|The motion depicted above represents waves traveling through a solid material such as the Earth's interior. Imagine that the balls represent atoms and that the springs represent the chemical bonds that attach the atoms to one another. Though it is not apparent in this animation, P-waves always travel faster than do S-waves.
In fluid media (liquids and gases, for instance) transmission of these waves is more difficult because the particles are no longer attached to one another:
click to see animation