'Civilization exists by geologic consent'

Will Durant.

Faults are common and important geological structures across which the predominant displacement discontinuity is parallel to the plane of the structure. They are also referred to as shear fractures generally in the engineering literature. In the geological literature, faults have been traditionally classified based on the sense displacement as strike-slip, dip-slip, and oblique-slip faults (Figure 1), see for example, any structural geology text book.

Figure 1. Illustration showing dip-slip, strike-slip, and oblique-slip faults. Please enlarge to see detail. By Arjuna Multimedia from www.indiana.edu/~g103/G103/week9/faultypes.jpg (2005).

Except quarries and mines, it is very seldom that a fault can be seen in 3D as in the block diagrams in Figure 1. In fact, views of natural faults are commonly available on irregular surfaces at an arbitrary angle to the orientation of faults (Figure 2). This means that key offset markers commonly are not observable, and if they are, the offsets are 'apparent.' Fault surface features may reveal the actual direction of the slip across faults and this information would unambiguously define the fault type as illustrated in Figure 1. There are other features that reveal the kinematics of the fault motion. For example, the splay fracture orientation or pull-aparts can reveal the slip direction. Please see the section under 'Splay Joints', 'Splay Pressure Solution Seams', and 'Echelon Faults.' For large active faults with adequate seismological data, focal plane solutions may be used to determine the sense of motion.

Figure 2. Photo shows several small, predominantly dip-slip faults offsetting nearly flat-lying thin-bedded siliceous shale for a few millimeters to a few centimeters in a rock quarry. South-central coastal California. Note both centimeter (left-hand side) and inch (right-hand side) scales.

High quality seismic data from reservoirs provide the best 3D view of relatively larger faults. Unfortunately, these generally lack the exact slip directions and high enough resolution to image the fault zone and the associated damage. In essence, there is no single approach to extract all the essential fault properties. The best that one can do in each data set is to use the fault knowledgebase to make the best inferences possible.

Faults occur in a wide range of scales, from small ones with a few millimeters to a few centimeters offsets (Figure 2) to medium size faults with meters (Figure 3) to hundreds of meters of offsets (Figure 4). As the fault offset increases, so does the fault complexity. Finally, large scale faults with a few 1000 kilometers in length and over 100s of kilometers offsets, like the San Andreas Fault (Figure 5), are truly crustal scale and form tectonic plate boundaries (Wallace, 1990).

Figure 3. A small normal fault zone with about 3 m offset, in shale (dark colored) embedded with thin limestone layers (light colored) exposed in a nearly vertical road cut. Note that slivers of limestone pieces are imbricated along the fault zone. From Aydin (1973).

Figure 4. An aerial photograph of a series of strike-slip faults in Aztec Sandstone exposed at Valley of Fire State Park, Nevada. The faults are predominantly left-lateral as indicated by the offsets of the boundaries between the red and white sandstone units. The left-lateral offset across the largest fault zone on the right-hand side is about 170 m. From Flodin and Aydin (2004).

Figure 5. Aerial photograph of the San Andreas Fault, a major plate boundary fault in the western United States. Photo from the Carrizo Plain, central California shows bending and offset of drainages across the fault zone. From USGS website, http://pubs.usgs.gov/gip/earthq1/how.html.

Detailed properties of the components, geometry, patterns, and scaling of faults are discussed under various classes similar to other structural elements discussed earlier. For the time being, we note that faults commonly consist of components referred to as cores, slip surfaces, fault rocks (gouges), damage zones, and splays. Faults change the petrophysical properties of the host rocks in which they occur, like permeability, porosity, and capillary pressure and these are discussed under 'Properties of Faults.' The formation of faults is presented under the 'Mechanisms and Mechanics of Faults.' 'Case Studies' also include faults and assemblages of faults with excellent images and maps.