🌍  Earthquakes, Folds & Faults – DGMS Structural Geology Notes

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🧭 1. Introduction 

 In mining geology, understanding structural features of the Earth’s crust is crucial for safe and efficient mine planning.
Earthquakes, folds, and faults reveal the tectonic stresses acting on rocks — which directly influence mine design, support systems, and hazard assessment. DGMS often includes these topics in Legislation, Safety, and General Paper (Geology sections) for both Coal and Metal First/Second Class Manager examinations.

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🌋 2. Earthquakes in Mining Context ⚙️ Causes of Earthquakes

• Tectonic movements due to stress accumulation and release along fault planes.
• Volcanic activity near subduction zones.
• Human-induced seismicity – mine blasts, dewatering, caving, and stope collapse.

📈 Seismic Waves

• P-waves (Primary): Fastest, compressional, travel through all media.
• S-waves (Secondary): Shear, travel only through solids.
• Surface waves: Cause major destruction.

🪨 DGMS Relevance

• Underground metal mines often experience tremors during blasting or caving.
• Mines in seismically active regions (e.g., Jharia, Raniganj) require special design of supports and shaft linings.
• Monitoring via seismographs and ground vibration meters is recommended under DGMS guidelines.

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🏔️ 3. Folds 

  🧩 Definition 

 Folds are bends or curves in rock layers formed due to compressive forces acting on strata.
They affect the position of ore bodies, coal seams, and strata inclination—critical for mine planning. 🪞 Types of Folds

1. Anticline: Upward arch; oldest rocks at core.
2. Syncline: Downward trough; youngest rocks at core.
3. Monocline: Single flexure or step-like fold.
4. Overturned Fold: One limb tilted beyond vertical.
5. Recumbent Fold: Nearly horizontal axial plane.

🧭 DGMS Significance

• Incorrect recognition of folds may cause unexpected seam encounters.
• Affects drainage patterns, roof control, and ventilation layout.
• Important for borehole correlation and underground survey planning.

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🌐 4. Faults 

  A fault is a fracture or break in the rock strata along which displacement has occurred.

🔍 Types of Faults

Type	Description	Movement
Normal Fault	Tension causes hanging wall to move down.	Vertical
Reverse Fault	Compression pushes hanging wall up.	Vertical
Thrust Fault	Low-angle reverse fault.	Inclined
Strike-slip Fault	Lateral movement along strike.	Horizontal
Oblique Fault	Combination of vertical and horizontal.	Mixed

⚒️ DGMS Importance

• Faults may displace ore seams, cause water inrush, and affect roof stability.
• Accurate fault mapping reduces risk of accident and misalignment in drivage.
• DGMS expects candidates to identify fault types from field sketches or drill logs.

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📘 5. Quick One-Liners

• Focus zone of earthquake → Hypocentre
• Point on surface → Epicentre
• Normal fault → hanging wall moves down
• Reverse fault → hanging wall moves up
• Folds form due to compression; faults due to fracture
• DGMS expects sketch-based questions on folds & faults
• Mohorovičić discontinuity lies between crust & mantle

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✍️ 6. Descriptive Model Answer 

  Q: Explain folds and faults with examples and their importance in mining. 

  Answer:
Folds are bends in strata caused by compressive forces, such as anticlines and synclines, which affect ore body orientation and mine planning.
Faults are fractures with displacement, formed due to stress failure. They may cause displacement of seams, water inflow, and roof instability. In mining:

• Folds affect gradient and alignment of drives.
• Faults require precautionary drilling and water control measures.
  DGMS mandates mapping of structural features in all mine plans and sections to ensure safety and stability.

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🎯  25 MCQs – Structural Geology (DGMS Focus) 

Q1. The point within the Earth where an earthquake originates is called:
A. Epicentre
B. Hypocentre
C. Focus zone
D. Rift zone
E. Lithosphere
Answer: B.
Solution: Hypocentre is the true origin point; epicentre is directly above it.

Q2. In a normal fault, the hanging wall:
A. Moves up
B. Moves down
C. Remains stationary
D. Slides horizontally
E. Rotates only
Answer: B.
Solution: Tension causes the hanging wall to move downward.

Q3. Folds are formed due to:
A. Tension
B. Compression
C. Erosion
D. Deposition
E. Weathering
Answer: B.
Solution: Compressive forces bend strata into folds.

Q4. The topmost layer of the Earth is called:
A. Mantle
B. Core
C. Crust
D. Moho
E. Asthenosphere
Answer: C.
Solution: The crust is the outermost, thinnest solid layer.

Q5. The boundary between crust and mantle is known as:
A. Gutenberg discontinuity
B. Lehmann discontinuity
C. Moho discontinuity
D. Asthenosphere limit
E. Fault plane
Answer: C.
Solution: Mohorovičić discontinuity separates crust and mantle.

Q6. Anticlines have:
A. Youngest rocks at the core
B. Oldest rocks at the core
C. Uniform rocks
D. No age relation
E. Random sequence
Answer: B.
Solution: Anticline exposes older rocks at its centre.

