Types of Faults

A fault is a fracture surface within the earth on which slip or displacement has taken place. The total displacement on a fault may be less than a few centimeters or may be measured in hundreds of kilometers. Large displacements are commonly achieved by a series of sudden slips associated with earthquakes, but under some conditions involving slow slip, called creep. Many possible fault configurations are possible; the fracture surface may be planar or curved, and the slip may be uniform everywhere or may change from place to place, as in a rotational displacement or a displacement that becomes smaller and smaller and finally dies out. In this report we will focus on those portions of faults with uniform displacement on planar fracture surfaces and will not discuss complex faults or the details associated with the edges or intersections of faults.

The three fundamental fault types are normal, reverse, and strike-slip. Normal faults involve a dipping fracture surface on which the block above the fault plane, the hanging-wall block, is downthrown with respect to the block below, called the footwall block. Normal faults are common in regions of crustal extension. In contrast, reverse fault displacements, which are common in regions of compression, are such that the block above the fracture surface is uplifted with respect to the block below.
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Strike-slip faults generally involve no vertical motion, but instead are produced by two blocks that are sliding laterally past one another. The sense of lateral motion can be right lateral (dextral) or left lateral (sinistral). Imagine that you are standing on one side of the fault. If the other side has moved to the right, as may be indicated by offset streams, ridges, roads, fences, or other features that cross the fault, it is a right-lateral fault. If the other side has been offset to the left, the fault is left lateral.
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Few faults are, in fact, purely normal, reverse, or transverse, but instead combine transverse motion with either normal or reverse motion. This combined motion is termed oblique slip.

Complex fault types

When faults extend to the Earth's surface, displacing parts of the landscape, landforms are developed or modified. The portion of the fracture surface that is exposed by faulting is called the fault scarp (figure 3). Fault scarps may initially be angular and well defined, but over time they are modified by weathering and erosion on the upper portions while the lower portions become buried by eroded debris (talus). If a region is sliced by a series of subparallel normal faults with sufficient displacement, horst-and-graben topography may develop. A horst is a block that has remained high relative to those on either side, whereas a graben is depressed relative to the adjacent blocks (figure 4).