How do earthquakes build up?

The shifting of the Earths Techtonic Plates is what causes earthquakes. The earths crust is floating on top of a layer of magma. When two of earths techtonic plates shift and rub up against eachother, they get caught, because they are rigid, not smooth. When they finally get uncaught, they move very fast and that is what causes the shaking, as does the rubbing up against eachother.