Model Answer
0 min readIntroduction
Faults are fractures in the Earth’s crust where rocks on either side have moved relative to each other. They are fundamental features of tectonically active regions and provide insights into the stress regimes operating within the Earth. Understanding fault mechanisms requires analyzing the stress field, which is often represented using a stress ellipsoid. The stress ellipsoid is a geometric representation of the state of stress at a given point in the Earth’s crust, defined by three principal stresses: maximum compressive stress (σ1), intermediate stress (σ2), and minimum compressive stress (σ3). The orientation of these stresses dictates the type of fault that forms. This answer will diagrammatically explain the formation of normal, strike-slip, and thrust faults using the stress ellipsoid.
Understanding the Stress Ellipsoid
The stress ellipsoid is a 3D representation of stress at a point. Its axes represent the principal stresses. The orientation of these axes determines the type of deformation and, consequently, the type of fault that develops. The pole to the plane of maximum shear stress is the direction of σ3, while σ1 is perpendicular to the plane of maximum shear stress.
Normal Faults
Normal faults occur due to extensional stress, where the crust is being pulled apart. This is characterized by σ1 being vertical (or nearly vertical) and σ3 being horizontal. The fault plane dips at an angle, allowing the hanging wall (the block above the fault) to move down relative to the footwall (the block below the fault).
Diagram Explanation: In the diagram, σ1 is the shortest axis (vertical), representing the minimum compressive stress. σ3 is the longest axis (horizontal), representing the maximum tensile stress. The fault plane is oriented such that the hanging wall moves downward along it.
Strike-Slip Faults
Strike-slip faults result from shear stress, where forces are acting horizontally and parallel to each other. In this case, σ1 and σ3 are horizontal and perpendicular to each other, and σ2 is vertical. The fault plane is typically vertical, and the movement is predominantly horizontal.
Diagram Explanation: Here, σ1 and σ3 are horizontal, and the fault plane is vertical. The movement is along the strike of the fault, hence the name "strike-slip." The diagram illustrates right-lateral strike-slip, where the block on the opposite side moves to the right.
Thrust Faults
Thrust faults are a type of reverse fault with a low angle of dip (typically less than 45 degrees). They form under compressional stress, where the crust is being squeezed together. This is characterized by σ1 being horizontal and σ3 being vertical. The hanging wall moves up and over the footwall.
Diagram Explanation: In this diagram, σ1 is the longest horizontal axis, representing the maximum compressive stress. σ3 is the shortest vertical axis. The fault plane is inclined at a low angle, and the hanging wall is pushed up and over the footwall.
Comparative Table of Fault Types
| Fault Type | Stress Regime | σ1 Orientation | σ3 Orientation | Movement |
|---|---|---|---|---|
| Normal Fault | Extension | Vertical | Horizontal | Hanging wall down |
| Strike-Slip Fault | Shear | Horizontal | Horizontal (perpendicular to σ1) | Horizontal (along strike) |
| Thrust Fault | Compression | Horizontal | Vertical | Hanging wall up |
Conclusion
In conclusion, the formation of normal, strike-slip, and thrust faults is directly linked to the orientation of principal stresses within the Earth’s crust, effectively visualized through the stress ellipsoid. Normal faults arise from extension, strike-slip faults from shear, and thrust faults from compression. Understanding these relationships is crucial for interpreting tectonic settings and assessing seismic hazards. Further research into stress field complexities and fault interactions is vital for improving 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.