Fault (geology)

Lore & Background

Faults are classified by dip angle and slip direction. Strike-slip faults have predominantly horizontal offset, with left-lateral (sinistral) or right-lateral (dextral) motion. Transform faults are a special class forming plate boundaries. Dip-slip faults include normal faults, where the hanging wall moves down relative to the footwall, and reverse faults, where the hanging wall moves up. Thrust faults are reverse faults with a dip less than 45°, forming ramps, flats, and fault-bend folds. Oblique-slip faults combine both strike-slip and dip-slip components.

Reader's Guide

Faults are fundamental to understanding Earth's tectonic activity and earthquake generation. The distinction between normal and reverse faults reveals the stress regime—extensional or compressional—acting on the crust. The concept of asperities explains why faults lock and then rupture, releasing strain energy as seismic waves. Fault zones, with clusters of parallel faults or crushed rock, illustrate the complexity of deformation. The terminology of hanging wall and footwall, derived from mining, remains essential for describing fault geometry. Large thrust faults, including subduction zones, produce the largest earthquakes. The study of faults also informs resource exploration, as fault-related structures can trap hydrocarbons or minerals.

Did You Know?

• The terms hanging wall and footwall come from mining: the miner stood with the footwall under his feet and the hanging wall above him.
• A fault zone can be a cluster of parallel faults or the zone of crushed rock along a single fault.
• Transform faults are considered 'conservative' plate boundaries because lithosphere is neither created nor destroyed.
• Drag folding near a fault likely arises from frictional resistance to movement on the fault.