What is the name for the source of an earthquake?
The source of the earthquake at depth is the focus and its projection at surface is the epicentre.
Earthquakes are generated by a sudden release of energy along deep fractures in the Earth's crust called "seismogenic faults".
The point where a mechanical rupture along the seismogenic fault happens is called focus or hypocentre. It can be several kilometers deep below the surface. It is the very source of the earthquake
The point located directly above the focus (its projection on the Earth's surface) is called epicentre. This is the area where the effects of the earthquake are the strongest.
Generally a shallower focus allows more energy to reach the surface thus the generated earthquake is more damaging.
Using the arrival time of the seismic waves at different detection stations it is possible to calculate the position of the epicentre and the depth of the focus through some trigonometric calculation.