Here's why:

* Stress and Strain: Rocks in the Earth's crust are constantly under stress from tectonic plate movement. This stress causes the rocks to deform, which is known as strain.

* Elastic Limit: Rocks can withstand a certain amount of stress before they break or permanently deform. This point is called the elastic limit.

* Faulting: When the stress exceeds the elastic limit, the rocks fracture and move along a fault. This sudden movement releases energy in the form of seismic waves, which we experience as an earthquake.

So, it's the passing of the elastic limit, not the "what," that triggers an earthquake.