Grades 3–5 · Earth & Space
Why can a fault move suddenly in an earthquake?
Earth’s outer rigid layer is divided into moving plates that include both crust and the uppermost mantle. Along a fault, blocks of rock can move relative to one another. They do not always slide smoothly as the larger plate system moves.
In the diagramA locked fault can store deformationBefore slipAfter slipSome stored energy leaves as seismic wavesThese idealized snapshots do not predict an earthquake.
Contact and friction can keep parts of a fault stuck while nearby rock slowly deforms. Energy is stored in that deformation. The change may be too small and slow to notice at the surface, even though forces are building.
If the fault can no longer hold, part of it can slip suddenly. Nearby rock changes shape, and some stored energy travels outward as seismic waves. Those waves can shake the ground some distance from the place where slipping began.
This links a slow process with a sudden event. It does not mean a visible crack always opens, or that plates float on a global ocean of molten rock. Much of the mantle is solid but can deform over long periods; its slow behavior differs from a fault’s rapid slip.
About this explanation
Starting knowledge: Understand that rocks can press against each other and a material can store energy while deforming.
Sources
- U.S. Geological Survey — The Science of Earthquakes