How do geologists locate the epicenter of an earthquake?

Geologists locate the epicenter of an earthquake by taking measurements from three seismograms. These are measured at seismic stations and give the distance that the earthquake's waves traveled in order to reach the station. These three distances are then used to triangulate the epicenter.

The following shows the steps used:

1. Get three seismograms
The first step to locate the epicenter of an earthquake is to get three seismograms. They must come from seismic stations located in different places.
2. Measure the distance
For each of the seismograms it is necessary to measure the S - P time interval. "S" stands for secondary waves, and "P" stands for primary waves. Then, the S - P interval is used to measure the distance that the waves traveled to reach the seismic station. This will not indicate the direction it came from though, just the distance the waves traveled.
3. Triangulate
To get the epicenter, it is necessary to triangulate the results worked out in the previous step. This is done by drawing a circle around the location of the seismic station that has an equal radius between the station and the distance the waves traveled. This should be done with all three and it should produce an overlap. The epicenter of the earthquake is in the area represented by the overlap.