An epicenter is a point on the Earth's crust directly above the focus or hypocenter of an earthquake. People often mistakenly refer to the epicenter as the point of origin of an earthquake, but in fact this is not the case. The earthquake's origin lies below the surface, with the epicenter being the point on the surface directly above the origin. Earthquake damage tends to be most intense at the epicenter, although sometimes an earthquake behaves unexpectedly and the damage is heaviest elsewhere.
The hypocenter, also known as the focus, is the place inside the Earth's crust where a rupture occurs as a result of geologic stresses. The movement of a fault at the hypocenter causes a tremendous release of energy which spreads through the Earth, and can vary in magnitude. As one might imagine, the site on the surface directly above the rupture can experience significant shaking as a result of the release of energy. Finding the epicenter is important for geologists because it will help them locate the hypocenter, and they can use that information to learn more about that particular earthquake as well as earthquakes in general.
To locate the epicenter, scientists need readings from at least three seismographs in the region. They use the data from each seismograph to determine how far away it was from the epicenter when the earthquake occurred, and this data is used to triangulate to find the site on the Earth above the hypocenter. Computer programs are available to do this today, although historically it could be done with a compass and a map, by drawing circles around the location of each seismograph and looking for the point where the circles intersected.
When information about an earthquake is released, the data usually includes the site of the epicenter. Earthquake maps, updated on a regular basis in geologically active regions, show all of the earthquakes which have occurred within a set period, and point to the location of each epicenter for the convenience of people consulting the maps. Patterns on an earthquake map can also reveal trends which may be important, such as increased activity along a particular fault.
Knowing the location of the epicenter can also be important for disaster relief efforts, as it tells people where they should concentrate their energies. It can also be valuable when trying to make predictions about tsunamis and aftershocks, both of which can follow an earthquake and endanger relief workers and citizens.