A simple procedure is proposed for estimating the strong-motion duration so and the rms strong-motion acceleration tσo of earthquake ground motion records. The strong-motion duration is found to be nearly proportional to the quantity Io/a2max where amax is the maximum ground acceleration, and Io is the integral over time of the squared accelerations. A less important factor influencing the relationship between so and Io/a2max is the predominant period To of the strong phase of the accelerogram. The following simplified expression is also derived for the strong-motion duration: so = 7.5 Io/a2max.
Duration-related statistics are presented for a set of 140 horizontal components of earthquake ground motion recorded in California. The dependence of duration upon magnitude, epicentral distance, and local condition is examined. The proposed measure of duration provides an important link between the major spectral representations of earthquake ground motion: the Fourier amplitude of acceleration, the acceleration spectral density function, and the response spectrum.