Here's why:
* Crust: This is the outermost, rigid layer of the Earth. It's broken into large pieces called tectonic plates.
* Upper Mantle: This layer is partially molten and contains a zone called the asthenosphere, which is very hot and behaves like a viscous fluid.
Earthquakes occur when tectonic plates move, creating stress and pressure on the rocks along their boundaries. When this stress exceeds the strength of the rocks, they suddenly break, releasing energy in the form of seismic waves that cause the ground to shake. This process can happen both within the crust and at the boundary between the crust and the upper mantle.
