Here's why:

* Brittle deformation occurs when rocks break under stress. Imagine a piece of chalk. If you bend it gently, it will deform elastically, meaning it will return to its original shape when you release the stress. But if you bend it too far, it will snap – that's brittle failure.

* Earthquakes are caused by the sudden release of energy stored in rocks that have been subjected to stress. This stress can come from tectonic plate movement, which causes rocks to bend, stretch, and compress over long periods.

* Elastic rebound theory: The leading theory explaining earthquakes is the elastic rebound theory. It states that as rocks are deformed, they store energy like a stretched rubber band. When the stress exceeds the rock's strength, the rock breaks, releasing the energy in the form of seismic waves, which we experience as an earthquake.

Other types of deformation:

* Ductile deformation occurs when rocks bend and flow under stress. This is common in deeper parts of the Earth's crust where the pressure and heat are higher. While ductile deformation doesn't directly cause earthquakes, it can play a role in building up stress that eventually leads to brittle failure.

In summary: Brittle deformation, where rocks break under stress, is the primary cause of earthquakes. This occurs when the stress from tectonic plate movements exceeds the strength of the rocks, resulting in a sudden release of energy in the form of seismic waves.