epicentre, point on the surface of the Earth that is directly above 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 to the time of travel of seismic waves from the focus to each station. The point of intersection of the arcs marks the epicentre. Equivalent algebraic analysis is conducted in computer programs.