Geology > Structural Geology > Faults
Description
Structural Geology is a sub-discipline of geology focused on the study of rock formations and the processes that shape them. It examines the three-dimensional distribution of rock units with respect to their deformational histories. One critical aspect of Structural Geology is understanding faults, which are fractures in the Earth’s crust along which there has been displacement of the sides relative to one another parallel to the fracture.
Faults
Faults are significant structural features formed due to the tectonic forces acting upon the Earth’s crust. They are categorized based on the direction of the displacement and the nature of the stresses causing them. These categories include normal faults, reverse (or thrust) faults, strike-slip faults, and oblique-slip faults.
Normal Faults: These occur under extensional stress where the crust is being pulled apart. The block above the fault (hanging wall) moves downward relative to the block below the fault (footwall).
Reverse Faults: These develop under compressional stress where the crust is being pushed together. In this case, the hanging wall moves upward relative to the footwall. Thrust faults are a subtype of reverse faults with a low angle of inclination.
Strike-Slip Faults: These faults are characterized by horizontal movement. The displacement along the fault is parallel to the fault’s strike. Strike-slip faults can be categorized further into left-lateral (sinistral) and right-lateral (dextral), based on the direction of relative movement.
Oblique-Slip Faults: These faults exhibit both vertical and horizontal movements. They are created by a combination of shear and tensional or compressional stresses.
Mathematically, faults can be analyzed using various parameters such as the fault plane’s orientation, given by its strike and dip:
- Strike: The orientation of a horizontal line on the fault plane, usually measured as an angle relative to the north.
- Dip: The angle of inclination of the fault plane relative to the horizontal surface.
The behavior and formation of faults can be described using Mohr’s Circle for Stress, a graphical method to determine the relationship between normal and shear stresses acting on a particular plane.
The fundamental equation for a fault in terms of stress is:
\[ \tau = \sigma_n \tan(\phi) + C \]
where:
- \(\tau\) represents the shear stress,
- \(\sigma_n\) denotes the normal stress,
- \(\phi\) is the angle of internal friction,
- \(C\) is the cohesion of the material.
Understanding faults is crucial for several practical applications, including earthquake risk assessment, evaluation of oil and gas reservoirs, and the study of regional tectonics. Faults can create pathways or barriers to fluid flow in the Earth’s crust, influencing groundwater movement and hydrocarbon migration. Their activity is also a primary cause of seismicity, making them a focal point in earthquake engineering and hazard mitigation.