Like wax, rock gets softer as higher temperatures At temperatures that occur 100-150 km deep, rock is soft enough to flow. Layer are Earth above 100-150 km, where rock is rigid, is lithosphere. Layer of Earth below 100-150, where rock is plastic, is asthenosphere. The seismic belts suggest the lithosphere is not a single coherent shell. These move with respect to one another.
Earthquakes occur where plates grind and rub against each other at their edges.
Plate tectonics is the underlying concept that explains many geologic phenomena.