A fault is an area of stress in the earth where broken rocks slide past each other, causing a crack in the Earth's surface.
Typically faults are associated with or form around the boundaries between Earth's tectonic plates.
Large faults within the Earth's crust result from the action of tectonic forces.
Energy release associated with rapid movement on active faults is the cause of most earthquakes.
A fault line is the surface trace of a fault, or the line of intersection between the fault plane and the Earth's surface.
Since faults do not usually consist of a single, clean fracture, geologists use the term fault zone when referring to the zone of complex deformation associated with the fault plane.
The two sides of a non-vertical fault are known as the hanging wall and footwall.
By definition, the hanging wall occurs above the fault and the footwall occurs below the fault.
This terminology comes from mining: when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall hanging above him.
In an active fault, the pieces of the Earth's crust along a fault move over time causing earthquakes to occur.
Inactive faults had movement along them at one time, but no longer move.
The type of motion along a fault depends on the type of fault.
There are three major types of faults: dip-slip normal, dip-slip reverse, strike-slip, and oblique-slip.