Abstract: The earthquake ground motion is modeled as a twice filtered gaussian white noise random process with zero mean. The high frequency content of the white noise is reduced by the first filter;the low frequency content of the white noise is cut down by the second filter. On the basis of power spectra of earthquake records, the parameters of the power spectral density function of the twice filtered Gaussian white noise random process are determined by using the least-squares procedure of a nonlinear function.