The Earth's lithosphere, or crust, is divided into tectonic plates. These plates are constantly in motion due to the convection currents of the Earth, which are caused by heat cycles in the mantle, the fluid layer beneath the Earth's crust. At the bottom of the mantle, the fluid gets extremely hot due to its proximity to the core. The hot liquid rises to the top of the mantle, where it then cools. Once it cools, it returns to the bottom, completing the cycle. This movement creates what is known as convection currents, which causes the tectonic plates to move.
Earthquakes occur at three types of plate boundaries, or fault lines: convergent, divergent, and transform.