Which term is useful for describing the depth of an earthquake?

Correct answer: Focal depth

The governing concept is the relationship between an earthquake’s focus and its depth. Focal depth is the vertical distance from Earth’s surface to the focus, the underground point where rupture begins. Thus, option A correctly describes earthquake depth. The epicentre is the surface point directly above the focus, a seismic wave is energy travelling through Earth, and the Richter scale refers to an older method of expressing magnitude, not depth.