Types of Geological Faults: Normal, Reverse & Strike-Slip

The 3 main types of geological faults are: normal faults (hanging wall moves down due to tension), reverse/thrust faults (hanging wall moves up due to compression), and strike-slip faults (blocks slide horizontally past each other). Faults are classified by the direction of relative movement along the fault plane.

Geological faults are fractures in Earth's crust where movement has occurred. The three main types — normal, reverse, and strike-slip — are defined by the direction of movement along the fault plane.

Normal Faults

Normal faults occur when tensional forces pull the crust apart. The block of rock above the fault plane (hanging wall) moves downward relative to the block below (footwall). Normal faults create grabens (down-dropped blocks forming valleys) and horsts (uplifted blocks forming ridges). They are characteristic of divergent plate boundaries and rift zones.

How it forms: Extensional (tensional) tectonic forces stretch and thin the crust. When stress exceeds rock strength, the crust fractures and the hanging wall drops down.

Examples: East African Rift faults, Basin and Range Province (western USA), Rhine Graben (Germany), Wasatch Fault (Utah)

Reverse (Thrust) Faults

Reverse faults occur when compressional forces push the crust together. The hanging wall moves upward relative to the footwall. When the fault plane is at a low angle (less than 45°), it is called a thrust fault. Thrust faults can push older rocks on top of younger rocks and are responsible for building many mountain ranges.

How it forms: Compressional forces at convergent plate boundaries squeeze the crust. The rock fractures and one block is pushed up and over the other.

Examples: Himalayas (Main Central Thrust), Rocky Mountain thrust belt, Alps, Cascadia subduction zone

Strike-Slip Faults

Strike-slip faults occur when blocks of rock slide horizontally past each other with little or no vertical movement. They are caused by shear forces. Strike-slip faults produce some of the world's most destructive earthquakes because they occur on land and at shallow depths. They are classified as right-lateral or left-lateral depending on the direction of offset.

How it forms: Shear forces cause blocks to slide horizontally past each other. Friction locks the blocks together until stress exceeds the rock's strength, causing a sudden slip (earthquake).

Examples: San Andreas Fault (California), North Anatolian Fault (Turkey), Alpine Fault (New Zealand), Dead Sea Transform

Oblique-Slip Faults

Oblique-slip faults have both vertical and horizontal components of movement — they combine elements of normal/reverse faults with strike-slip motion. Most real-world faults are oblique-slip rather than purely one type, though one component usually dominates.

How it forms: Occur where tectonic forces act at an angle to the fault plane, producing simultaneous vertical and horizontal displacement.

Examples: San Fernando fault (California, 1971 earthquake), many faults in complex tectonic settings

Key Facts

• The San Andreas Fault is about 1,300 km long and has been active for about 28 million years.
• Faults can range from a few meters to thousands of kilometers in length.
• The deepest earthquakes occur on faults about 700 km below Earth's surface, at subduction zones.
• Geologists identify faults by offset rock layers, scarps, and displaced stream channels.

Fun Facts

• Los Angeles is slowly moving toward San Francisco along the San Andreas Fault at about 46 mm per year — they will be neighbors in about 15 million years.
• The 1906 San Francisco earthquake caused up to 6 meters of horizontal displacement along the San Andreas Fault.
• The Denali Fault in Alaska ruptured for 340 km in a single earthquake in 2002.

Summary

The three main fault types — normal, reverse, and strike-slip — reflect the different tectonic forces acting on Earth's crust. Understanding fault types is critical for earthquake hazard assessment and building safer communities.