Focus, Hypocenter and Epicenter of an Earthquake

The terms Epicenter, Hypocenter and Focus are used a lot to describe earthquakes and earthquake related events and can sometimes lead to confusion. The two terms Focus and Epicenter actually carry two different meanings.
The focus of an earthquake is the point where the fault begins to rupture. It is where the strain energy stored in the rock is released and is often located underground. Earthquakes are classified according to their focal depths as, shallow-focus earthquakes, mid-focus earthquakes and deep-focus earthquakes.
The term hypocenter means the same as focus, although hypocenter is also used to refer to a nuclear explosion site (ground zero)
The epicenter is the the point on the earth's surface directly above the focus. Thus the epicenter represents a geographic location on the earth's surface. This means that the focus lies at a focal depth below the epicenter.

source : http://earthquakesandplates.files.wordpress.com/2008/05/epicenter.gif

source : http://en.wikipedia.org/wiki/File:Epicenter.png

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