Seismologists locate earthquakes by measuring the difference between the arrival of the P and S waves in a seismogram.
It takes at least three seismograms to locate exactly where an earthquake occurred. One seismogram can only tell how far away the earthquake occurred from that station. The earthquake could be located anywhere on a circle of a radius equal to this distance and centered on the observation station. By measuring the S-P times at three or more stations, these circles can be drawn around each station and where they meet indicates the epicenter of the earthquake.
The picture below shows how these circles are plotted.
Now let's do an exercise and see if we can locate the epicenter of an earthquake. Click HERE to begin.