Chronology: The Beginnings of the Seismological Age

+ 1883: The English seismologist John Milne hypothesized that with the proper equipment, it should be possible to detect seismic waves from a large earthquake occurring anywhere on Earth.

+ 1889: Milne's 1883 hypothesis was proven correct when E. von Rebeur Paschwitz used delicate pendulum seismographs to record the April 18, 1889, Tokyo earthquake in Potsdam and Wilhemshaven, Germany.

+1897 Richard Dixon Oldham noticed that seismograms from earthquakes consistently showed three different disturbances, the first and second "preliminary tremors" (now known as P waves and S waves, respectively) and the "large waves" that followed the preliminary tremors, and that the difference in arrival time between the "large waves" and the "preliminary tremors" increased in a predictable fashion with increasing distance from the earthquake.

+ 1900: Oldham established that the "preliminary tremors" (P and S waves) have travel paths that take them through the body of the Earth (we now call them body waves), and that the "large waves" (now called surface waves) travel along Earth's surface.

+ 1906: Oldham used evidence from earthquake waves to demonstrate the existence of a large central core at a depth of about 3,821 km beneath the surface.

+ 1909: Andrija Mohorovicic, a Croatian seismologist, used seismic waves to discover a discontinuity at a depth of about 50 km beneath the surface. This marks the boundary between what we now call the Earth's crust and the underlying mantle. In his honor, we call the boundary separating the crust from the mantle the Mohorovicic discontinuity, or the Moho for short.

+ 1914: Beno Gutenberg used an extensive data set of earthquake wave travel times to compute the average distance to the top of the core at about 2,900 km.

+ 1926: Harold Jeffreys' measurements of tides in the solid Earth suggested that the Earth was less rigid than had been previously assumed. This led to the assumption that the core is fluid.

+ 1936: Inge Lehmann, Danish seismologist, demonstrated the presence of an inner core.