4. Seismic waves

Sound waves travel very easily underground, often for many thousands of kilometres. These are usually referred to as a kind of seismic wave and are most often triggered by earthquakes, which result from a sudden slip of tectonic plates, down to about 700 kilometres below the Earth’s surface. ‘Seismic waves’ describes the four types of seismic wave generated by earthquakes: P-waves (primary waves), S-waves (shear waves), Love waves (usually the most powerful and destructive of seismic waves), and Rayleigh waves, which are created when P and S waves reach the Earth’s surface together, combining to form undulating ground rolls. Free vibrations and star waves are also described.