Intensity of ground motion during strong earthquakes

A measure of the surface intensity of the ground motion during an earthquake is defined which is proportional to the maximum stresses produced in structures. Intensities are computed for fourteen strong-motion shocks and these are compared with the Modified-Mercalli Intensities for the same earthquakes. A method is developed for computing the magnitude of an earthquake from the intensity and vice versa which permits the intensities to be calculated over the area affected by the earthquake. The maximum intensity and the maximum ground acceleration likely to be experienced by a California city are estimated. The frequency with which strong earthquakes are likely to occur in California is determined and an estimate is made of the frequency with which a California city is likely to experience severe ground. motion.