A fault is a planar fracture or zone of fractures in the Earths crust where there has been significant displacement as a result of rock-mass movements). Faults allow blocks of rock to move relative to each other, and this movement may occur in different ways depending on the type of fault. Some key features of faults include:

• Fault plane: This is the plane that represents the fracture surface of a fault).
• Fault trace: This is a place where the fault can be seen or mapped on the surface, and it is also the line commonly plotted on geologic maps to represent a fault).
• Fault zone: This is a cluster of parallel faults, or the zone of crushed rock along a single fault. Prolonged motion along closely spaced faults can blur the distinction, as the rock between the faults is converted to fault-bound lenses of rock and then progressively crushed).

There are different types of faults, including:

• Normal fault: This type of fault develops where the crust stretches apart, and the hanging wall slides down relative to the footwall.
• Thrust fault: This type of fault is found at subduction zones, and the rocks on one side of the fault are pushed up and over those on the other side.
• Strike-slip fault: This type of fault involves horizontal movement of the rocks on either side of the fault, and it is caused by shear stress.

Faults can have a large influence on the mechanical behavior of soil and rock masses in, for example, tunnel, foundation, and geotechnical engineering).