The Science Book

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189
This model of
an earthquake
shows seismic
waves passing
through Earth
and the “shadow
zones” of the
primary (P) waves
and secondary
(S) waves.
See also: James Hutton 96–101 ■ Nevil Maskelyne 102–03 ■ Alfred Wegener 222–23
A CENTURY OF PROGRESS
travel faster than S waves, and
can travel through solids, liquids,
or gases. S waves can travel only
through solid materials.
Shadow zones
Later, Oldham studied seismograph
records for many earthquakes
around the world, and noticed that
there was a P-wave “shadow zone”
extending partway around Earth
from the earthquake location.
Hardly any P waves from an
earthquake were detected in this
zone. Oldham knew that the speed
at which seismic waves travel
inside Earth depends on the
density of the rocks. He concluded
that properties of the rocks change
with depth, and the resulting
changes in speed cause refraction
(the waves followed curved paths).
The shadow zone is therefore
caused by a sudden change in
the properties of rocks deep
within Earth.
Today, we know that there is
a much larger shadow zone for
S waves, which extends across
most of the hemisphere opposite
the focus of the earthquake. This
indicates an Earth interior that has
very different properties than those
of the mantle. In 1926, American
geophysicist Harold Jeffreys used
this evidence from S waves to
suggest that Earth’s core is liquid,
since S waves cannot pass through
liquids. The P-wave shadow zone is
not completely “shadowed,” since
some P waves are detected there.
In 1936, Danish seismologist Inge
Lehmann interpreted these
P waves as reflections from an
inner, solid core. This is the model
of Earth we use today: a solid inner
core surrounded by liquid, then the
mantle with crustal rocks on top. ■
Richard Dixon Oldham
Born in Dublin in 1858, the
son of the superintendent of
the Geological Survey of India
(GSI), Richard Dixon Oldham
studied at the Royal School of
Mines, before joining the GSI
himself and became
superintendent as well.
The GSI’s main work
involved mapping the rock
strata, but it also compiled
detailed reports on earthquakes
in India, and it is for this aspect
of his work that Oldham is best
known. He retired on health
grounds in 1903 and returned to
the United Kingdom, publishing
his ideas about Earth’s core in

1906. He was awarded the Lyell
      Medal by the Geological Society
      of London, and was made a
      Fellow of the Royal Society.

Key works

1899 Report of the Great

Earthquake of 12th June 1897

1900 On the Propagation

of Earthquake Motion to

Great Distances

1906 The Constitution of the

Interior of the Earth

The seismograph,

recording the unfelt motion

of distant earthquakes,

enables us to see into

the earth and determine

its nature.

Richard Dixon Oldham

Focus of earthquake

S waves P waves

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Inner

core

Outer

core

Mantle