M A S T E R P A G E
Chronology:
The Beginnings of the Seismological Age
3.2b
Q
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.
Q
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.
Q
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.
Q
1900: Oldham established that the “preliminary tremors” (P and S waves) have
travel paths that take them through the body of Earth (we now call them body
waves), and that the “large waves” (now called surface waves) travel along
Earth’s surface.
Q
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.
Q
1909: Andrija Mehorovicic, 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.
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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.
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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.
Q
1936: Inge Lehmann, Danish seismologist, demonstrated the presence of an
inner core.
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