To locate an earthquake, you need the data from at least three seismometer stations. The process is known as triangulation and is described in more detail below. The seismometer records the time when the P and S-waves arrive at the recording station. P-waves travel faster through the earth than S-waves and so they arrive at the seismometer station before the S-waves and are recorded by the seismometer first. The difference in arrival time between the two types of seismic wave can be used to calculate the distance of the earthquake's epicentre from the seismometer, as the further away an earthquake is, the greater the lag time between the detection of the S waves relative to the P waves (imagine two cars racing against each other. They both set off at the same time from the same place, but