Fault traces in outcrops as well as in seismic sections across large regions are discontinuous, consisting of a number of segments at apparently every scale. Figure 1 shows an example of each fault type with multiple segments: (a) a left-lateral strike-slip fault with about 80 cm slip in sandstone, (b) a thrust fault with calcite filled pull-aparts between the neighboring segments, and (c) a series of normal fault segments in cross section.

Figure 1. Traces of an incipient right-lateral strike-slip fault with 80 cm slip in sandstone, Valley of Fire State Park, Nevada, showing discontinuous nature of the fault geometry and the resulting segments. Simplified from de Joussineau and Aydin (2007).

Figure 2 illustrates several examples of fault segmentation from the San Andreas plate boundary zone in increasing size. Images of strike-slip faults at depth are difficult to obtain in the absence of high resolution seismic data and reliable earthquake locations. Therefore, down-dip geometry of these faults and the geometry of their segments at depth are generally uncertain. However, there is little doubt that the geometric complexities mentioned above also occur at subsurface. Figure 3 shows a seismic image of a strike-slip fault system with distinct segments at subsurface. High resolution earthquake locations also indicate large scale discontinuities along strike-slip faults in down-dip direction.

Figure 2. Maps illustrating the segmentation along the faults of the San Andreas system in increasing sizes. From Aydin and Schultz (1990).

Figure 3. Epicenters (A) and hypocenters (B), projected on a transverse plane, of the 1979 Coyote Lake earthquake on the Calaveras Fault, California. From Reasenberg and Ellsworth (1982). Solid lines define regions for comparing distributions in both plots. The alignments in both figures define segments in map view as well as at depth. For more information, see Aydin and Nur (1985).

Dip-slip faults also have segments. Figure 4 illustrates a number of normal faults at various sizes with multiple segments and Figure 5 is a seismic dip line section showing an example of an array of normal fault segments from the Niger Delta.

Figure 4. Segmentation along normal faults of various sizes. (a) A small normal fault in the Entrada sandstone, Arches National Park, Utah; (b) an intermediate size fault in tuff, Volcanic Tableland, California; and (c) Wasatch fault zone, a large active fault in Utah. (*) denotes segment boundaries inferred from paleoseismological data. (d) defines the terminology. From Willemse at al. (1996) and Willemse (1994).

Figure 5. Major fault segments along a normal fault in Niger Delta based on seismic dip line and wells-log data. The segments are color-coded and are numbered from 1 at the base to 7 at the top. Simplified from Koledoye et al. (2003).

There are many mechanisms for fault segmentation. One is that, similar to opening mode fractures, faults may branch into segments as they grow. This notion is common in seismology in which the seismic signature of faults is believed to be generally simpler at depth than near the surface. Another is that vertical anisotropy such as alternating brittle-ductile units may cause vertically segmented faults. For example, the segmented normal fault array in Figure 6 was interpreted this way. One of the most common mechanisms for segmentation is inherited from the earlier structures whether they are opening, closing, or mixed mode structures. See the segmentation of joints and pressure solution seams for faults which form by shearing of these structures. Once started from different planes of weakness, fault interaction becomes the dominant factor for determining the ultimate echelon geometry of the segmented faults. See 'Fault interaction' for details.

Figure 6. Pattern of a series of thrust faults along the Alberta Front Ranges illustrating discontinuous nature of their geometry. Simplified from Dahlstrom (1969).

Similarly, both lateral and down-dip segmentation also occur in thrust faults as shown by Figure 6 and Figure 7.

Figure 7. Segmented geometry of a series of thrust faults in cross section. Inset showing details of a small section. From Grant (1990).

It should be noted that there is also a trend opposite to the segmentation trend in fault growth such that a newer continuous fault may link previous segments thereby simplifying the fault geometry. This new, so-called 'through-going' fault forms by cataclastic deformation along already weakened zones of fragmented and comminuted rock material. See the section 'Growth of Faults by Linkage and Coalescence' for more information on this process.

In the same manner, the length of fault segments and the size of the steps between them are observed to follow certain forms of distribution. See the sections 'Scaling between Fault Length and the Number of Segments per Unit Length' and 'Fault Segment Linkage.'