Illustration of a seismic P-wave, one of the two types of body wave produced by an earthquake. The P-wave, (primary or pressure wave) is a longitudinal wave that causes ground displacement (compression or dilation) in the same at direction as the wave's movement (red arrow). P-waves are analogous to sound waves in air. Secondary waves (S-waves) have displacement at right angles to the direction of movement. These waves radiate from the earthquake's focus (or hypocentre) and travel through the entire Earth. Their behaviour has allowed the structure of the Earth to be determined. As well as body waves, earthquakes produce surface waves, which travel just under the Earth's surface.