Seismic body waves. Artwork of the two types of body wave produced by an earthquake. The Primary wave (P wave, top) is a longitudinal wave that causes ground displacement (blue arrows) in the same at direction as the wave's movement (yellow arrow). P waves are analogous to sound waves in air. Secondary waves (S waves, bottom) have displacement at right angles (blue arrows) 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. See image E360/020 for these waves.