| Summary: | 碩士 === 國立中央大學 === 土木工程研究所 === 90 === Abstract
In this study, a three-dimensional finite element method is employed to simulate the response of site induced by microtremor in time domain. We investigate the suitability of Nakamura’s method in evaluating the fundamental frequency of soil stratum.
Cases investigated are the irregular ground and a two-layer model. The irregular ground includes a hunch rock in bottom bedrock and rock mass. Two types of the two-layer model are discussed：one is that the upper layer is softer than the lower layer, while the situation is reversed for the second type.
Based on the results obtained, it is found that if the soil stratum meets the requirement for the one-dimensional analysis, the Nakamura method can give almost the same fundamental frequency as the one-dimensional analytical solution. For a hunch rock in bottom bedrock, we would realize a blind spot that many researchers used one-dimensional analytical solution to prove Nakamura’s result. When rock mass locates between source and measured site, the size of rock mass would influence Nakamura’s method accuracy. For two-layer model, the empirical formulas given in the code behave well only in low impedance contrast； the Nakamura method ,however, gives good estimation for all the impedance contrast considered in this study but a caution has to be taken in the low frequency range where the spectral ratio plot shows peak which may lead to wrong judgment. In the second model, Nakamura’s method is only suitable in low-impedance contrast.
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