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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.

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

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 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.

The visible surface can be an exhumed fault surface, rather than an unrelated hillslope that happens to intersect a buried plane. Downward motion of the hanging wall exposes the footwall face; repeated displacement and removal of loose material can reveal a substantial length of the same fault plane. Where exposure is renewed faster than weathering, erosion and collapse erase it, that planar surface remains part of the hillside. The less weathered lower face and rougher, vegetated upper ground are consistent with differential exposure, though the photograph alone does not date individual events.

At greater distance or over longer times, erosion modifies the slope and the hillside will cross or obscure the fault plane. The correspondence is therefore a transient competition between fault slip, exhumation and surface processes, not a requirement that all hillslopes follow faults.

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.

After removing orbital, topographic and atmospheric contributions, an earthquake InSAR radar interferogram would show elongated displacement lobes and generally closed or nested fringes associated with uplift and horizontal shortening of the hanging wall. Each full phase cycle represents ${\lambda }_r/2$ of line-of-sight displacement, where ${\lambda }_r$ is radar wavelength; equivalently $\Delta \varphi =\pm 4\pi d_{\mathrm{LOS}}/{\lambda }_r$, with sign fixed by processing convention.^[ref]

If primary slip reaches the surface, expect a displacement discontinuity across the surface trace: fringes can terminate or change abruptly there, and large near-fault gradients or disturbed ground may decorrelate. For a blind reverse fault, the elastic displacement field is continuous at the ground surface and spread over a broader zone, giving smoother gradients and fringes over the uplifted fold. A blind rupture can still create sharp local gradients or secondary faulting.

The exact colour order, number of fringes and sign of the displacement lobes cannot be fixed from this shaded relief alone. They depend on slip, depth and radar look direction; uplift is only one contribution to line-of-sight displacement.

Use normalized image coordinates $(u,v)$ within the map rectangle, with $(0,0)$ at its upper left and $(1,1)$ at its lower right. A principal candidate thrust fault trace follows the sharp, approximately diagonal southwestern edge of the dissected high ground on the right: a practical tracing guide passes nearMark the upland, upper-right side as hanging wall and the smoother, lower-left basin side as footwall for a thrust dipping beneath the uplands. Use broken lines through the gaps, not one asserted continuous exposed rupture. A subsidiary candidate frontal segment follows the basin-facing edge of the isolated dissected ridge on the left, approximately $(0.04,0.32)$ through $(0.23,0.47)$ to $(0.34,0.61)$; place teeth toward its locally higher ridge side.

These are geomorphic interpretations of mountain fronts and scarps, not measured subsurface dip directions. On a conventional north-up reading the principal trend is broadly northwest–southeast, but the supplied map has no north arrow: the image-relative coordinates are the reproducible identification. Southwestern illumination means southwest-facing slopes are bright; brightness is a slope-orientation clue, not an elevation contour. Some aligned relief may be a fold scarp or an eroded lithological contact, so field confirmation and topographic profiles would test the candidate traces.

The offset layered sequence indicates reverse slip on the west-dipping fault: its western hanging wall moved up relative to the eastern footwall. With the fault strike perpendicular to the east–west image plane, an Andersonian faulting regime has ${\sigma }_1$ horizontal east–west, ${\sigma }_2$ horizontal north–south, and ${\sigma }_3$ vertical. Draw opposing compression arrows east–west and a smaller opposing pair vertically; the intermediate pair points into and out of the page.

For example, in east, north, up coordinates, a compression-positive effective stress tensor consistent with a $30^{\circ }$ reverse fault and zero cohesion isThese magnitudes illustrate one possible stress tensor, rather than a measurement from the photograph. With $\mu =1/\sqrt{3}$, the ${\sigma }_1$–${\sigma }_3$ Mohr circle is tangent to the Mohr-Coulomb failure criterion at a plane $30^{\circ }$ from horizontal. The original failure-mechanics figure above sketches these principal stress directions.

A reverse fault can create a sharp mountain front by raising its hanging wall beside a lower footwall. Drainages entering the rising block may be deflected along the front or cut narrow gorges across it. In cross-section, draw a dipping fault with the hanging wall displaced up-dip and a surface-height contrast above the trace.

In Attachment A, mark the approximately east–west to west-northwest–east-southeast break between the southwestern mountain block and the central basin. Its approximate image coordinates run from $(0.03,0.56)$ through $(0.18,0.61)$ and $(0.34,0.65)$ to $(0.45,0.69)$. The abrupt topographic change and diversion of drainage along this front support the reverse faulting interpretation. It is the southern margin of the central basin, not the much straighter oblique valley farther northeast. The original interpretive figure gives a schematic section.

