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.