Study on effects of ground surface conditions like irregular shaped ground, trees and buildings on earthquake ground motions

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The purpose of this study is to clarify the effects of the conditions of the ground surface on the earthquake ground motions. Irregular shaped ground, lots of trees and buildings, which are existing ordinally at the ground surface, were considered in this study. Even if it has been pointed out previously that the ground with an irregular surface may cause complicated seismic responses, the process of generating this complex response has not yet been clarified. In this study, such a process was also examined.Regarding the effects of the irregular ground surface, for example, the seismic responses of ground models with sinusoidal-shaped irregularity, i.e., with various heights and numbers of sinusoidal-shapes, were analyzed. The boundary element method was developed in SH wavefield for numerical analyses, and it was utilized to estimate the frequency response functions at the ground surface. Also, scattered-wave contribution was newly introduced to examine the frequency response functions. The examination of the numerical results revealed the followings :1)the frequency response at the trough varies sharply with frequency, whereas the response at the peak shows large amplification over a wide frequency range ;2)the predominant frequencies at the trough depend on the amplitude of the surface irregularity and do not depend on the number of fluctuations of the ground ; and3)these characteristics can be understood as the results of the interference of scattered waves coming from the irregular surface.