Statistical Properties of Earthquake Accelerograms and Equivalent Earthquake Excitation Pattern

Abstract

In order to analyze the earthquake responses of structures, it is first very important to predict reasonably an earthquake excitation pattern referring to the model of a structural system and the measures of aseismic safety. It is not possible, however, to know the excitation properties of future earthquakes so assuming that future earthquakes will have approximately the same properties as past earthquakes, it is necessary to analyze the properties of the accelerograms of past earthquakes. Most of such accelerograms are likely to be random time functions. And their statistical quantities are, in details, different from each other depending upon the individual past earthquake, the recording place of accelerograms and other conditions. In this paper, the auto-correlation functions and amplitude probability density distribution functions are estimated for a number of past strong earthquake accelerograms. It is also shown that an earthquake accelerogram has a spectral density with a few peaks and non-Gaussian probability density distribution. On the other hand, the statistical model for the equivalent earthquake excitation pattern is presented by considering the common properties based on the above statistical results. Although it is difficult for the earthquake response analyses to give the dimensional quantities-intensity, frequency characteristics and so on-of earthquake excitations relating to individual soil conditions, the dimensionless parameters which define the statistical model of the equivalent earthquake excitation pattern can be determined in the case of the assumption that the influence of the ground-structure coupling being small. Then it is found that the spectral density of a simplified equivalent earthquake excitation pattern may consist of the band limited white noise spectrum and the delta functions corresponding to a noise component and periodic components. It should be noted that this paper was presented to the First Chilean Sessions on Seismology and Earthquake Engineering held in July, 1963.