The epicenter was the central origin point of an earthquake.
In 2269, the USS Enterprise changed the epicenter of an quake on Argo to the planet's north polar seas by targeting unpopulated areas with phaser fire. This was done so that the Aquans wouldn't be affected by it. (TAS: "The Ambergris Element")
In 2368, two earthquakes on Penthara IV had their epicenters beneath the two drill sites furthest to the south. (TNG: "A Matter Of Time")