Faults

In all types of faults, the different blocks of rock push very tightly together, creating a good deal of friction as they move. If this friction level is high enough, the two blocks become locked -- the friction keeps them from sliding against each other. When this happens, the forces in the plates continue to push the rock, increasing the pressure applied at the fault.

If the pressure increases to a high enough level, then it will overcome the force of the friction, and the blocks will suddenly snap forward. To put it another way, as the tectonic forces push on the "locked" blocks, potential energy builds. When the plates are finally moved, this built-up energy becomes kinetic. Some fault shifts create visible changes at the earth's surface, but other shifts occur in rock well under the surface, and so don't create a surface rupture.

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