報告書番号

N04006

タイトル（和文）

Study on Rupture Process by Waveform Inversion Using Simulated Annealing and Simulation of Broadband Ground Motions

タイトル（英文）

Study on Rupture Process by Waveform Inversion Using Simulated Annealing and Simulation of Broadband Ground Motions

概要 （図表や脚注は「報告書全文」に掲載しております）

従来，推定が困難とされてきた震源からの高周波地震動励起特性を明らかにするために，比較的短時間で精度のよい波形合成が可能な経験的グリーン関数法と，強度の非線形問題に対して実績のある焼きなまし法を組み合わせた探索アルゴリズムを開発し，1997年伊豆半島東方沖地震の震源近傍の強震記録を用いて開発手法の適用可能性を検討した．震源パラメータのうち，特に高周波地震動を規定するとされる実効応力と，低周波地震動に比例する地震モーメントを独立に探索した結果，周波数5Hzまでの速度波形を精度よく再現できる不均質震源モデルを得た．さらに，インバージョンで求められた実効応力と地震モーメントの空間分布に対し，その統計的な不均質性を矩形のアスペリティ領域と背景領域で表現する手順（特性化手法）を導出するとともに，実地震の広帯域震源特性を直接に反映したシナリオ地震の強震動評価手法を提案した．

概要 （英文）

A source inversion method using very fast simulated annealing (VFSA), which is the variation of simulated annealing (SA) classified into the heuristic search techniques, is proposed for estimating rupture process radiating broadband strong ground motions. The empirical Green's function (EGF) method is used for a forward process of computing synthetic ground motions. From the displacement motions, the released moment, rise time and rupture starting time at each sub-fault are estimated in this procedure. From velocity motions the spatial distributions of the effective stress is estimated with those of the released moment and rupture time. A smoothing constraint to the rupture starting time is introduced for the stability of solutions in both inversions. In the numerical test VFSA shows a good performance for quick and deep search in the model space compared with other heuristic techniques. The developed inversion method is applied to the strong-motion data recorded at bedrock stations during the 1997 Izu-Hanto Toho-Oki earthquake (MJMA5.9). The upper limit of the frequency range used in the inversion reaches 5Hz to deduce the higher-frequency wave radiation from the target source. Results of the displacement inversion indicate that the large moment released area is located at the shallow part just above the hypocenter and extends to the deep northern area. Similar results are obtained from the velocity inversion for the moment release distribution and rupture propagation. The spatial distribution of the effective stress on the target fault also exhibits a similar behavior to the moment distribution. In addition to these results we estimate an overall source model that minimizes the sum of misfits calculated from displacement, velocity and acceleration waveforms and acceleration envelopes with equal weights, assuming constant rupture propagation and the proportional relation between the effective stress and the released moment at each sub-fault. Obtained overall source model provides rather good fit between synthetic and observed motions including the amplitude levels of acceleration motions. Finally we reconstruct the characterized source model that consists of the rectangular asperity area with uniform and high effective stress and the background area with low effective stress from the results of the displacement and velocity inversions. The broadband ground motion simulation in the frequency range up to 10 Hz demonstrates that the characterized source model successfully reproduces the observed ground motions as well as the source model directly derived from the velocity inversion. This suggests our proposed inversion method makes it possible to obtain the characterized source model that directly reflects the high-frequency radiation from the actual seismic source.

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