Elastic Rebound Theory: How Earthquakes Happen

Most earthquakes follow a repeating cycle:

1. Locking: Two sides of a fault are stuck together by friction. No motion occurs, but the plates on either side continue to move slowly.
2. Stress buildup: Because the fault is locked, the rock around it bends and stores elastic strain energy — like a drawn bow.
3. Rupture: When stress exceeds friction, the fault suddenly slips. The rock snaps back to its unstrained shape, releasing the stored energy as seismic waves.
4. Aftershocks: The main shock redistributes stress, which can trigger smaller slips on nearby parts of the fault or on adjacent faults.

The cycle then begins again. For large earthquakes, the interval between major ruptures can be centuries or millennia.

When the fault slips, the rock on both sides does not simply slide in silence. It springs back toward its original, unstressed shape. That sudden spring-back jostles the surrounding rock, sending out vibrations — seismic waves — in all directions. It is the rebound of the rock, not the sliding itself, that generates the waves we feel as shaking.

The amount of energy released depends on how much rock slipped, how far it slipped, and how stiff the rock is. A long fault with large slip releases far more energy than a small fault with tiny slip — which is why magnitude scales with fault area and displacement.

Not all earthquakes are isolated events. Many are preceded by smaller foreshocks — tiny slips on nearby patches of the fault that hint at the stress redistribution to come. The largest event is the main shock. Afterward, the region experiences aftershocks — smaller quakes as the crust adjusts to the new stress state.

Aftershocks can continue for months or years after a large main shock, gradually decreasing in frequency. The Parkfield segment of the San Andreas Fault produces moderate earthquakes roughly every 22 years, while the Cascadia subduction zone has remained locked for centuries, storing stress for a future great earthquake.