Put the reverse fault along the mountain-front trace identified in part (ii), with the mountainous southern/southwestern block as hanging wall and the central basin to its north/northeast as footwall. Draw teeth on the southern hanging wall side of the line. In the simplest cross-section the fault dips south and the southern block moves northward and up over the basin edge. The image constrains the surface trace and relative uplift more securely than its subsurface fault dip; the precise dip cannot be measured from this map alone.

The network partitions deformation into oblique horizontal displacement on the strike-slip fault and more directly north–south crustal shortening on the approximately east–west reverse fault. On the oblique strand, northeastward motion of the southeastern side relative to the northwestern side has a northward component, reducing separation in the direction of ${\sigma }_1$. On the reverse fault, northward and upward motion of the southern hanging wall shortens and thickens the crust.

Sketch north–south opposing compression arrows around the plan view, left-lateral arrows along the oblique trace, and teeth on the southern side of the reverse fault trace. A section normal to the latter shows horizontal shortening converted into tectonic uplift. Where strands meet, folding, distributed strain or subsidiary faulting can maintain displacement compatibility; a strike-slip fault terminating abruptly in undeforming rock would not. This is an interpretation involving transpression, and the map does not by itself give a unique partition of slip rates between the strands.

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.

A reverse or thrust fault earthquake is the expected end member: a hanging wall moves upwards relative to its footwall, accommodating north–south tectonic shortening. A plausible double-couple seismic source focal mechanism has an approximately east–west fault strike, a moderately north- or south-dipping fault plane, and up-dip fault slip. For example, right-hand-rule strike $090^{\circ }$, southward fault dip about $32^{\circ }$, and fault rake $+90^{\circ }$ specify a pure thrust mechanism.

In a lower-hemisphere equal-area plot, with shaded regions denoting compressional P first motions, this focal mechanism has a shaded central region and two unshaded outer lobes. The P axis is approximately horizontal north–south and the T axis approximately vertical for a symmetric idealization; for a single $32^{\circ }$ plane their axes are slightly tilted. The P and T axes of a double-couple seismic source need not exactly equal the regional principal stress axes. The nodal curves represent both the fault plane and its auxiliary plane; additional evidence is needed to choose between them.^[ref]

The geomorphology of active normal faulting records a competition between accumulated tectonic displacement and the ability of erosion, transport and deposition to modify it. A hanging wall moves down relative to a footwall, producing a fault scarp, mountain-front relief, and accommodation space for a sedimentary basin. Repeated earthquakes can build a large front even though each individual surface break is much smaller.^[ref]

The principal controls and their observable effects are:

Useful observations therefore include scarp profiles, repeated offsets of dated surfaces, drainage organization, and continuity along strike. A high rugged front is not by itself evidence of a high present fault slip rate; conversely, a muted or buried scarp need not mark an inactive fault.

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.

Excavate across the mapped fault scarp, approximately perpendicular to its strike, at a site where young sediments record and preserve deformation. On cleaned trench walls log bedding, soil horizons, shear zones, fissure fills, buried surfaces and colluvial wedges. Correlate the same marker beds across the fault before reconstructing individual displacements.

Look for event horizons, not merely a stack of sediments:

In the drawing, A was faulted in event 1; W1 accumulated; B buried and stabilized that surface. Event 2 displaced A, W1 and B, after which W2 accumulated and C sealed the fault tip. Thus A records two events, B one and C none. This combination of offset and sealing relationships is stronger evidence than identifying two wedge-shaped units alone.

Bracket event ages with multiple samples just below and above the event horizons. Suitable short-lived organic material can be dated by radiocarbon dating; optically stimulated luminescence dating can constrain deposition of appropriate mineral grains. Reworked old charcoal, younger roots, incomplete bleaching, erosional gaps and time elapsed between rupture and wedge deposition must enter the age interpretation. For calendar dates, $t_{\mathrm{youngest}\mathrm{cut}}<t_{\mathrm{event}}<t_{\mathrm{oldest}\mathrm{seal}}$ only when those deposits genuinely bracket the rupture; a radiocarbon age in years before present reverses the numerical ordering.^[ref]

One earthquake can generate several debris pulses and wedges; repeated earthquakes may reuse one strand without leaving separable deposits. Erosion, bioturbation, nondeposition, lateral facies changes and upward-dying splays can hide events or mimic terminations. Confirm relationships on both trench walls and, when possible, in adjacent exposures. The result is a minimum supported event sequence with age intervals, not an assumption of perfectly periodic recurrence or one earthquake per wedge.

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 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.