Why is there a minimum number of stations necessary to locate an epicenter?

Finding the epicenter of an earthquake requires coordination between at least three seismographs separated by, ideally, hundreds of miles. This is because a seismograph is only capable of registering the strength and amplitude of an earthquake, which together give the distance to the epicenter, not its direction.

A single seismograph can register the occurrence of an earthquake. Analysis of the waves can yield the distance between the sensor and the epicenter. This describes a circle with the seismograph at the center. A second seismograph, which can be hundreds or thousands of miles away, can generate a similar plot. The circles from these two stations cross each other at two points. Adding data from a third station pinpoints the epicenter as the only place where all three circles intersect.