Fault and Fault Terminology in Geology

What is Fault in Geology?

A fault is a fracture or region of fractures between two blocks of rock. Faults enable the blocks to pass relative to every other. This motion may appear rapidly, in the form of an earthquake - or may also happen slowly, in the shape of creep. 

Figure 1 Types of faulting in tectonic earthquakes,

Publisher
Encyclopædia Britannica, 2015

Faults may additionally vary in size from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary attitude in between.

Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip alongside the fault to classify faults. Faults which move along the direction of the dip airplane are dip-slip faults and described as either ordinary or reverse (thrust), depending on their motion. Faults which cross horizontally are regarded as strike-slip faults and are categorized as both right-lateral or left-lateral. Faults which exhibit each dip-slip and strike-slip action are recognized as oblique-slip faults.

Hanging wall motion determines the geometric classification of faulting. Dip-slip motion takes place when the hanging wall moved predominantly up or down relative to the footwall. If the motion was once down, the fault is called a normal fault, if the motion was up, the fault is called a reverse fault.

Geometry of faults:

Normal Fault, Reverse Fault and Strike Slip Fault

Figure 2 (a) Normal Fault (b) Strike slip Fault  (c) Reverse Fault, Haakon Fossen

Rock mass which moves in the same direction of dip called hanging wall block, but the rock mass which move in reverse direction of dip are called foot wall block .

Non-vertical faults separate the placing wall from the underlying footwall . Where the hanging wall is decreased or downthrown relative to the footwall, the fault is a normal fault. 

The opposite case, the place the hanging wall is upthrown relative to the footwall, is a reverse fault. When the dip angle is shallow, a reverse fault is often described as a thrust fault.If the movement is lateral, . in the horizontal plane, then the fault is a strike-slip fault.