Many years ago when I was an undergraduate, the late Arthur F. Buddington of Princeton University pointed out to a group of us students in his petrology class that, at least once in every lifetime, such revolutionary new ideas are introduced into each field of science that well-established ideas must be abandoned and the whole view of the field thought out afresh. Such revolutions have affected seismology three times, each conveniently marked by a very large earthquake.

Previous to the Lisbon, Portugal earthquake of 1755, earthquakes were viewed largely as “acts of God” Imposed on mankind in retribution for misbehavior; afterward, they were studied more as natural phenomena, and knowledge of them grew gradually but steadily as a result of careful observations. This change in the way natural phenomena were viewed was not unique to seismology but had been developing in all aspects of science at least since the time of such men as Rene Descartes (1596–1650) in France and Gottfried Wilhelm Leibnitz (1646–1716) in Germany. For nearly a century and a half after 1755, understanding of earthquakes was limited to what could be learned by visual observation. This was due to lack of an adequate means of measuring ground motion. It was not until the development of sensitive seismographs toward the end of the nineteenth century that seismograms became good enough to recognize the various types of pulses that are propagated.