Fault (geology)

A fault is a break or crack in a rock mass where the two sides have shifted significantly relative to each other. The largest faults in the Earth's crust are created by the movement of tectonic plates, and the biggest ones mark the boundaries between plates—examples include the megathrust faults found in subduction zones and transform faults. When movement along an active fault happens quickly, it releases energy that causes most earthquakes. However, faults can also move slowly, a process known as aseismic creep.

The fault plane is the flat surface of the fracture itself. Where this plane meets the ground, it is called a fault trace or fault line, and this line is what geologists often draw on maps. A fault zone can refer to a group of parallel faults, but the term also describes the crushed and broken rock along a single fault. Over time, movement along closely spaced faults can blur these definitions, as the rock between them gets broken into lens-shaped pieces and eventually pulverized.

**Mechanisms of faulting** Friction and the stiffness of rocks often prevent the two sides of a fault from sliding smoothly past each other, so movement can stop entirely. The spots along a fault plane where friction is especially high and the fault locks are called asperities. While a fault is locked, stress builds up. When that stress exceeds the rock's strength, the fault ruptures, and the stored strain energy is released partly as seismic waves—an earthquake. Strain can build up gradually or be released instantly, depending on whether the rock is ductile or brittle. The ductile lower crust and mantle deform slowly through shearing, while the brittle upper crust fractures suddenly, causing motion along the fault. Even in ductile rocks, a fault can release energy instantly if the strain rate becomes too high.

**Slip, heave, throw** Slip is the relative movement of rock on either side of a fault plane, measured as a vector (a displacement with direction). The sense of slip describes how one side moves relative to the other. The vertical part of this displacement is called the throw, and the horizontal part is called the heave—for example, "throw up and heave out." The direction of slip can be estimated by looking at drag folding, which is the bending of rock layers near a fault caused by friction during movement. To measure the exact heave and throw, geologists need to find matching points (piercing points) on both sides of the fault. In practice, it is usually only possible to determine the slip direction and an approximate heave and throw vector.

**Hanging wall and footwall** On a non-vertical fault, the block above the fault plane is the hanging wall, and the block below is the footwall. These terms come from mining: a miner working a tabular ore body would have the footwall underfoot and the hanging wall overhead. They are key for distinguishing dip-slip faults: in a reverse fault, the hanging wall moves up; in a normal fault, it moves down. Knowing which type is present helps determine the stress regime that caused the fault movement. The pressure from the hanging wall can cause severe stress and rock bursts, as seen at Frood Mine.

**Fault types** Faults are classified mainly by the angle of the fault plane relative to the Earth's surface (the dip) and the direction of slip along it. Based on slip direction, faults are: - **Strike-slip**: offset is mostly horizontal, parallel to the fault trace. - **Dip-slip**: offset is mostly vertical or perpendicular to the fault trace. - **Oblique-slip**: a combination of strike-slip and dip-slip.

**Strike-slip faults** In a strike-slip fault (also called a wrench, tear, or transcurrent fault), the fault plane is usually nearly vertical, and the footwall moves sideways with little vertical motion. Left-lateral strike-slip faults are called sinistral; right-lateral ones are dextral. The direction is defined from the perspective of an observer on the opposite side of the fault. A special type is the transform fault, which forms a plate boundary. These are linked to offsets in spreading centers like mid-ocean ridges, or less commonly within continental lithosphere—for example, the Dead Sea Transform in the Middle East or the Alpine Fault in New Zealand. Transform faults are considered "conservative" plate boundaries because lithosphere is neither created nor destroyed.

**Dip-slip faults** Dip-slip faults are either normal (extensional) or reverse. The terms "normal" and "reverse" come from coal mining in England, where normal faults are more common. Over time, regional stress can switch between tension and compression, causing faults to reactivate with their block movement reversed—this is called fault inversion. So a normal fault can become a reverse fault, and vice versa.

**Normal faults** In a normal fault, the hanging wall moves downward relative to the footwall. The dip of most normal faults is at least 60 degrees.