Seismometers are devices that chart the arrival times of the seismic waves from the earthquake.  The first initial wave on the seismometer is called the primary (P) wave.  The second wave is called the secondary (S) wave.  Seismologists measure the interval time of S-P to find the distance from the seismometer to the epicenter.  Using three different seismometers, specialists can use the three different distances to find the epicenter using triangulation. 

Triangulation uses the three seismometers as centers of three different circles.  Each circle has a radius that matches the distance measurement from the seismometer to the epicenter.  By drawing three circles, each around a seismometer, the point where the circles intersect is the epicenter of the earthquake.

You can try to find the epicenter of an earthquake using the interactive website at sciencecourseware.org. The following illustration is taken from this website.  It shows the intersection of three circles at the epicenter, just south of San Francisco.

Image reproduced with permission of Geology Labs Online