Natural Fractures

	Fault slip and fault names



	This section reviews basic fault nomenclature. The diagram shows the basic fault-slip parameters. The blue arrow indicates the right-hand rule strike, the green arrow dip. The *hangingwall* is the fault block above the fault plane; the *footwall* is the fault block below the fault plane.

Figure 1: Fault slip

Fault heave and throw can be deceptive because:

Heave and throw only show one component of fault slip. A fault with a small heave or throw could have a large amount of strike-slip movement.
Vertical throw is very different from stratigraphic throw. This distinction may be very important in directional drilling.

Figure 2: Vertical throw vs. stratigraphic throw

Basic fault names based on slip direction. Fault names indicate the rake of the movement direction of the hangingwall. Rake is the angle of the hangingwall slip-vector measured in the fault plane. There are many different rake notations, but this website measures rake from the dip vector. Positive rakes are clockwise (as seen looking down on the fault plane) and rake can range either from +180° to -180° or from 0°-360° depending on your preference. I prefer the +180° to -180° scheme because rakes with absolute values <90° indicate normal slip, >90° indicate reverse slip, positive rakes indicate right-lateral movement, and negative rakes indicate left-lateral movement.

The next diagram shows how fault names relate to the hangingwall-slip rake. The diagram looks perpendicularly down onto the fault plane. The green arrow is the dip-vector; the blue arrow is the right-hand rule strike. In normal usage faults with slip-vectors lying within 10°-15° of the dip or strike orientation (not direction) are termed normal/reverse or right-/left-lateral faults, respectively. Faults with slip-vectors outside of these ranges are given compound names. The vertical or horizontal slip component of a vertical or horizontal fault is named arbitrarily.

Right-lateral vs. left-lateral. Imagine that you are standing on one side of a steeply dipping fault as it moves. If objects on the opposite block appear to be moving to your right, then the fault has a right-lateral component. If objects on the opposite block appear to be moving to your left, then the fault has a left-lateral component. The apparent movement is the same whether you are standing on the hangingwall or footwall.

Figure 3: The fault name describes the rake of the slip vector.

Special fault names. Geologists use many different names for special types of faults. These names have complex and far-reaching implications that depend to some extent on the user. When in doubt, use a simple name. Some important types of specialized fault names:

Wrench vs. strike-slip fault. These terms are synonyms and indicate a fault with a slip-vector closely parallel to the fault strike. Some geologists reserve the term wrench for large, regional strike-slip faults, steeply-dipping regional strike-slip faults or as a synonym for tear fault.

Thrust fault. This name once meant any reverse fault with a dip-angle of 30° or less. Now the term indicates faults with an originally low dip-angle that formed during regional compressional deformation. A single thrust fault may change its orientation as it crosscuts different lithologies. Folding can reorient thrusts so that they may have a variety of angles today.

Detachment fault. A regional, low-angle, listric normal fault formed during crustal extension.

Tear fault. Often used to indicate a steeply-dipping wrench fault that bounds or cuts the hangingwall of a thrust or normal fault, also used for mode III faults.

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