Seismometers are devices that measure the size and location of earthquakes by detecting vibrations in the Earth due to seismic waves, pulses of energy produced by the Earthquake.
The time delay between different types of seismic wave - primary and secondary waves - allows the location of the earthquake to be calculated.
Primary (P) waves travel faster than secondary (S) waves, so they are the first waves detected by seismometers. Scientists use the delay between P waves and S waves to calculate how far the earthquake is from that seismograph.
P waves can move through any part of the Earth's interior but S waves can only move through the solid parts.
One seismograph wouldn’t give us an accurate location as you can only say that the earthquake is a certain distance away, but not in which direction.
To locate the earthquake, you need at least three points. Each circle shows the distance of the earthquake from the seismograph at its centre. Can you work out where the earthquake has taken place in the diagram?
Answer: the point where the three circles intersect is the epicentre of the earthquake. This method of working out the location of a pulse of energy is known as triangulation.
(Left): primary, or P waves, travel the fastest and can move through solids and liquids. This means they can travel through any part of the Earth’s interior including the liquid outer core. They cause relatively little damage as they only stretch and compress the ground like an accordion or spring.
(Right): secondary, or S waves, are slightly slower and can only move through solids.They make the ground move up and down or side to side so they cause a lot more damage.