Proposed by H.F. Reid, this theory is the fundamental explanation for how tectonic earthquakes occur. Imagine stretching a rubber band until it snaps; the energy released and the “snap back” represent the earthquake and the crustal adjustment. In the Earth, tectonic forces apply stress to brittle crustal rocks. Instead of breaking immediately, these rocks bend and store “elastic strain energy.” Eventually, the stress overcomes the friction holding the rocks together at a fault. The rocks fracture at the focus, releasing the energy that we feel as tremors and the displaced blocks settle into a new equilibrium.