A third type of seismic wave is generated by the interaction of the P- and S-waves with the surface and internal layers of the Earth.
The speed of the waves depends on wave type and the properties of the rock; the denser the rock, the faster the waves travel. In the Earth’s crust P-waves travel at around 6–7 km per second, while S-waves travel at around 3.5–4 km per second.
P-waves travel fastest. They consist of successive contractions and expansions, just like sound waves in air.
The motion of the particles in the rocks that the waves travel through is parallel to the direction of the wave.
S-waves are slower than P-waves. They are transverse waves, which means that the particle motion is at right angles to the direction of travel. S-waves cannot travel through air or liquids.