The effect of the variability in earthquake parameters on response of structure is sufficiently important to require careful consideration in design. A theoretical relation is presented between the seismological Fourier amplitude spectrum and the mean squared value of the elastic response which is defined by a Gaussian distribution. By shifting a general process to its mean value, spectrum of the mean squared value of the displacement is computed from the Fourier amplitude spectrum and the real part of the relative displacement transfer function of the single-degree-of-freedom elastic oscillator. For illustrating the proposed theoretical relation, the mean squared values of drift have been calculated for earthquake ground motions with different focal distance and soil condition. In this paper, the contribution of these sources of variability to the uncertainty in drift is examined using stochastic analysis, Monte Carlo simulation, to compare the effect these earthquake ground motion parameters. Analytical results show that the influence of earthquake magnitude, fo-cal distance and soil condition are significant.
