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

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 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 fluid-filled fracture can open perpendicular to the least compressive principal stress. In a compression-positive contact approximation, effective stress is ${\sigma }_n^{\prime }={\sigma }_n-P_f$. Opening an initially intact plane of tensile strength $T_0$ requiresA sufficiently weak pre-existing crack can open near zero effective compression. Increasing isotropic pore fluid pressure translates all effective principal stresses without rotating their directions, so an opening fracture and a frictional normal fault can record compatible stress axes at different pressure episodes. Subsequent mineral precipitation creates a mineral vein; white appearance alone does not identify its filling mineral.

The boundary between a mountain block and adjacent lower terrain. Its geometry records interacting tectonic uplift, faulting, erosion and deposition. A straight front, triangular facets and displaced alluvial fans together can support an active normal fault interpretation.

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.

A lies on the continuation of the offset outside the two ridge tips: it is the fracture zone, whose two sides are carried in the same overall plate direction. A bathymetric step can remain because the two sides have different ages and thermal histories, without continuing plate-boundary strike-slip faulting.

B is the black segment connecting the two white spreading ridges. Construct the plate motion from divergence away from those ridge axes. Between their tips, the upper side of B belongs to the plate west of the right-hand ridge and moves approximately west-northwest; the lower side belongs to the plate east of the left-hand ridge and moves approximately east-southeast. Looking across B from either side, the opposite side moves to the observer's left. B is therefore a left-lateral strike-slip faulting on a transform fault. The offset preserved outside the active segment must not be read as the sense of present slip.

The grey arrows in the source image are pointers to detachment faults, not plate-motion vectors. The original publication identifies white lines as spreading ridges, black lines as transform faults, and arrows as pointers to detachments. Its Figure 1b is the unlettered source of the exam's Figure 3.^[ref]

C lies on the white spreading ridge extending down from the left-hand ridge-transform intersection. Relative motion there is extensional, so its tectonic faults have normal slip. Dike intrusion also accommodates extension at a spreading ridge; “normal slip” is the appropriate category among the four offered, not a claim that all spreading is achieved by a single normal fault.

A normal fault is the plausible interpretation. The photograph shows a steep, exposed planar rock face with lower ground on its open side; this geometry is consistent with an exhumed footwall and a down-dropped hanging wall. A steep dip alone is not a unique indicator of shear sense, and the photograph does not supply a displaced marker proving the sense independently. Under an Andersonian faulting regime, a plausible maximum compression ${\sigma }_1$ is approximately vertical, with ${\sigma }_3$ horizontal in the extension direction and ${\sigma }_2$ approximately along strike.

Assuming pure dip-slip faulting, use an east–west striking, $65^{\circ }$ north-dipping normal fault, with the northern hanging wall moving down dip. In a right-hand-rule strike convention this is strike $270^{\circ }$, dip $65^{\circ }$, rake $-90^{\circ }$. The auxiliary nodal plane strikes east–west and dips $25^{\circ }$ south. The focal mechanism has a predominantly vertical P axis and a predominantly north–south horizontal T axis; the steep central direction is dilatational.

The sketch is a double-couple seismic moment tensor calculation, not a claim that first motions uniquely identify which nodal plane slipped. The mapped north-dipping surface supplies that additional geological choice.

For an Andersonian faulting regime appropriate to the stated normal fault, take ${\sigma }_1$ vertical, ${\sigma }_2$ north–south along strike, and ${\sigma }_3$ east–west. Thus the minimum horizontal compression is normal to the fault’s north–south strike. In an east–up cross-section, draw ${\sigma }_1$ vertically and ${\sigma }_3$ horizontally, with ${\sigma }_2$ perpendicular to the page. These are plausible regional principal stress directions, rather than measured directions uniquely recoverable from one photograph.

A blind reverse fault can shorten and bend its cover into a fault-propagation fold or other growing anticline. At the fold crest, bending can create shallow extension and secondary normal faults or tension fissures, even though the regional deformation is shortening. Bedding-parallel flexural slip, local backthrusts and fold scarps can also break the surface.

Thus a shallow surface break need not be the upward continuation of the main seismogenic thrust fault. The expected association is distributed deformation related to the uplifted fold; its detailed pattern depends on cover layering, fold geometry and shallow strength. The broad frontal relief in Attachment A is compatible with this possibility, but the image alone does not establish a unique earthquake rupture history.

