A site specific estimation of Fourier amplitude spectra - an alternative to standard response spectra

The paper discusses various shortfalls of the site independent technique. In particular, a review of peak acceleration as a measure of strong ground motion over the entire frequency band and amplitude range of engineering interest is presented. Further, the use of broad response spectra in areas of low to intermediate seismicity, subject only to local earthquakes, is questioned. A method is outlined for determining site dependent Fourier amplitude spectra for use as input data to soil-structure interaction calculations in the frequency domain. The method is based on a direct multiplication of a crystalline basement acceleration density spectrum (Fourier spectrum) with the transfer function applicable to the particular site. The crystalline basement acceleration density spectrum is obtained from simple seismic source models. The far field radiation emanating from such models is characterised by a long period level proportional to the seismic moment, a corner frequency, proportional to source area and a high frequency spectral decay. The far field, high frequency shear radiation for the γ=2 model in the presence of anelastic attenuation may be considered as band limited finite duration white noise in acceleration. Such an interpretation leads to a flat acceleration density crystalline basement spectrum. The synthetic spectra are tested against those from the 1978 Swabian Jura earthquake series in Germany. (orig./HP)