lished geologic basis, for example, and they would remain mysterious until the plate-tectonic revolution of the 1960s (19).
In 1883, the English mining engineer John Milne suggested that “it is not unlikely that every large earthquake might with proper appliances be recorded at any point of the globe.” His vision was fulfilled six years later when Ernst von Rebeur-Paschwitz recorded seismic waves on delicate horizontal pendulums at Potsdam and Wilhemshaven in Germany from the April 17, 1889, earthquake in Tokyo, Japan. By the turn of the century, the British Association for the Advancement of Science was sponsoring a global network of more than 40 stations, most equipped with instruments of Milne’s design (21); other deployments followed, expanding the coverage and density of seismographic recordings (22). Working with records of the great Assam earthquake of June 12, 1897, Oldham identified three basic wave types: the small primary (P or compressional) and secondary (S or shear) waves that traveled through the body of the Earth and the “large” (L) waves that propagated across its outer surface (23).
Milne investigated the velocities of the P, S, and L waves by plotting their travel times as a function of distance for earthquakes whose location had been fixed by local observations. From curves fit to these travel times, he could then determine the distance from the observing stations to an event with an unknown epicenter, and he could fix its location from the