What is the primary difference between the focus and the epicentre of an earthquake?

The primary difference between the focus and the epicentre of an earthquake lies in their definitions and locations. The focus, also known as the hypocentre, is the actual point within the Earth where the earthquake originates. This point is located below the Earth’s surface, often found within the crust or upper mantle. On the other hand, the epicentre is the point on the Earth's surface that is directly above the focus. It is the location where seismic waves first reach the surface and is often reported in news about earthquakes.

In this context, the correct answer highlights that the focus is located within the Earth's crust or deeper, while the epicentre is situated on the surface. This distinction is vital for understanding how seismic waves travel and the impact they have on different locations relative to the earthquake's origin.