Q7. Synclines are recognized by:
A. Outward dip
B. Inward dip
C. Horizontal layers
D. Fault traces
E. Vertical joints
Answer: B.
Solution: Synclines dip inward toward the axis. Q8. The angle between fault plane and horizontal is:
A. Dip
B. Strike
C. Pitch
D. Plunge
E. Gradient
Answer: A.
Solution: Dip measures inclination of fault plane.

Q9. Strike-slip faults show:
A. Vertical movement
B. Horizontal displacement
C. Upward fold
D. Downward slip
E. None
Answer: B.
Solution: Strike-slip fault movement is horizontal along the strike.

Q10. Folds with both limbs dipping in same direction are:
A. Symmetrical
B. Monoclines
C. Recumbent
D. Overturned
E. Isoclinal
Answer: D.
Solution: Overturned folds have both limbs dipping in same direction.

Q11. The angle of dip in a fault measures:
A. The direction of strata strike
B. The inclination of the fault plane to the horizontal
C. The depth of displacement
D. The gradient of bedding plane
E. None of the above
Answer: B.
Solution: Dip angle defines the inclination of the fault plane relative to the horizontal plane. Q12. The vertical displacement along a fault plane is known as:
A. Heave
B. Throw
C. Slip
D. Drag
E. Pitch
Answer: B.
Solution: The vertical component of fault displacement is called the throw.

Q13. The horizontal displacement along a fault is termed:
A. Throw
B. Heave
C. Drag
D. Dip
E. Roll
Answer: B.
Solution: Heave is the horizontal displacement along a fault plane.

Q14. Which type of stress generally causes normal faults?
A. Tensional
B. Compressional
C. Shear
D. Torsional
E. None
Answer: A.
Solution: Normal faults result from tensional stress when rocks are pulled apart.

Q15. Which of the following faults is associated with compressional stress?
A. Normal
B. Reverse
C. Strike-slip
D. Oblique-slip
E. None
Answer: B.
Solution: Reverse faults form due to compression where hanging wall moves upward.

Q16. Which type of fault commonly results in landslides or roof falls in mines?
A. Thrust fault
B. Normal fault
C. Strike-slip fault
D. Oblique fault
E. Recumbent fault
Answer: B.
Solution: Normal faults cause downthrow and instability leading to slips and roof failures.

Q17. In thrust faults, the hanging wall:
A. Moves upward over the footwall
B. Moves downward
C. Remains stable
D. Slides horizontally
E. Rotates
Answer: A.
Solution: Thrust faults are low-angle reverse faults where hanging wall moves upward.

Q18. The term “focus” in an earthquake refers to:
A. Surface rupture
B. Centre of destruction
C. Point of origin within Earth
D. Epicentre location
E. Seismograph station
Answer: C.
Solution: Focus or hypocentre is the true origin point of an earthquake within the Earth.

Q19. The Richter scale measures:
A. Frequency of earthquake
B. Duration of shaking
C. Magnitude of energy released
D. Distance from epicentre
E. Wave speed
Answer: C.
Solution: Richter scale measures the magnitude of energy released during an earthquake.

Q20. The type of fault where displacement is parallel to strike is called:
A. Normal fault
B. Reverse fault
C. Strike-slip fault
D. Oblique fault
E. Thrust fault
Answer: C.
Solution: Strike-slip faults have horizontal displacement along the strike direction.

Q21. Which structural feature often traps mineral deposits and petroleum accumulations?
A. Faults
B. Folds
C. Joints
D. Cleavage planes
E. Bedding planes
Answer: B.
Solution: Anticlines and domes (folds) form traps for oil, gas, and ore deposits.

Q22. Which type of fold has both limbs dipping at equal angles in opposite directions?
A. Symmetrical fold
B. Asymmetrical fold
C. Overturned fold
D. Recumbent fold
E. Isoclinal fold
Answer: A.
Solution: Symmetrical folds have equal dip angles on both limbs.

Q23. The intensity of an earthquake is measured by:
A. Richter scale
B. Mercalli scale
C. Seismograph amplitude
D. Wave frequency
E. Fault length
Answer: B.
Solution: The Mercalli scale measures intensity (damage effect) of earthquakes.

Q24. The geological structure that may cause sudden inrush of water in underground mines is:
A. Fold
B. Fault
C. Joint
D. Cleavage
E. Bedding plane
Answer: B.
Solution: Faults may intersect aquifers and cause water inflow hazards.

Q25. DGMS emphasizes fault detection in mine plans primarily for:
A. Increasing production
B. Identifying ore grade
C. Preventing hazards due to displacement and water inflow
D. Surface drainage
E. Mapping vegetation
Answer: C.
Solution: DGMS requires fault detection to ensure safety from falls, inflow, and misaligned drivages. 

                         🧾 Conclusion 

Understanding Earthquakes, Folds, and Faults is essential for mine stability, safety, and planning.
For DGMS exams, focus on:

• Identification of fold/fault types in field sketches.
• Fault displacement impacts on drivage.
• Earthquake zones affecting mine construction.

 

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