The point below Earth's surface where rock breaks under stress and tension is called the focus of an earthquake.

Here's why:

* Focus: This is the actual point within the Earth where the rupture starts. It's like the origin of the earthquake.

* Epicenter: This is the point on the Earth's surface directly above the focus. It's where the earthquake's effects are often felt most strongly.

So, while the rock breaks at the focus, the epicenter is the point on the surface where the seismic waves reach first and cause the most shaking.