Abstract
Power spectral density function upon a ground motion which describes the frequency content of the corresponding motion is considered to be one of the most significant properties to be taken into account when one generates ground motions by use of stochastic processes. Using a smoothed and normalized Fourier amplitude spectrum, frequency contents for components of motion along a set of principal axes are determined. In the diagrams representing a Fourier amplitude spectrum with moving-window technique, time-depencency of frequency contents can be observed for motions produced by the San Fernando earthquake of February 9, 1971. A mathematical model to simulate ground motion processes is introduced hereby which can produce nonstationarity both in intensity and in frequency content. Using the mathematical model and reflecting the results of principal transformation and frequency content of motions along principal axes obtained by analysis of real motions recorded during the San Fernando earthquake, three-dimensional ground motion processes are synthetically generated. The resulting properties of the simulated motions yield general characteristics of prescribed properties while they reveal complexities similar to the characteristics observed in real accelerograms. Therefore, the model presented herein is considered to be adequate.
