When an earthquake occurs there are seismograms taken from a number of locations, all at the same time. Each seismogram looks different, depending on the location of the epicenter of the earthquake and how close it was to the seismograph. All of the data from the seismographs is collected, and by having at least three different seismograms, you can locate the epicenter of the earthquake.

The United States Geological Survey monitors earth movements and activity. Check out how geologists use maps, pictures, and seismograms to keep track of earthquakes. U.S. Geological Survey

Watch a map of California during an earthquake to discover how scientists pinpoint the exact location of the epicenter. Triangulating the Location