--- title: "Fault classification, evidences, documentation" subject: "Engineering Geology I" chapter: 5 key: "semester_3/engineering_geology_i:fault-classification-evidences-documentation" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fault-classification-evidences-documentation site: BCT Engineering --- # Fault classification, evidences, documentation Engineering Geology I · Chapter 5: Structural Geology - Asked in: 2059 Chaitra (old course), 3+2+3 marks - Video: [Classification of Faults](https://www.youtube.com/watch?v=qlk7IfYMufs) — GeoScience Videos - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fault-classification-evidences-documentation - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5 ## Question Write down the genetic classification of faults. List out the physiographic evidences to recognise the faults in the field. Why is the documentation of faults essential in civil engineering practice? ## Short answer Genetic classification: normal, reverse/thrust, strike-slip, oblique-slip, rotational, horst/graben. Physiographic evidence: fault scarps, triangular facets, linear valleys, offset streams, waterfalls, line of springs, sag ponds, abrupt vegetation/rock changes, earthquake epicentre alignment. Documentation is essential to locate weak zones for site selection and treatment, identify active faults for seismic design, correlate boreholes, predict seepage, assess slope stability, and estimate costs. ## Answer #### Genetic classification of faults **By relative movement** 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. #### Physiographic evidence to recognise faults - Fault scarps (straight cliffs) and triangular facets on a hillside. - Linear valleys, straight stream courses and sudden bends in streams. - Offset or beheaded streams, ridges and spurs; waterfalls and rapids in a line. - A line of springs, seepages, sag ponds and hot springs. - An abrupt change in vegetation or topography along a straight line. - A sudden change in rock type, and alignment of earthquake epicentres. #### Why documentation of faults is essential - To locate weak, crushed zones and avoid them when selecting dam, tunnel and bridge sites, or to design their treatment. - To find **active faults** that can cause earthquakes and ground rupture, for seismic hazard design. - To correlate beds and rock depth correctly in borehole logs, since faults repeat or omit beds. - To predict seepage and water inflow in reservoirs and tunnels. - To assess slope stability and plan support, drainage and grouting. - Documentation is also needed for the cost estimate of the design and for future maintenance. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a reverse fault, how does the hanging wall move? A. Down relative to the foot wall, due to tension B. Horizontally, due to shear C. Up relative to the foot wall, due to compression D. It does not move Answer: C. Reverse faults form by compression. (Key point: Reverse fault) 2. Which fault has one block rotating about a point? A. Oblique-slip fault B. Normal fault C. Horst D. Rotational (hinge) fault Answer: D. Pivotal faults involve rotation. (Key point: Rotational fault) 3. Why is documenting active faults essential? A. To design for earthquakes and ground rupture B. To find good building stone C. To raise bearing capacity D. To estimate sand volume Answer: A. Active faults give seismic hazard. (Key point: Active faults) 4. Why do faults complicate borehole logs? A. They add extra fossils B. They repeat or omit beds C. They make beds horizontal D. They fix the bed thickness Answer: B. Missing or repeated beds spoil correlation. 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