Fault (geology)

A fault in geology is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement can occur rapidly, in the form of an earthquake, or slowly, as aseismic creep. Faults are caused by the stresses exerted by the tectonic forces that drive plate tectonics.

Faults can range in length from a few millimeters to hundreds of kilometers. The surface along which the blocks move is called the fault plane. The orientation of the fault plane is described by its dip (the angle between the fault plane and a horizontal plane) and strike (the direction of a horizontal line on the fault plane). The movement along a fault is called the slip.

Faults are classified based on the direction of relative movement between the blocks:

• Normal Fault: The hanging wall (the block above the fault plane) moves down relative to the footwall (the block below the fault plane). Normal faults are typically associated with extensional tectonic regimes.

• Reverse Fault: The hanging wall moves up relative to the footwall. Reverse faults are typically associated with compressional tectonic regimes. A reverse fault with a low angle (less than 45 degrees) is called a thrust fault.

• Strike-Slip Fault: The movement is predominantly horizontal and parallel to the strike of the fault. Strike-slip faults are typically associated with transform plate boundaries. They are described as either right-lateral (if, when facing the fault, the opposite side moves to the right) or left-lateral (if, when facing the fault, the opposite side moves to the left).

The area where a fault is located is often characterized by features such as:

• Fault Scarps: Cliffs formed by the displacement of the ground surface along the fault.
• Fault Breccia: Rock fragments formed by the crushing and grinding of rocks during fault movement.
• Gouge: A clay-rich material formed by the alteration of rocks during fault movement.
• Slickensides: Polished and grooved surfaces created by the rubbing of rocks against each other along the fault plane.

Faults play a significant role in earthquake generation, mountain building, and the formation of sedimentary basins. They are also important conduits for fluids, influencing the migration of groundwater and hydrocarbons. Understanding the location and characteristics of faults is crucial for assessing seismic hazards and managing natural resources.