A hypocenter (or hypocentre; ) is the point of origin of an

earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that ...

or a subsurface nuclear explosion
Nuclear may refer to:
Physics
*Nuclear engineering
Nuclear engineering is the branch of engineering
Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including ...

. In seismology, it is a synonym of the focus. Outside seismology, the term ''hypocenter'' is also used as a synonym for ''ground zero
In relation to nuclear explosion
Nuclear may refer to:
Physics
*Nuclear engineering
Nuclear engineering is the branch of engineering
Engineering is the use of scientific method, scientific principles to design and build machines, str ...

'', the surface point directly beneath a nuclear airburst.
Earthquakes

An earthquake's hypocenter is the position where the strain energy stored in the rock is first released, marking the point where thefault
Fault commonly refers to:
*Fault (geology), planar rock fractures showing evidence of relative movement
*Fault (law), blameworthiness or responsibility
Fault(s) may also refer to:
Arts, entertainment, and media
* "Fault", a song by Taproot from ...

begins to rupture.''The hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the Earth. Also commonly termed the focus.'' This occurs directly beneath the epicenter
The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter, hypocenter or focus, the point where an earthquake or an underground explosion originates.
Surface damage
In most earthqua ...

, at a distance known as the focal or hypocentral depth.
The focal depth can be calculated from measurements based on seismic wave
Seismic waves are waves
The United States Naval Reserve (Women's Reserve), better known as the WAVES (for Women Accepted for Volunteer Emergency Service), was the women's branch of the United States Naval Reserve
The United States Navy ...

phenomena. As with all wave
In physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. " ...

phenomena in physics
Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succession of eve ...

, there is uncertainty in such measurements that grows with the wavelength
In physics, the wavelength is the spatial period of a periodic waveâ€”the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adjac ...

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.
Computing the hypocenters of foreshocks, main shock, and aftershocks of earthquakes allows the three-dimensional plotting of the fault along which movement is occurring. The expanding wavefront from the earthquake's rupture propagates at a speed of several kilometers per second, this seismic wave is what is measured at various surface points in order to geometrically determine an initial guess as to the hypocenter. The wave reaches each station based upon how far away it was from the hypocenter. A number of things need to be taken into account, most importantly variations in the waves speed based upon the materials that it is passing through. With adjustments for velocity changes, the initial estimate of the hypocenter is made, then a series of linear equations is set up, one for each station. The equations express the difference between the observed arrival times and those calculated from the initial estimated hypocenter. These equations are solved by the method of least squares
The method of least squares is a standard approach in regression analysis to approximate the solution of overdetermined systems (sets of equations in which there are more equations than unknowns) by minimizing the sum of the squares of the resid ...

which minimizes the sum of the squares of the differences between the observed and calculated arrival times, and a new estimated hypocenter is computed. The system iterates until the location is pinpointed within the margin of error for the velocity computations.
References

External links

* *{{Commons category-inline, Hypocenters Seismology Geometric centers