An earthquake releases energy that travels through the Earth as seismic waves. These are detected by seismometers at stations around the world.P waves (primary waves) are longitudinal. They are the fastest seismic waves, so they arrive first, and they can travel through solids and liquids.S waves (secondary waves) are transverse. They are slower than P waves and cannot travel through liquids, because a liquid cannot support the shearing motion a transverse wave needs. S waves are not detected on the far side of the Earth from an earthquake. This S-wave shadow zone is evidence that the Earth's outer core is liquid.
Surface waves travel along the surface. They are the slowest but have large amplitudes, so they cause most of the damage.Because P and S waves travel at different speeds, the time gap between their arrivals tells you how far away the earthquake was. For a station a distance d from the epicentre:Δt = d/vS − d/vP Worked example: vP = 8.0 km s-1, vS = 4.0 km s-1 and the S wave arrives 50 s after the P wave. Then d(1/4.0 − 1/8.0) = 50, so d/8.0 = 50 and d = 400 km.One station only gives a distance. Distances from three stations, drawn as circles on a map, cross at one point: the epicentre.
Surface waves travel along the surface. They are the slowest but have large amplitudes, so they cause most of the damage.Because P and S waves travel at different speeds, the time gap between their arrivals tells you how far away the earthquake was. For a station a distance d from the epicentre: