報告書番号

N17

タイトル（和文）

地震動評価のための硬質地盤における減衰評価とその体系化

タイトル（英文）

Attenuation measurements of Hard rock ground and its systematization for earthquake ground motion estimation

概要 （図表や脚注は「報告書全文」に掲載しております）

本報告書では，いわゆる「10Hz付近の変移」と「大きな減衰」といった減衰の評価・モデル化において明らかにしておくべき，地表に近い岩盤で指摘されている特性に着目し，それを目的とした減衰評価を行った。ここで用いた評価法は，評価地点の鉛直アレイ地震観測記録を用いる減衰評価，岩石コアの超音波試験よる減衰評価，そしてPS検層データを用いた不均質性に基づく評価であり，各評価法から得られる結果の特徴とそれぞれの関係性についても併せて明確にした。さらに，これらの検討結果に基づき，本研究で適用した減衰評価法を効果的に組み合わせることによる1次元地盤モデル作成における減衰(Q値)の設定方法を提案した。

概要 （英文）

Attenuation characteristics of near-surface rock ground, such as low-Q, were investigated based on borehole recordings of earthquake at rock site and laboratory measurements of rocks sampled near the borehole. Spectral ratios with the frequency from 1 Hz to 20 Hz using the multiple reflected S waves between different recording levels were well explained by the bi-linear frequency-dependent attenuation model considered transition frequency of 19.5 Hz. Whereas, spectral ratio with the frequency from 18 Hz to 30 Hz using the direct upgoing S wave between GL-50 m and GL-340 m were well modeled with the frequency-independent attenuation. These results clarified that the near-surface attenuation of rock was independent of frequency over about 19.5 Hz, which was consistent with the existence of 10 Hz transition. Then we estimated the attenuation of unfractured rocks by the laboratory measurements of ultrasonic wave. From the comparison of damping factors, it was demonstrated that the attenuation from borehole recordings was low-Q rather than the one from the laboratory measurements using unfractured rock. Furthermore, the standard deviation of heterogeneity (heterogeneity index) was determined from the borehole velocity-log, and a good relationship between heterogeneity index and attenuation was obtained. It therefore was suggested that the additional attenuation effect caused from the existence of rock heterogeneity and fracture was included in the estimated attenuation by using borehole recordings. Furthermore, it was demonstrated that the additional damping factor had same effect as heterogeneity modeling on site-response transfer-function estimation. Finally, the damping factor determination flow chart for the earthquake ground motion estimation was proposed.

報告者

キーワード