Shadow zones...

The figure below shows the structure of the Earth. P and S waves travel very differently through the Earth. Initially P and S waves travel in all directions from the epicentre of an earthquake outwards. They are refracted as they travel from the epicentre and follow arcs. However, S waves cannot travel through the liquid outer core of the Earth.

Notice:
  1. the large shadow zone for the S waves on the opposite side of the earth from the epicentre.
  2. the two smaller shadow zones for P waves.

Note that there is a considerable change in density from the solid mantle to the liquid outer core.

Consider a P wave at the point A where it just enters the liquid outer core at a very small angle.
This wave experiences a large increase in refraction and undergoes a significant change in direction at this interface.
Again when this wave leaves the outer core back into the mantle there is a change in direction.
This wave gives the extent of the P wave shadow zone.

By finding the angles at which the P and S waves both disappear we can calculate the radius of the liquid core of the earth.

 Shadow zone

 

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