Field identification criteria of fault

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 be a few millimetres to hundreds of kilometres.

Evidences for recognition in the field

Direct evidences

• Displacement or offset of beds, dykes, veins, streams, roads or ridges.
• Repetition or omission (missing) of beds in a sequence.
• Slickensides (polished, grooved surfaces) and striations on the fault plane showing the direction of movement.
• Fault breccia, fault gouge or mylonite along the fault zone.
• Drag folds (bending of beds near the fault plane).

Physiographic (topographic) evidences

• Fault scarps (straight steep cliffs) and triangular facets.
• Linear valleys, straight stream courses and sudden changes in stream direction.
• Offset or beheaded streams, waterfalls and rapids along a straight line.
• Line of springs and seepages, and a sharp line of vegetation change.
• Alignment of sag ponds, lakes and hot springs.
• Abrupt change of rock type across a straight line, and earthquake epicentres in a line.

Note

No single evidence is final; the presence of several (such as slickensides, breccia and offset beds together) is used to confirm a fault.