Fault

Fault

Earthquakes typically occur along faults, which are fractures in the Earth’s crust where two blocks of rock have moved past each other. When the stress on the fault exceeds the friction on the fault, the fault breaks and slips, resulting in an earthquake. 

The main parts of a fault are: 

• Fault Plane: The surface along which the rocks have slipped. In all above diagrams this is outlined in red. 
• Fault Line: The line where the fault intersects the Earth’s surface. 
• Hanging Wall: The block of rock above the fault plane. 
• Footwall: The block of rock below the fault plane. 

The main types of faults are: 

Dip Slip Fault: 

A fault in which the slip direction is the same as the dip direction. This type of fault includes normal faults, reverse faults, and thrust faults: 

1. Normal Fault: 

• Definition: In a normal fault, the hanging wall (the block of rock above the fault plane (in red)) moves downward relative to the footwall (the block of rock below the fault plane. 
• Stress Type: Tensional stress, which pulls rocks apart. 
• Location: Common at divergent plate boundaries where tectonic plates are moving away from each other. 
• Example: The East African Rift. 

2. Reverse Fault: 

• Definition: In a reverse fault, the hanging wall moves upward relative to the footwall. A reverse fault is a steeper version of a thrust fault. 
• Stress Type: Compressional stress, which pushes the rocks together. 

• Location: Common at convergent plate boundaries where tectonic plates are moving towards each other. 
• Example: The Himalayas, where the Indian Plate is colliding with the Eurasian Plate. 

3. Thrust Fault: 

• Definition: A type of reverse fault with a low-angle fault plane. The hanging wall moves up and over the footwall at a gentle angle. 
• Stress Type: Compressional stress. 
• Location: Found in regions of intense compressional forces, often associated with mountain-building processes. 
• Example: The Canadian Rockies. 

Strike-Slip Fault: 

• Definition: In a strike-slip fault, the blocks of rock on either side of the fault slide past each other horizontally. There is little vertical movement. 
• Stress Type: Shear stress, which causes horizontal displacement. 
• Location: Typically occurs at transform plate boundaries where tectonic plates slide past one another. 
• Example: The San Andreas Fault in California. 

Oblique-Slip Fault: 

• Definition: A fault that shows both vertical and horizontal movement, combining elements of both normal/reverse faults and strike-slip faults. 
• Stress Type: A combination of tensional, compressional, and shear stresses. 
• Location: Occurs in regions where the stress is complex, leading to combined types of displacement. 
• Example: Some faults in the Basin and Range Province in the western United States. 

Transform Fault: 

• Definition: A special type of strike-slip fault that occurs along plate boundaries and accommodates horizontal motion between tectonic plates. Often this transform fault is between two spreading ridges, as in the diagram in red. 
• Stress Type: Shear stress. 
• Location: Found where tectonic plates slide past each other without significant vertical movement. 
• Example: The North Anatolian Fault in Turkey.