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Fault: causes, classification, slope effects

Short answer

A fault is a fracture with relative displacement. Caused by tectonic stress, plate movements, isostatic adjustment, volcanic activity, and gravity sliding. Classification: normal (tension), reverse/thrust (compression), strike-slip (shear), oblique-slip, rotational, horst/graben. Faults reduce slope strength by creating weak crushed zones, providing sliding surfaces when dipping out of slope, increasing water pressure, accelerating weathering, and enabling earthquake-triggered movement.

What is fault? Write down its causes. Classify fault with suitable diagram. State the effects of fault on the strength of the slope as a whole.

A fault is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may range from a few millimetres to hundreds of kilometres.

Causes of faulting

  • Tectonic stress (compression, tension, shear) exceeding the rock strength.
  • Plate movements, and uplift and subsidence of crustal blocks.
  • Isostatic adjustment, volcanic activity and magma intrusion.
  • Release of strain energy during earthquakes; also gravity sliding.

Classification

By relative movement (genetic classification)

  1. Normal fault - the hanging wall moves down relative to the foot wall; caused by tension.
  2. Reverse fault - the hanging wall moves up relative to the foot wall; caused by compression. A reverse fault with a dip of less than 45 degrees is a thrust fault.
  3. Strike-slip (lateral) fault - the blocks move horizontally along the strike of the fault; caused by shearing. It may be right-lateral (dextral) or left-lateral (sinistral).
  4. Oblique-slip fault - the movement has both dip-slip and strike-slip components.
  5. Rotational (hinge/pivotal) fault - one block rotates about a point on the fault.
  6. Horst and graben - a horst is a block raised between two normal faults; a graben is a block dropped between them.
 NORMAL FAULT (hanging wall moves down)
   foot wall    \       hanging wall
  ==== A ========\
                  \
  ---- B ----------\ ==== A ====
                    \
                     \ ---- B ----

 REVERSE FAULT (hanging wall moves up)
   foot wall    \ ==== A ====   hanging wall
                 \
  ==== A =========\ ---- B ----
                   \
  ---- B -----------\

 (\ = fault plane dipping to the right; the right-hand block is the hanging wall)

Effects of a fault on the strength of a slope

  • The rock in the fault zone is crushed (gouge, breccia) and weak, so the strength of the slope is reduced.
  • A fault plane dipping out of the slope is a ready-made sliding surface, leading to planar or wedge failure.
  • Fault zones carry water, which increases pore pressure and reduces shear strength.
  • Differential weathering along the fault produces deep weathered zones.
  • Movement may be renewed by earthquakes, triggering landslides.

✓ Answer checked by Claude Opus 5.5 (high effort)

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