Fault classification, evidences, documentation

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.