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The great majority of earthquakes are caused by the movement of faults. Two terms are used to determine the location of an earthquake, focus and epicenter. The focus of an earthquake is the point along a fault where the rocks slipped and released the energy previously stored during the elastic phase of deformation. Because faults represent brittle deformation, the highest frequency of earthquakes occurs at Earth's surface and decreases with depth as rocks become less brittle and more plastic. The deepest earthquake foci occur at depths of about 640 km. ( 400 mi.) which is the deepest penetration of subducting oceanic plates. The epicenter of an earthquake is the point on the surface immediately above the focus and is the point at which the surface waves originate. Being the point on the surface closest to the origin of the energy, the epicenter of an earthquake generally experiences the maximum amount of damage. Note that while earthquake foci can occur anywhere from Earth's surface to a depth of 640 km., the epicenter of an earthquake is always at Earth's surface.