3 次元不均質地盤モデルを用いた地震動シミュレーションに基づく地震動の空間変動モデルの構築

Ground motions could cause spatial variation in phase and amplitude in propagating the heterogeneous subsurface soil, even at adjacent points. It has been pointed out that the spatial variation has the effect of reducing the input motion to the building, as well as the effect of exciting the rotation and twist of the building. In this study, we performed ground motion simulations using 3-D heterogeneous soil models and analyzed the characteristics of the coherence of ground motions at adjacent points. We also suggested the coherence model of ground motions at adjacent points based on the scattering theory. In the ground motion simulation, 3-D heterogeneous soil models were used, varying the heterogeneous parameters such as the correlation distance of the propagation velocity of the ground motion, the coefficient of variation, to analyze the relation of the heterogeneous parameters and the coherence. The response waves were extracted from the surface of the soil models and these response waves were used to evaluate the effect of each heterogenous parameter on the coherence at adjacent points. As a result, it was clarified that the coherence tends to increase as the horizontal correlation distance a₁ increases, whereas the coherence tends to decrease as the vertical correlation distance a₂ increases. And it was indicated that the coherence decreases as the size of the soil model is increased in the depth direction, but the coherence becomes stable even if the soil model is deepened from a certain depth. Also, the modelling of the coherence evaluation was performed by deriving the correlation of the wave field at adjacent points from the wave equation based on the scattering theory, considering the variation of the propagation velocity. In order to verify the applicability of this coherence evaluation model, we compared the coherence of the evaluation model and the results of ground motion simulation using 3-D heterogeneous soil models. As a result, the coherence from the evaluation model corresponds well with the coherence by the ground motion in the examination case whose a₂/a₁ of the heterogeneous soil model is small, but in the examination case whose a₂/a₁ is large, the coherence by the ground motion simulation becomes higher significantly than the coherence by the evaluation model as the depth of the soil model gets larger. In addition, the coherence from the ground motion simulation tends to be larger than the result of the evaluation model in the case the separation distance increased to some extent. Based on these phenomena, we proposed the correction coefficient with a₂/a₁, separation distances and the depth of the soil model as parameters, so that the coherence of the ground motion simulation could be evaluated appropriately.