Hypocenter

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The hypocenter or hypocentre (literally: 'below the center' from the Greek υπόκεντρον), may refer 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.

[edit] Earthquakes

In connection with an earthquake, strictly speaking, the term hypocenter is redundant. It is the same as the earthquake's centre or focus. That is the location inside the Earth's crust where the earthquake originates; however, the term is used to mean the point at which the earthquake originated. It is directly below the epicenter at a depth known as the focal depth. It is the position where the energy stored in the strain in the rock is released.

The location of an earthquake's hypocenter 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 under consideration so the depth of the source of these long-wavelength (low frequency) waves is difficult to determine exactly.

This is important, because very strong earthquakes radiate a large fraction of their released energy in seismic waves with very long wavelengths. A stronger earthquake involves the release of energy from a larger mass of rock. As with the larger mass of air in a larger organ pipe, it produces a lower 'note'.

[edit] Air burst explosions

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.