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Faults form mainly in the upper, brittle, part of the crust in response to external stresses, resulting mainly from the interaction between tectonic plates. Most large faults develop within the seismogenic layer, just above the brittle-ductile transition zone, where the strength of the crust is at its greatest. Below this, higher temperatures lead to ductile behaviour becoming dominant and brittle faults are replaced with shear zones or broader areas of distributed strain.
Once formed, faults move mainly by periodic slip, as fault planes typically have irregularities that have higher levels of friction, known as asperities. When a fault is unable to slip because of asperities, it is known as "locked". The stresses in the surrounding rock mass increase until there is enough stored elastic strain energy to break through the asperities, releasing the stored energy, partly as seismic waves, forming an earthquake.