focus

…the Earth, is called the focus, or hypocentre. The point at the surface immediately above the focus is known as the epicentre.

…the underground point (called the focus) where fault rupture commences, producing an earthquake. The effects of the earthquake may not be most severe in the vicinity of the epicentre. The epicentre can be located by computing arcs from each of three or more seismic observatories, with the arcs’ radii proportional…