Answer:
Earthquakes occur when tectonics plates shift and brush against each other, as they move a couple of a couple of inches a year. Whenever there is a sudden movement (such as a large amount of movement (in centimetres), or a plate falling under another one, a earthquake will occur. Why do they occur at fault lines more often then not? Because those places are set so that 1) they can touch, and 2) there are areas where one plate can or have already slid under another plate, resulting in guaranteed earthquakes in the area.