Seismic waves
Vibrations that travel through the Earth from earthquakes and explosions; seismographs record them and reveal where they came from.
When rock suddenly breaks along a fault, or a large explosion goes off, the ground vibrates and the vibrations spread outwards as seismic waves. Earthquakes happen because stress builds up slowly as plates move, bending the rock on either side of a locked fault like a stiff spring. When the fault finally slips, the rock snaps back within seconds, a process called elastic rebound, and the land can be permanently raised, lowered or shifted.
The fastest waves, primary or P-waves, squeeze and stretch the rock along their direction of travel, rather like sound. In the crust they usually travel at up to about 6 kilometres a second, and they arrive first. Secondary or S-waves shake the rock from side to side at a little more than half that speed. Surface waves, which travel along the ground, come later and often do the most damage.
A seismometer records the difference between a heavy mass that tends to stay still and the ground moving beneath it. Because P-waves outrun S-waves, the gap between their arrivals grows with distance, a little like the gap between lightning and thunder. Measure the gap at three or more stations, draw a circle of the matching distance around each, and the circles meet at the earthquake's epicentre.
Seismographs pick up more than earthquakes. Mine explosions and nuclear tests show up clearly, which is how distant blasts are detected and located.
