Influence of local soil and geologic site conditions on fourier spectrum amplitudes of recorded strong motion accelerations, 1987

The idea that the local soil and geological site conditions may influence the amplitudes of recorded seismic waves has been around for many years and has been investigated by many researchers. This has been studied theoretically (e.g. Haskell, 1960; Tsai, 1969; Trifunac, 1971) and experimentally (e.g. Kanai, 1949,'1957; Gutenberg, 1957; Duke, 1958; Medvedev, 1955) by considering various overall measures of strong shaking or its effects on different types of structures. Beginning in the 1960's and 1970's these studies took on more detailed and more complete nature because of the increased availability of recorded strong motion accelerograms. Through comparison of the shapes of the Fourier and response spectrum amplitudes, it became possible to describe the effects the local soil conditions have on the local site response (Seed et al., 1974) and to extend the results of Gutenberg (1957) about the effects of the local geologic conditions to the high frequency spectral amplitudes (Trifunac, 1976). Through the 1970's and early 1980' s these studies were further refined by detailed regression analyses which were made possible by still larger numbers of well documented records of strong ground motion (Trifunac, 1976,1979; Trifunac and Lee, 1987a,b,c).; However, to this date these studies considered either the local soil or the local geologic site conditions and never combined the simultaneous effects of both media in the development of one and more general scaling relation. Since the typical dimensions of the local soil versus the local geologic site conditions are so different one might expect that their effects would be reflected in the recorded spectral amplitudes in high and in low frequencies respectively. If both of these effects can be shown to contribute significantly to the variation of spectral amplitudes between 0.05 and 25 Hz, the frequency range of interest to earthquake engineering, then both soil and geologic site condition should be considered simultaneously. The purpose of this report is to investigate this and to find how these effects should be used in the empirical scaling of Fourier amplitude spectra.