Scientists use triangulation to find the epicenter of an earthquake. When seismic data is collected from at least three different locations, it can be used to determine the epicenter by where it intersects. Every earthquake is recorded on numerous seismographs located in different directions. Each seismograph records the times when the first (P waves) and second (S waves) seismic waves arrive. From that information, scientists can determine how fast the waves are traveling. Knowing this helps them calculate the distance from the epicenter to each seismograph.
To determine the direction each wave traveled, scientists draw circles around the seismograph locations. The radius of each circle equals the known distance to the epicenter. Where these three circles intersect is the epicenter.
This post is part of Exploring Earthquakes, a rich collection of resources co-presented by the California Academy of Sciences and KQED. This material is also available as a free iBooks textbook and iTunes U course.
Don’t miss Earthquake, an interactive exhibit at the Academy exploring the seismic forces that impact us today and featuring the Shake House, an earthquake simulator.