In order to qualitatively study the Fourier spectral characteristics of strong-motion earthquake records on bedrock, the Fourier spectra of 17 accelerograms (4.3 ≦ M ≦ 7.1), which were recorded in Japan, are analyzed. The shear-wave velocities (Vs) for these records of the sites are as follows: site A1, Vs = 1.3 km/sec and site B1, Vs = 1.6 km/sec. The acceleration spectra on bedrock are estimated by statistically analyzing the above 17 accelerograms. Method of least-square is used to obtain the amplitude of the Fourier spectrum (amplitude = a + b · T) between period 0.1 sec and Tm sec (where Tm = 100.39M−1.7 sec). The distribution of coefficient {b} is investigated. As a result of this investigation, the confidence interval for the population mean of {b} is established.

The result of this investigation suggests that the averaged Fourier amplitude spectra on bedrock can be considered to flat statistically during the above period range. This quantitative result agrees well with the characteristics of theoretical spectra calculated by propagating fault model. The period Tm and the corner frequency (Savage, 1972) also agree well for magnitude ≧6.0.

Kanai (1957), in his study on the characteristics of spectra on bedrock, concluded that the velocity spectrum seemed to be flat. However, he had computed the velocity spectrum from the response of a single degree of freedom system. His conclusions agree quite well with the results of this study.

From this paper, it can be concluded that the shape of the averaged spectra on bedrock should be flat during periods 0.1 sec and Tm sec [for magnitude < 6.0, the corner frequency (Savage, 1972) should be replaced with Tm]. In other words, the results indicate that the averaged Fourier spectra of accelerograms observed at a given site should be similar to the transfer function of that site. Hence, the site dependent spectra proposed by many investigators (Housner, 1952; Newmark et al., 1973; Seed et al., 1974; Kiremidjian and Shah, 1978) represent the transfer function of the site.

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