The seismic energy released by an earthquake courses through the Earth in the form of waves that cause the tremors experienced as earthquakes at the surface. These waves can be measured by seismometers (see the question "What is a seismometer and how does it work?").
Seismic waves can be divided into two main types: body waves (P and S waves) and surface waves (Love and Rayleigh waves).
The hypocentre and epicentre of an earthquake can be calculated because these seismic waves are propagated at different velocities (see the questions "What is a hypocentre?", "What is an epicentre?" and "How is an earthquake's place of origin determined?").
P waves (primary waves) are propagated spherically from the hypocentre in the form of body waves (see the question "What is a hypocentre?") and spread by alternatively pushing (compressing) and pulling on the geological substrate.
The propagation velocity of P waves varies between roughly 6 and 8 km/s, depending on the rock strata through which they pass. Being the fastest seismic waves, P waves are the first to reach seismic stations.
S waves (secondary waves) also propagated spherically from the hypocentre in the form of body waves (see the question "What is a hypocentre?") and spread by moving the particles of rock through which they pass perpendicularly to their direction of travel, either horizontally (from side to side) or vertically (up and down).
S waves spread through any rock strata at a propagation velocity of roughly 3 to 4 km/s and therefore always reach seismic stations after P waves.
Love waves are surface waves propagated from the epicentre (see the question "What is an epicentre?"). They are produced when P and S waves reach the surface. Love waves move rock parallel to the surface of the Earth (horizontally, from side to side).
Love waves are slower than S waves but faster than Rayleigh waves.
Rayleigh waves are also surface waves propagated from the epicentre (see the question "What is an epicentre?") and are likewise produced when P and S waves reach the surface. Rayleigh waves roll with an elliptically rotating motion, rather like the ripples caused when a stone is thrown into water. The amplitudes of Rayleigh waves are often far greater than other seismic waves, which is why they tend to cause the strongest tremors and greatest damage (see the question "What do the terms 'period', 'wavelength' and 'amplitude' mean?“).