Model Answer
0 min readIntroduction
A fault is a fracture or zone of fractures between two blocks of rock. Faults occur due to the brittle failure of rocks when subjected to stress exceeding their strength. Stress, in geological terms, is the force acting per unit area on a rock. These stresses are categorized into principal stresses – σ1 (maximum compressive stress), σ2 (intermediate stress), and σ3 (minimum compressive stress). The orientation of these stress axes dictates the type of fault that forms. Understanding fault types is crucial for seismic hazard assessment and interpreting regional tectonic history.
Types of Faults and Stress Axis Orientation
Faults are broadly classified into three main types based on the relative movement of the blocks and the orientation of the stress axes. These are: Normal Faults, Reverse (Thrust) Faults, and Strike-Slip Faults.
1. Normal Faults
Normal faults occur when the hanging wall (the block above the fault plane) moves down relative to the footwall (the block below the fault plane). This happens when the tensile stress (extension) dominates, meaning σ1 is vertical and σ3 is horizontal. This results in crustal extension and often leads to the formation of grabens and horsts.
Stress Axis Orientation: σ1 (vertical, tensile), σ2 (horizontal), σ3 (horizontal)
2. Reverse (Thrust) Faults
Reverse faults occur when the hanging wall moves up relative to the footwall. This is caused by compressive stress, where σ1 is horizontal and σ3 is vertical. A thrust fault is a reverse fault with a low angle of dip (less than 45 degrees). These faults are common in convergent plate boundaries and contribute to mountain building.
Stress Axis Orientation: σ1 (horizontal, compressive), σ2 (vertical), σ3 (horizontal)
3. Strike-Slip Faults
Strike-slip faults occur when the movement is predominantly horizontal and parallel to the strike of the fault plane. This happens when σ1 and σ3 are both horizontal but oriented differently. Right-lateral strike-slip faults have movement where the block on the opposite side appears to move to the right, while left-lateral faults have movement to the left. These faults are commonly found at transform plate boundaries.
Stress Axis Orientation: σ1 (horizontal), σ2 (vertical), σ3 (horizontal)
The following table summarizes the key differences:
| Fault Type | Relative Movement | Dominant Stress | σ1 Orientation |
|---|---|---|---|
| Normal Fault | Hanging wall down | Tension (Extension) | Vertical |
| Reverse/Thrust Fault | Hanging wall up | Compression | Horizontal |
| Strike-Slip Fault | Horizontal, parallel to strike | Shear | Horizontal |
Conclusion
In conclusion, the type of fault that develops is directly related to the orientation of the principal stress axes acting on the rock. Normal faults indicate extension, reverse/thrust faults indicate compression, and strike-slip faults indicate shear stress. Understanding these relationships is fundamental to deciphering the tectonic history of a region and assessing potential seismic hazards. Further research into the complexities of stress fields and fault interactions is crucial for improved earthquake prediction and mitigation strategies.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.