Map the continuous break near the base of the rugged mountain front, following the low scarp across the smoother fan surfaces, rather than the ridgeline. With the same normalized image coordinates, an approximate trace follows $(0.04,0.56),(0.22,0.52),(0.43,0.60),(0.55,0.64)$, steps toward $(0.62,0.43)$, and continues via $(0.72,0.49),(0.88,0.64),(0.98,0.61)$. Some parts are much clearer than others and should be drawn as inferred or dashed where obscured.

The mountainous upper side of the image is the uplifted footwall; the smoother basin/fan side below is the downthrown hanging wall. A section perpendicular to the front has a normal fault dipping toward the basin. Attachment B has no printed north arrow, so these image-relative labels avoid assigning an unsupported compass direction. The original interpretive figure marks the trace and the central fault step-over.

An overlapping pair of basin-dipping normal fault segments can first accommodate displacement through an unbreached relay ramp. Fault throw tapers toward each tip and transfers between the strands; the intervening block tilts and bends while retaining a continuous ramp-like surface. Draw overlapping parallel traces in plan view and a tilted connecting surface between their different fault throws.

With continued extension, the ramp can be breached by a linking fault. An oblique connector joins the segment tips and takes up a combination of dip-slip and strike-slip displacement; the system becomes a more continuous, zigzag normal fault. Draw this connector diagonally through the overlap, with downthrown arrows on the basin side. Distributed small faults and folding can accompany either arrangement. The central bend around $u\simeq 0.6$ in Attachment B is the candidate transfer zone; the image alone does not uniquely establish the three-dimensional connection at depth.

A surface-reaching normal fault earthquake commonly produces a down-facing step towards the hanging wall, accompanied by extension across the break. A fresh free face can expose bedrock, polish or striations, roots or offset soil. The actual rupture is usually irregular: fault slip changes along strike, separate strands overlap or step, and tension fissures, small grabens and secondary cracks distribute part of the extension.

Exposed competent rock or very thin cover: a relatively narrow, sharp break can closely follow the principal fault plane. A steep free face can be recognizable immediately, then weather and shed debris. The face need not be vertical or maintain the same height along strike.

Loose granular or poorly consolidated cover: the main shear can splay upwards through alluvium or regolith. Open fissures, step-like scarps, rotated blocks, synthetic and antithetic normal faults, and a small intervening graben can form across a wider zone. Collapse and raveling quickly build colluvium at the toe. A single rupture event can therefore create several parallel breaks; counting them does not count earthquakes.

Thick weak or cohesive cover: deeper fault slip can be transferred into distributed shear strain, tilted beds, broad warping or monoclines, with tension cracks above a propagating tip. A discrete break may be attenuated or absent if the fault remains blind. Cover thickness alone is not enough to predict the response: cohesion, grain size, layering, saturation, rupture displacement and fault depth also matter. Saturated loose sediment can additionally develop soil liquefaction, sand blows or lateral spreading; these are secondary shaking effects and do not independently locate the tectonic rupture.

The observed vertical step, crack aperture and total width of disturbed ground should be measured separately. Reconstruct the total displacement across all strands and account for the original ground slope. Landslides or gravitational spreading can imitate normal-fault scarps, so map connections to the main fault and displaced markers rather than assuming every open crack is primary tectonic slip.

For a surface-breaking reverse earthquake, expect a fault scarp or disrupted belt with the hanging wall displaced upward and commonly over the footwall. Small thrust faults, splays and backthrusts may repeat or override shallow deposits. Shortened soil, roads or other markers may buckle, while soft sediment can form pressure ridges and compression folds. Near the scarp, gravitational collapse and local bending can also produce tensile fissures and small normal faults; a reverse event need not create compression everywhere at the surface.

For a blind reverse rupture, expect coherent uplift and warping, newly tilted surfaces, or a fold scarp without a throughgoing primary fault offset. Small flexural-slip surfaces along bedding, secondary thrust faults or backthrusts, and opening cracks or small normal faults over the bent outer arc of the anticline can accompany the folding. Deformation may be distributed or localized at fold hinges. Thus a blind main fault does not imply an entirely unbroken ground surface, and secondary cracking must be distinguished from the principal rupture by its geometry and displacement pattern. Shaking-triggered landslides or soil liquefaction can accompany either case and should not automatically be mapped as primary tectonic rupture.^[ref]

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.

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.