Why do earthquakes happen?

Part of Hall of Planet Earth.

Why Do Earthquakes Happen? AMNH/R.Mickens

In 1906, the seismologist Henry Reid developed the “elastic rebound theory” to explain earthquakes. When rocks begin to press against each other, they initially bend, like a spring, to accommodate the opposing forces. Eventually, when the rocks reach a point where they cannot bend further, they break. The bent rocks snap back, or rebound, to their original shape. The break is the fault itself, and the shock waves emanating from the rebound are the earthquake. The shock waves vibrate through the Earth, making it “ring” like a bell.