Geological fault

A geological fault is a fracture or fracture zone across which rock has undergone relative displacement. Its local geometry is described by a fault plane, fault strike, fault dip and fault rake. A locked fault can store elastic strain while surrounding rock deforms; some faults also accommodate displacement by slow creep.

A lateral separation between adjacent or overlapping fault strands. Displacement transfer can be accommodated by distributed deformation, a tilted relay ramp, or a connecting fault. Its releasing or restraining effect depends on both the stepping direction and the sense of fault slip.

A polished or striated surface on a fault, produced or modified by sliding. Striation direction constrains the orientation of fault slip; determining the sense of motion can require additional asymmetric indicators.

A finite-width region containing the main slip surface, subsidiary fractures and mechanically or chemically modified rock associated with a fault. It can include a comminuted core and a surrounding damaged region. Porous permeability can be enhanced along connected fractures while fine fault gouge reduces cross-fault flow.

One of a pair of fault orientations that can form under a common principal stress field with opposite senses of resolved shear stress. For newly formed, optimally oriented faults obeying the same Mohr-Coulomb failure criterion, their acute intersection angle is $90^{\circ }-\varphi$, where $\varphi$ is the friction angle. The maximum compressive principal stress bisects that acute angle. This permits an estimate $\mu =\tan \varphi$ only under the common-formation and optimal-orientation assumptions, rather than for arbitrary inherited faults.

A local principal stress model associated with Ernest Masson Anderson in which one principal stress is vertical, as suggested by negligible shear stress on a flat free surface. With compression positive and ${\sigma }_1\ge {\sigma }_2\ge {\sigma }_3$, vertical ${\sigma }_1$ gives normal faulting, vertical ${\sigma }_2$ gives strike-slip faulting, and vertical ${\sigma }_3$ gives reverse faulting. The other two principal stresses are horizontal. Topography, mechanical layering and local fault geometry can violate the idealization.

For a cohesionless, optimally oriented normal fault with vertical maximum compression and horizontal minimum compression,where $\delta$ is fault dip and $\phi$ the internal friction angle. This is a geometric inference under the model, not a measurement of friction from dip alone.

For a compression-positive Mohr circle, put $C=({\sigma }_1^{\prime }+{\sigma }_3^{\prime })/2$ and $R=({\sigma }_1^{\prime }-{\sigma }_3^{\prime })/2$. Tangency to the cohesionless Mohr-Coulomb failure criterion $\tau ={\mu }_f{\sigma }_n^{\prime }$ gives $R/C=\sin \phi$. The contact point is $({\sigma }_n^{\prime },\tau )=(C{\cos }^2\phi ,C\sin \phi \cos \phi )$. The physical plane-normal angle to ${\sigma }_1^{\prime }$ is half the corresponding central angle, giving the stated dip. Equivalently the plane itself makes $45^{\circ }-\phi /2$ with ${\sigma }_1^{\prime }$.

The associated stress ratio is ${\sigma }_1^{\prime }/{\sigma }_3^{\prime }=(1+\sin \phi )/(1-\sin \phi )$. A reactivated nonoptimal plane, nonzero cohesion or unknown principal stress directions defeats the simple dip-only inversion.

An opening crack formed by local extension. Near a normal fault it may accompany primary shear, but shaking, slope instability or compaction can also produce fissures.

Rock whose structure and composition record deformation within a fault zone.

A cohesive fault rock produced predominantly by mechanical grain fragmentation and granular rearrangement. It differs from a loose fault breccia or fault gouge in cohesion, while sharing a brittle crushing mechanism rather than requiring bulk melting.

Coarse angular rock fragments produced by crushing during fault slip, commonly in a finer matrix. Fine-grained, poorly cohesive comminuted material is fault gouge; cohesive material formed by similar grain-size reduction can be called cataclasite.

Deformation involving slip on geological faults.

A major continental strike-slip fault system in California; its strength has been studied with boreholes, heat flux and seismic observations.

Fine-grained fault rock produced by comminution and alteration; its mineralogy and fluid state can strongly affect friction.

A crustal fault system beneath the Seattle region capable of producing damaging earthquakes and local tsunamis.

The block below an inclined fault plane. It is paired geometrically with the hanging wall.

The block above an inclined fault plane. In reverse faulting it moves upward relative to the footwall.

A topographic step associated with displacement across a surface-reaching fault. Erosion and distributed deformation can modify its height and shape.

