Shadow zone
A shadow zone is an area in which an S-Wave (secondary seismic wave) is not detected due to it not being able to pass through the outer core of the earth due to it being liquid. When an earthquake occurs, seismographs near the epicenter, out to about 90° distance, are able to record both Primary and Secondary waves, but those at a greater distance no longer detect the S-wave. This is due to the fact that shear waves cannot pass through liquids. This was how Oldham proved that the Earth had a liquid outer core. The Moon has been proven by seismic testing to have a solid core, because it conducts shear waves.
The S wave shadow zone occurs between 105 degrees and 180 degrees latitude (with the point of origin as zero and due south of origin as 180 degrees).
P-Waves (primary seismic waves) also have a shadow zone as they are refracted downwards on entering the Earth's core as the liquid outer core lowers their speed. This shadow zone occurs between 104 and 140 degrees.
See also
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