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SEEING INTO
THE EARTH
RICHARD DIXON OLDHAM (1858–1936)
T
he shaking caused by
earthquakes spreads out
in the form of seismic
waves, which we can detect using
seismographs. While working for
the Geological Survey of India
between 1879 and 1903, Richard
Dixon Oldham wrote a survey of an
earthquake that struck Assam in- In it he made his greatest
contribution to plate tectonic
theory. Oldham noted that the
quake had three phases of motion,
which he took to represent three
different types of wave. Two of
these were “body” waves, which
traveled through Earth. The third
type was a wave that traveled
around the surface of Earth.
Wave effects
The body waves Oldham identified
are today known as P waves and
S waves (primary and secondary—
the order in which they arrive at
a seismograph). P waves are
longitudinal waves; as the wave
passes, rocks are moved backward
and forward in the same direction
as the waves are traveling. S waves
are transverse waves (like the
waves on the surface of water); the
rocks are moved sideways to the
direction of the wave. P wavesIN CONTEXT
BRANCH
GeologyBEFORE
1798 Henry Cavendish
publishes his calculations
of the density of Earth. The
value is greater than the
density of the surface rocks,
showing that Earth must
contain denser materials.1880 British geologist John
Milne invents the modern
seismograph.1887 Britain’s Royal Society
funds 20 earthquake
observatories worldwide.AFTER
1909 Croatian seismologist
Andrija Mohorovicic identifies
the seismic boundary between
Earth’s crust and the mantle.1926 Harold Jeffreys claims
that the core of Earth is liquid.1936 Inge Lehmann argues
that Earth has a solid inner
core and a molten outer core.There are different types
of seismic wave....therefore rocks
inside Earth must be
deflecting the paths
of the waves.P waves are not
detected at certain distances
from an earthquake...Earth’s core has
properties that are
different from those
in Earth’s upper layers.