The earth's crust is made up of continents and oceans composed of very large and small moving rock layers called crustal plates. Crustal plates are said to float over a liquid rock called asthenosphere that allows the crustal plates to move. Great forces acting on them cause the rocks to break apart and collide.
When a collision occurs, energy is released. This is felt on the earth's surface as earthquake tremor. Tremors vary in intensity. They may be weak or strong. That's how earthquakes are born.