報告書番号

N10004

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４）－地表に近い岩盤における減衰定数の評価と速度の揺らぎとの関係－

タイトル（英文）

Attenuation characteristics of seismic motion based on earthquake observation records Part 4 Estimation of damping factor at near surface of rock and its relationship with random velocity fluctuation

概要 （図表や脚注は「報告書全文」に掲載しております）

硬質地盤のモデル化で重要となる地表に近い岩盤の減衰定数について，おもに波動伝播に基づいた異なる手法により総合的な評価を行った．1-20Hzが対象の地震記録のスペクトル比の逆解析から，バイリニア型周波数依存モデルで同定した結果，検討地点における花崗岩層全体の減衰定数の下限値は0.035(Qs=14.2)，であり，この値は地殻での花崗岩の一般的な値(Qs=70-150と比較して大きい減衰定数であった．しかしながら，既往のPS検層による評価結果(30Hz程度)は，同定された下限値と一致し，鉛直アレイ観測記録のS波上昇波のスペクトル比から評価した結果も，対象範囲(18-30Hz)において，同定された下限値とほぼ等しい．以上から，減衰定数の下限値については，異なる手法による結果を合わせて議論でき，検討地点の減衰定数については3%程度と考えられる．また，検討地点近傍におけるPS検層結果から，速度の揺らぎ大きさを推定した結果，同定された下限値と速度の揺らぎ大きさに正の相関関係がみられ，またこの関係は，既往の研究による関係とも調和的であった．これは，地表付近の減衰定数の相対的な大小関係の把握に不均質強度を指標とした評価が有効である可能性を示唆する．

概要 （英文）

In this study, characteristics of near surface attenuation in rock (granite) are investigated using earthquake records from borehole array observations at depths of 1, 50, 140 and 340m. Spectral ratio inversion using bi-linear-type frequency dependent damping factor model, having a frequency independent lower limit (hmin), is mainly applied to the investigation. The lower limit of damping factor includes mainly intrinsic attenuation, but also scattering attenuation due to short wavelength heterogeneity. From spectral ratio inversion, hmin of 0.035(Qs=14.2) is identified in the depth between 7 and 340 m, which is the whole part of granite in the borehole. In the other case of inversion, damping factors of shallower and deeper part of granite are identified as hmin of 0.039 (Qs=12.7) in the depth less than 172 m, 0.017(Qs=30.2) in the depth between 172 and 340 m. These identified hmin of near surface granite are larger (lower Q) than the ones in deep crustal granite (e.g. Qs=70-150). However, the lower limit of damping factors are in very good agreement with the other estimations at the same borehole using PS-logging and spectral decay of up-going initial S-wave. These results suggest that the damping factor of near surface granite is about 0.03 at the borehole site, and show that the lower limit of bi-linear-type frequency dependent damping factor model is appropriate to an estimation of attenuation using borehole array records. Furthermore, the standard deviation of random velocity fluctuation (heterogeneity) of rock is determined by an inversion of von-Karman type auto correlation function using the suspension PS-logging data of same granite obtained at near the borehole site. From comparisons of the relationship between determined heterogeneity with the identified damping factor (hmin), a simple proportional relationship between damping factor and the heterogeneity is demonstrated including the one from previous study. This result suggests that a standard deviation of random velocity fluctuation (heterogeneity) of rock gives one of valuable information to estimate the level of near surface attenuation of rock.

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