Fault Stress Types: Normal, Reverse, and Strike-Slip Faults Explained

Fault Classification by Stress Regime

Faults are fractures in Earth's crust where movement has occurred. The type of stress determines the fault geometry and motion.

1. Normal Fault (Extensional / Tensile Stress)

• Stress: Pulling apart (extension).
• Motion: Hanging wall moves down relative to footwall.
• Setting: Divergent boundaries, rift zones (e.g., Basin and Range, East African Rift).
• Surface expression: Horst and graben topography.

2. Reverse Fault (Compressional Stress)

• Stress: Pushing together (compression).
• Motion: Hanging wall moves up relative to footwall.
• Thrust fault: Low-angle reverse fault (<30° dip); common in mountain belts.
• Setting: Convergent boundaries, subduction zones, collisional orogens (e.g., Himalayas, Rockies).

3. Strike-Slip Fault (Shear Stress)

• Stress: Horizontal shearing (sliding past).
• Motion: Blocks move horizontally past each other.
• Types: Right-lateral (dextral) or left-lateral (sinistral).
• Setting: Transform boundaries (e.g., San Andreas Fault), intraplate shear zones.

4. Oblique-Slip Faults

Combine dip-slip (normal/reverse) and strike-slip components. Common in transpressional or transtensional regimes.

Key Concept: Anderson's Theory of Faulting

Fault orientation relates to principal stress axes (σ1 > σ2 > σ3):

• Normal fault: σ1 vertical, σ3 horizontal.
• Reverse fault: σ1 horizontal, σ3 vertical.
• Strike-slip: σ1 and σ3 horizontal, σ2 vertical.