1. Focus: The point on the fault where slip starts is the focus. It is also known as hypocentre
[Ref. Fig. 20.1].
2. Epicentre: The point vertically above the focus on the surface of the earth is the epicentre.
3. Focal Depth: The depth of focus from the epicentre is called the focal depth.
4. Epicentral Distance: Distance from epicentre to any point of interest on the surface of earth is called epicentral distance.