The advent of analog computers in the 1940s provided the first simulations of structural vibrations induced by the recorded ground motions (166) and allowed the automation of strong-motion spectral analysis (167). These early calculations showed that the spectra of earthquake accelerations are similar to “white noise” over a limited range of frequencies, a pivotal observation in the study of earthquake source processes. However, the immediate implication for earthquake engineering was the lack of a “dominant ground period” that might be destructive to particular structures (168). Without a characteristic frequency, earthquake engineering was recognized to be complex, requiring a comprehensive analysis of coupled vibrations between earthquakes and structures. George Housner outlined the issues in 1947:
In engineering seismology, the response of structures to strong-motion earthquakes is of particular interest…. During an earthquake a structure is subjected to vibratory excitation by a ground motion which is to a high degree erratic and unpredictable…. Furthermore, the average structure, together with the ground upon which it stands, is an exceedingly complex system from the viewpoint of vibration theory. It is apparent the problem divides itself into two parts; first a determination of the characteristics of strong motion earthquakes, and second a determination of the characteristics of structures subjected to earthquakes.
Following an earlier suggestion by M.A. Biot, Housner put forward the concept of the response spectrum, the maximum response induced by ground motion in single degree-of-freedom oscillators (“buildings”) with different natural periods but the same degree of internal damping (usually selected to be 5 percent) (169) (Figure 2.17). At shorter periods the maximum induced acceleration exceeds the recorded ground acceleration, whereas for longer periods it is less. When multiplied by the effective mass of a building, the response spectrum acceleration constrains the lateral force that a building must sustain during an earthquake. Computing response spectra over a wide range of frequencies using data from a wide range of earthquakes significantly improved understanding of the damage potential of strong motion.
The availability of strong-motion data began to transform earthquake engineering from a practice based on pseudostatic force criteria to a science grounded in an understanding of the complex coupling between ground motions and building vibrations. By the 1950s, strong-motion records were combined with response spectral analysis to demonstrate that structures can amplify the free-field accelerations (recorded on open