The classification and mechanics of faults

Fault Classification

Faults are classified based on several criteria, including the direction of movement, the geometry of the fault plane, and the type of stress involved.

1. Based on Displacement Direction

• Normal Faults: These occur under tensional stress, resulting in the hanging wall moving down relative to the footwall. They are common in areas of extension, such as rift valleys.
• Reverse Faults: These form under compressional stress, where the hanging wall moves up relative to the footwall. A special type of reverse fault with a low angle (<45°) is called a thrust fault.
• Strike-Slip Faults: These faults involve horizontal movement along the fault plane. The San Andreas Fault in California is a classic example. They can be right-lateral (dextral) or left-lateral (sinistral).

2. Based on Geometry

• Dip-Slip Faults: Movement is primarily vertical along the dip of the fault plane (Normal and Reverse faults fall under this category).
• Strike-Slip Faults: Movement is primarily horizontal along the strike of the fault plane.
• Oblique-Slip Faults: These faults exhibit both dip-slip and strike-slip movement.

3. Based on Scale

• Major Faults: Large-scale faults that extend for hundreds of kilometers, often associated with plate boundaries (e.g., San Andreas Fault).
• Minor Faults: Smaller faults, often localized and associated with regional stress fields.

Fault Mechanics

The mechanics of faulting involve the interplay of stress, strain, friction, and rock strength.

1. Stress and Strain

Stress is the force per unit area acting on a rock. Strain is the deformation of the rock in response to stress. Different types of stress lead to different types of faulting:

• Tensional Stress: Pulls rocks apart, leading to normal faults.
• Compressional Stress: Squeezes rocks together, leading to reverse and thrust faults.
• Shear Stress: Causes rocks to slide past each other, leading to strike-slip faults.

2. Friction and Fault Strength

The strength of a fault is determined by the frictional resistance between the rocks on either side of the fault plane. The coefficient of friction (μ) is a measure of this resistance. The shear stress required to initiate faulting must overcome the static friction. Once faulting begins, the shear stress required to maintain movement is lower, governed by kinetic friction.

3. Fault Zones

Faults are rarely single, clean breaks. Instead, they are typically complex fault zones consisting of multiple, interconnected fractures, breccias, and gouge (pulverized rock). These zones can vary in width from a few centimeters to several kilometers. The presence of fluids (water, oil) within the fault zone can significantly reduce friction and promote fault movement.

4. Elastic Rebound Theory

Developed by H.F. Reid after the 1906 San Francisco earthquake, the Elastic Rebound Theory explains how earthquakes occur. Stress builds up in rocks along a fault due to tectonic forces. The rocks deform elastically until the stress exceeds the rock’s strength, causing a sudden rupture and release of energy in the form of seismic waves. The rocks then rebound to their original shape, creating the earthquake.

Fault Type	Stress Regime	Movement	Example
Normal Fault	Tensional	Hanging wall down	Basin and Range Province, USA
Reverse Fault	Compressional	Hanging wall up	Himalayan Frontal Thrust
Strike-Slip Fault	Shear	Horizontal	San Andreas Fault, California