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Rise of a depositional surface through net sediment accumulation. It can fill a hanging-wall basin and conceal an active fault scarp.

Slope-derived sediments transported chiefly by gravity and local slope processes. Collapse and raveling of an earthquake fault scarp can supply a colluvial wedge.

The vertical component of displacement between correlated markers across a fault. For pure dip slip of magnitude $D$ at fault dip $\delta$, the displayed formula applies. Erosion and ground slope can make surface fault scarp height different from throw.

For regional coverage, use stereo photogrammetry from overlapping satellite or airborne imagery, or a radar-derived digital elevation model where suitable data exist. Match the same terrain features in images taken from different known viewing positions, infer their three-dimensional positions by triangulation, and georeference and grid the result. Satellite stereo or interferometric radar can cover $10^4\mathrm{k}{\mathrm{m}}^2$ efficiently; choose pixel size and vertical precision fine enough for the fault scarps of interest, rather than assuming every regional model resolves small scarps.

InSAR topographic mapping uses the phase difference from a known viewing baseline; repeat-pass deformation work must separate that height contribution from actual motion. Vegetation, cloud, radar shadow and steep slopes can leave gaps or represent the canopy rather than bare ground.^[ref] Ground control and independent elevation checks are needed before interpreting small relief as active faulting.

Use airborne lidar with sufficient pulse density and ground-return classification. Laser travel times give ranges; the aircraft’s surveyed position and orientation place returns in three dimensions. Multiple returns include canopy and, where gaps permit, the ground. Classify ground points, remove vegetation returns and interpolate a bare-ground digital elevation model over the $20\mathrm{k}{\mathrm{m}}^2$ strip.

This specifically addresses the forest-obscured fault scarp, unlike ordinary optical stereo, which primarily models the visible canopy. Leaf-off acquisition can improve ground sampling where relevant. Dense understorey and a complete lack of ground returns still cause uncertainty; interpolation does not create observed ground heights. Inspect ground-point coverage and compare hillshades under several illumination directions.^[ref]

The rate of surface modification controls how quickly a fault scarp is erased. Strong rainfall, runoff, slope failure and easily eroded lithologies can round or dissect a scarp; dry conditions and resistant rock often preserve it longer. Alluvial fan deposition and aggradation can bury a scarp even if erosion is slow. A young marker surface can preserve an offset that an older, strongly modified surface no longer records.

The balance between fault slip rate, recurrence interval and marker age controls whether renewed displacement outruns this modification. Rapid or repeated normal faulting can maintain a clear front, whereas a long interval after a small displacement permits smoothing. A very young fan deposited after the event may have no offset, while an older surface may retain cumulative rather than single-event fault throw. These two controls explain why absence of an obvious scarp is not proof of an inactive fault.

Mark a fault scarp cutting an alluvial fan on the left-to-central smooth piedmont, approximately $(u,v)=(0.38,0.58)$. Its narrow, aligned height break continues across surfaces that are smoother than the mountains; repeated downthrow on the basin side produces the scarp. Sketch a once-continuous gently sloping fan surface cut by a basin-facing step.

Mark triangular facets on the mountain front in the right-central part, near $(0.78,0.48)$. These are roughly triangular planar mountain faces bounded laterally by incised valleys and below by the fault front. In a front view, draw repeated triangles separated by stream valleys. Triangular facets result from the interaction of normal faulting, uplift and erosion; neither they nor a sharp mountain front alone demonstrates a rupture in the particular historical event.

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.

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]