How do earthquakes start deep underground along a fault line?

Tectonic plates are always moving, but friction can lock sections of a fault together. As the plates continue to push or pull, elastic strain accumulates in the surrounding rock. When the stress exceeds the fault's strength, the rocks rupture and slip abruptly, often in a fraction of a second. That rupture point is the focus or hypocenter, and the point directly above it on the surface is the epicenter. The largest earthquakes, such as the 1960 Chile earthquake and the 2004 Sumatra-Andaman earthquake, release enormous amounts of energy over long fault segments.