PURPOSE:To inexpensively and highly accurately evaluate the earthquake vibration- amplifying characteristic of a surface layer in a short time by setting a vibrometer on the ground and finding the frequency response function of the surface layer based on the micromotion-observed data obtained front the vibrometer. CONSTITUTION:After a vibrometer 11 is set on the ground, the horizontal and vertical components of micromotions propagated through a surface layer 12 are observed and spectra are found at every component. Then an approximate value of the frequency response function of the layer 12 is obtained based on the average spectrum ratio between the horizontal and vertical components. Thereafter, the profile of the layer 12 is set from already known data and, at the same time, the frequency response function of the layer 12 is decided by presuming the dynamic physical property values of the layer 12 and comparing the frequency response function with the approximate value of the function calculated from the micromotion data. When the frequency response function is decided, the seismic response of the layer 12 can be analyzed and, when such data as incident waves, etc., are inputted, the surface and underground earthquake vibration amplifying characteristic of the layer 12 can be calculated.
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