Large crustal blocks, separated by these faults, are held in place by friction. As the pressure from the core continues to increase over time, a certain threshold is reached in which these frictional forces are overcome resulting resulting in sudden shifting, slippage or re-adjustment along the fault lines. This movement is what we call an earthquake. The point of origin of the quake is referred to as the focus or hypocenter. The epicenter is an imaginary reference point located on the surface directly above the focus.
Seismologists use the epicenter for determining site locations of quakes for mapping purposes.