報告書番号

N07013

タイトル（和文）

地震観測記録に基づく地震動の減衰特性（その３）－硬質地盤における減衰メカニズムの解明－

タイトル（英文）

Attenuation characteristics of seismic motion based on earthquake observation records Part3. -Attenuation mechanism of seismic motion of hard rock ground-

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，スペクトル比の逆解析により同定される地盤減衰のメカニズムの解明を目的として，材料減衰で説明しきれない減衰定数の周波数依存性と表層付近における特に大きな値について，それらを生じさせている物理的なメカニズムについての検討を行なった．
本報告書では，まず，検討地点における，鉛直アレイ観測記録のスペクトル比の逆解析から，地盤減衰の同定を行ない，同定結果が周波数依存性を示すこと，さらに表層付近の地盤減衰については，下限値で比較しても材料減衰の経験的な値と比べて大きな値となることを事例として示した．そして，検討地点における地盤減衰の同定結果を，本研究においてメカニズムを解明すべき対象と位置づける．そうした上で，まず，減衰定数の周波数依存性のメカニズムの解明を目的として，検討地点で実施されているPS検層結果から，散乱減衰の発生要因の一つと考えられている地盤のランダムな不均質性に関する統計的性質を求めた．次に，それを利用した2次元ランダム媒質における波動伝播の数値シミュレーションにより散乱減衰の評価を行なった．その結果，数値シミュレーションに基づき評価した散乱減衰と検討地点で実際に同定した地盤減衰の周波数依存性を比較し，相関距離2000m程度の地盤（S波速度）のランダムな不均質性に起因した散乱減衰により，同定された地盤減衰の周波数依存性が説明できることを示した．さらに，表層付近で特に大きな値となる地盤減衰のメカニズムについて，検討地点周辺における地形および地質構造に着目し，それを模擬した2次元ＦＥＭ解析を行なった．その結果，周辺地形の影響により，1次元の観測スペクトル比（伝達関数）に低減効果が生じ，その低減効果は実際に同定された地盤減衰と等価であることを明らかにした．

概要 （英文）

In this study, we try to clarify the physical mechanisms of the damping factor of hard rock ground (or the attenuation characteristic of seismic motion of hard rock ground) identified from spectral ratio inversion using vertical array observation records. The matter in question in the mechanisms of inverted damping factor of hard rock ground is that a frequency dependency and a larger damping factor near the surface layer were not explained by the experimental material damping of ground.
In this report, before we examined the physical mechanisms of the damping factor, it was identified that the damping factor of hard rock ground at the test site, and we showed the existence of a frequency dependency and a larger damping factor near the surface layer. The physical mechanisms of the identified damping factor of ground at test site are clarified in this study. The first mechanism to be clarified is whether the frequency dependency of the damping factor of hard rock ground can be explained by scattering attenuation due to random velocity fluctuation of ground. Then we first estimated the statistical property (especially correlation distance) of random velocity fluctuation of ground by P-S logging profile using suspension method at test site. Next, we estimated scattering attenuation due to the random velocity model of test site and the other models with different correlation distances, by 2D numerical simulations using finite difference method. As a result, it was clarified that the random velocity model with the correlation distance of about 2,000m was able to explain the frequency dependence of identified damping factor of ground at test site by comparing scattering attenuation based on numerical simulation and identified damping factor. The second mechanism to be clarified is whether the larger damping factor of hard rock ground near the surface layer can be explained by effects of surface topography and geology. Then we constructed 2D FEM model imitated around test site, and we conducted 2D FEM analysis by using the constructed model. As a result, it was clarified that the decreasing effect of surface topography and geology on spectral ratio (transfer function) was almost equivalent to the one of identified damping factor of ground at test site. Therefore it is reasonable to understand that the larger damping factor of hard rock ground near the surface layer was caused by effects of surface topography and geology.

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