Time-dependent power spectral density of earthquake ground motion*

This paper investigates the stationarity of earthquake accelerograms and shows that the strong motion segments constitute a locally stationary random process. For such a process, a time-dependent power spectral density may be formulated as the product of: (a) a mean-square acceleration; (b) a time scale factor describing the variation of the local mean square acceleration; and (c) a normalized power spectral density describing the frequency structure of the motion. A segmental time averaging procedure is proposed for obtaining a time scale factor, an envelope function, and a normalized power spectral density from the strong motion segment of accelerograms. The procedure is used to compute smooth normalized power spectral densities from a total of 367 horizontal and vertical components of accelerograms recorded on alluvium, rock, and alluvium underlain by rock.