Following the first steps at Caltech made by von Kármán and Biot in the early 1930s, the dynamic response of structures to earthquake shaking remained in the academic sphere of research for many years and did not gain widespread engineering acceptance until the early 1970s. There were two main reasons for this. First, computation of the response to earthquake ground motion, without digital computers, led to formidable numerical difficulties; second, there were only a few well-recorded accelerograms that could be used for that purpose. This started to change in the 1960s with the arrival of digital computers and the commercial availability of strong-motion accelerographs. By the late 1960s and early 1970s, however, the digitization of analog accelerograph records, organized by Hudson and his graduate students at Caltech, and the digital computation of ground motion and of the response spectra were developed completely. Then in 1971 with the occurrence of the earthquake in San Fernando, California, which was recorded by 241 accelerographs, the modern era of earthquake engineering was launched.
Don was among the first to recognize the significance of the availability of a comprehensive and accurate database for future developments of earthquake engineering. With T. Caughey, his former student and later a faculty colleague at Caltech, Don invested considerable effort to develop a special-purpose analog computer (Mark II) for computation of the response spectra from recorded strong-motion accelerograms, but the process was time consuming and the results were not accurate. In retrospect, it is clear and logical that in the mid-1960s Hudson decided to gather all important records of strong ground motion and to organize digitization, processing, and dissemination of digital strong-motion data, a conditio sine qua non for all subsequent developments in modern earthquake engineering. Von Kármán and Biot formulated the response spectrum method in earthquake engineering, but it was Don Hudson who made modern analyses possible by gathering and processing the data, thus providing a sound and realistic experimental basis for the theory.