An exposed fault plane revealed as fault slip, differential motion and removal of cover bring it to the surface. On a normal fault, lowering the hanging wall can expose a planar footwall face. Continued exposure may preserve a bedrock scarp while weathering, erosion and collapse progressively modify its older portions. A hillslope following the plane therefore need not be an independent erosional surface that accidentally has the same orientation.

A simple model of hillslope smoothing in which sediment flux is proportional to negative slope. For constant transport coefficient $\kappa$, an ideal initial step broadens on scale $\sqrt{\kappa t}$. Real free-face collapse, nonlinear transport, changing climate and deposition can invalidate the linear diffusion equation approximation.

The relationship between characteristic accumulated fault slip and fault-system length. Approximately $D\propto L$ follows in self-similar elastic models at comparable driving stress, but natural faults can accumulate displacement at nearly constant length over part of their history.

A fault with displacement predominantly parallel to its strike.

Dip-slip faulting is relative motion on a geological fault predominantly along the steepest direction within the fault plane. That direction is perpendicular to the fault strike and follows the fault dip. It differs from strike-slip faulting, whose motion is mainly along strike.

For an inclined plane, normal faulting lowers the hanging wall relative to the footwall; reverse faulting raises it. The surface step is only a projection of the full fault slip, so vertical offset alone is not the total displacement unless the geometry is known. Mixed motion belongs to oblique-slip faulting, and an observed focal mechanism must be interpreted with independent evidence for the actual fault plane.

Shortening accommodated by a hanging wall moving up relative to the footwall. The centre of its dip-slip focal mechanism is compressional under the usual lower-hemisphere convention. Low-angle reverse faulting is commonly called thrust faulting.

A fault on which the hanging wall moves up relative to the footwall, accommodating shortening. A low-angle reverse fault is a thrust fault. The fault dip is a geometric property and should be distinguished from the direction of fault slip.

A low-angle reverse fault. Near-surface splays may accompany either a surface-breaking or a blind main rupture.

A mechanically weak detachment along which an overlying package can move relative to its substrate. A décollement can follow weak bedding and connect to steeper thrust ramps, so its orientation need not equal the optimum dip for a new fracture in homogeneous rock.

A very large plate-interface thrust fault at a subduction zone.

A reverse fault dipping oppositely to the dominant thrust system and accommodating part of its shortening.

A fault with reverse slip whose upper tip remains beneath the ground surface. It can deform the overburden through folding, localized fold scarps, and secondary faults.

Extension accommodated by a hanging wall moving down relative to the footwall. Its focal mechanism has an extensional dip-slip pattern, with the centre unshaded under the convention that shaded quadrants denote compression.

A tilted region transferring displacement between overlapping normal fault segments. Opposing gradients of fault throw can produce an unbroken ramp. Continued extension can breach the ramp with a connecting fault, generating a linked segmented trace.

A relatively down-dropped block bounded by opposing normal faults. A small secondary graben can form within the distributed surface rupture zone of one earthquake.

A fault on which the hanging wall moves downward relative to the footwall. Local bending or collapse can create small normal faults even during a regional reverse-faulting event.

A long-lived low-angle normal fault that can exhume deep crust or mantle. Its mapped scarp need not be a plate-motion arrow.

Fault slip with both dip-slip and strike-slip components. The focal mechanism can have an off-centre nodal intersection, with the central polarity indicating its dip-slip sense under a specified shading convention. A reverse component with horizontal shear is compatible with transpression.

Transpression combines horizontal shearing with shortening across a deforming region. On individual geological faults, that combination can appear as oblique-slip faulting with a reverse component. At larger scales, different structures can partition the motion: some accommodate strike-slip faulting, while others accommodate reverse faulting or folding.

A restraining bend in an otherwise laterally moving fault system illustrates the mechanism: the geometry makes neighbouring blocks converge as well as slide past one another, promoting uplift and distributed deformation. A vertical surface displacement consequently does not mean that every part of the regional motion is vertical. The relevant comparison is between measured displacements, the fault strike and the full regional deformation pattern.

Predominantly horizontal relative motion along a fault. A pure strike-slip double couple has two vertical nodal planes; a focal mechanism gives both possible planes, and independent evidence selects the actual one.

Strike-slip faulting in which the opposite side moves to an observer's right when looking across the fault.

Strike-slip faulting in which the opposite side moves to an observer's left when looking across the fault. The sense is the same from either side.