The hypocenter or hypocentre (literally: 'below the center' from the Greek υπόκεντρον), refers to the site of an earthquake or to that of a nuclear explosion. In the former, it is a synonym of the focus; in the latter, of ground zero.
The location of an earthquake's hypocenter is the position where the strain energy stored in the rock is first released, and is the point where the fault begins to rupture. This occurs at the focal depth below the epicentre.
The focal depth can be calculated from measurements based on seismic wave phenomena. As with all wave phenomena in physics, there is an uncertainty in such measurements that grows with the wavelength so the focal depth of the source of these long-wavelength (low frequency) waves is difficult to determine exactly. Very strong earthquakes radiate a large fraction of their released energy in seismic waves with very long wavelengths and therefore a stronger earthquake involves the release of energy from a larger mass of rock.
The term hypocenter also refers to the point on the surface of the earth directly below an explosion above the ground, in the atmosphere. In principle, it applies to any such explosion but the term was not found to be necessary until the very large explosions of nuclear bombs became a reality. In this context, the term 'ground zero' was synonymous with hypocenter, though the ground zero term has been rendered less precisely useful, as it has been used by journalists and others ever more loosely.
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