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.

  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


Materials Teaching Educational Resources
1999 MATTER Project, The University of Liverpool

ESF flagSupported by the European Social Fund under
the Objective One programme for Merseyside