By looking at the seismograms from different recording stations we can find out the epicentre of the earthquake.
The signals arrive first at the closest station and last at the one furthest away.
The time difference between the P- and S-waves tells us the distance the earthquake is from the seismograph.
By measuring this at three stations we can work out where the epicentre is.
We need measurements from at least three stations to find the epicentre.
The intersection of the circles whose radius is equivalent to the distance from the earthquake gives the epicentre.