S and P Waves
A knowledge of how seismic waves travel through the Earth provides us with evidence of the Earth’s structure. Remember that:
p-waves, also called pressure waves, travel through solids and liquids, so they can travel through all of the Earth’s layers
s-waves, which are transverse, cannot travel through liquid rock, so they cannot travel through the outer core
The speed of p-waves and s-waves increases as they travel deeper into the mantle, since speed tends to increase with density. They travel through the Earth in curved paths, but they change direction suddenly when they pass through the boundary between substances in different states. The diagrams show what happens when p-waves and s-waves pass through the Earth:
By analysing the points on the Earth's surce at which s waves can be recorded after an earthquake, the size of the Earth's core can be deduced, and analysus of p waves can enable us to find the density of the Earth's core.