twice difference of 680 vs. 322 cm/s/s of peak acceleration between the No.1 and the No.6 reactor buildings was observed on the base mat. From the results of recent research, it is suggested that the deep sedimentary layers can be one of the important factors to elucidate these phenomena. In this study, at first, the applicability of microtremor array measurements for estimation of deep S-wave velocity structure (~Vs=3km/s layer) are discussed. Vertical microtremors were observed in three arrays at the Kashiwazaki-Kariwa NPS with the maximum station spacings of 3.04km, 1.49km and 0.75km, respectively. The Rayleigh wave phase velocity in a frequency range from 0.3Hz to 1.5Hz was estimated from frequency-wavenumber spectral analysis (F-K method). The phase velocity was inverted to a 1D S-wave velocity structure consisted of 9 layers using a heuristic search technique by simulated annealing. The estimated structure of deep sedimentary layers at the Kashiwazaki-Kariwa NPP is mainly characterized by the Nishiyama, Shiiya, Upper Teradomari, Lower Teradomari, Nnatani-Green tuff layers with S-wave velocities of 0.76 km/s, 1.2 km/s, 1.7 km/s, 1.9 km/s and 2.6 km/s, respectively. The basement depth is about 3.8 km, and the thickness of each layer was consistent with the previous survey around the Kashiwazaki-Kariwa NPP. Next, we discussed the applicability of SPAC method for the estimation of phase velocity using non-equilateral triangle array data. As a result, we demonstrated that the estimated phase velocity from SPAC method without average over azimuth using non-equilateral triangle array data was in good agreement with the above one from F-K method. Moreover, in order to explore the spatial variations of subsurface structure in Kashiwazaki-Kariwa NPS at the depth of around 1 km, we conducted microtremor array measurements with the maximum station spacings of 0.7~0.9 km at the north and south side of Kashiwazaki-Kariwa NPS. Our results showed that the estimated phase velocities from both array measurements using above SPAC method were consistent with the theoretical ones from P-S logging which were carried out under both array areas, respectively. Furthermore we discussed the applicability of phase velocity inversion technique to the estimation of the low velocity contrast (increasing gradually at the interfaces) S-wave velocity structure. As a results, we demonstrated that the estimated structure from the ordinary layered model inversion well explained the average of velocity structure and the stratigraphic interfaces. We also showed that the estimated structure from the proposed inversion scheme well explained the feature of low velocity contrast velocity structure.

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