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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.

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.

A channel system whose correlated reaches have been separated by fault slip. A fault-parallel connecting reach, abandoned reach or abrupt bend can reveal repeated strike-slip faulting. Reliable displacement measurement requires correlation of the same original channel rather than assigning every bend to tectonic displacement.

Ernest Masson Anderson related faulting to the orientations and relative magnitudes of principal stresses. Near an approximately horizontal, shear-free surface, the vertical direction is a principal direction, leading to the idealized Andersonian faulting regime.^[ref]

With compression taken positive and ${\sigma }_1\ge {\sigma }_2\ge {\sigma }_3$, vertical ${\sigma }_1$ corresponds to normal faulting, vertical ${\sigma }_3$ to reverse faulting, and vertical ${\sigma }_2$ to strike-slip faulting. Pre-existing weaknesses and rotated stress axes can produce departures from this simple classification.

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.

For the ray sketches in (i) and (ii), take a horizontally layered isotropic medium, a surface source and receiver, depth $z$ positive downwards, and wave speed $v(z)$. Snell's law conserves $p=\sin i/v$, where $i$ is measured from the vertical. A returning diving ray turns where $pv(z_t)=1$, withFor a uniform positive gradient $v=v_s+\beta z$, rays are circular arcs andThus ordinary travel time increases with distance but has a decreasing slope as deeper rays sample faster material. The following two leaves discuss the labelled changes to this reference structure.

For the OR alternative, the supplied fault-plane diagrams, assume the usual convention that shaded quadrants are compressional first motions on a lower-hemisphere projection. Each pair of nodal planes represents a fault plane and an auxiliary plane; first-motion polarity alone cannot choose which slipped. Geological mapping, aftershock locations or rupture observations can resolve that ambiguity.^[ref]

Diagram (i): normal faulting. The central unshaded band indicates the extensional dip-slip pattern. It is compatible with crustal extension, such as a continental rift or a spreading environment, with a hanging wall moving down relative to the footwall on the actual fault. Its two curved nodal traces are the possible fault and auxiliary planes.

Diagram (ii): strike-slip faulting. The two straight nodal diameters divide four alternating quadrants, as expected for two vertical candidate planes. It is compatible with a transform plate boundary or continental shear zone. One candidate is right lateral and the conjugate is left lateral; polarity alone does not select the actual plane. Without a north arrow, geographical strikes cannot be assigned.

Diagram (iii): oblique-slip faulting with a reverse component. The off-centre nodal intersection and curved traces distinguish it from pure vertical strike slip, while shading through the centre indicates the reverse component under the stated convention. It is compatible with transpression, such as oblique convergence at a plate margin or a compressional strike-slip bend. Both strike-slip and dip-slip components are required; it is not merely a rotated pure strike-slip pattern. Exact strike, dip, rake, and slip-plane selection cannot be recovered numerically from this uncalibrated printed sketch. Reversing the shading convention would reverse the normal/reverse interpretation, so the convention is essential.

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.

The fault dips are approximately $90^{\circ }$. The mapped senses of fault slip are predominantly strike-slip faulting. In an Andersonian faulting regime, the intermediate principal stress ${\sigma }_2$ is vertical and the maximum and minimum principal stresses ${\sigma }_1,{\sigma }_3$ are horizontal, so the preferred fault planes are vertical. Local bends can introduce oblique-slip faulting without changing this first-order interpretation.

Treat the two strands as newly formed conjugate faults obeying the same Mohr-Coulomb failure criterion, rather than merely inherited faults reactivated in a later stress field. A fault strike is an unoriented line, so $296^{\circ }$ is equivalent to $116^{\circ }$. The acute angle between the fault strikes is therefore $116^{\circ }-60^{\circ }=56^{\circ }$. The formal compressive acute bisector puts ${\sigma }_1$ at azimuth $88^{\circ }$ and ${\sigma }_3$ at $178^{\circ }$. This orientation must also be checked against the mapped senses of strike-slip faulting, as discussed below.

Let $\beta =28^{\circ }$ be the angle between either fault plane and ${\sigma }_1$. Tangency of Mohr's circle to $|\tau |=c+\mu {\sigma }_n^{\prime }$, where ${\sigma }_n^{\prime }$ is effective stress normal to the plane and $c$ is cohesion, gives $\beta =45^{\circ }-\varphi /2$ and $\mu =\tan \varphi$. ThusIn the Mohr circle, the fault planes are $112^{\circ }$ apart around the circle, because a physical plane rotation is doubled. The figure uses $c=0$ to display both tangencies; a common nonzero cohesion changes the intercept, not the inferred friction angle. The inference of $\mu$ requires optimally oriented conjugate faults at formation: the two fault strikes alone cannot measure a friction coefficient for arbitrarily reactivated faults.

Consistency limitation: the mapped westward motion of the Anatolian block corresponds to left-lateral strike-slip faulting on the East Anatolian strand and right-lateral strike-slip faulting on the North Anatolian strand. East–west maximum compression for the acute-bisector construction instead predicts the opposite pair. The stress orientation compatible with the mapped senses lies near the north–south, obtuse bisector; the planes are then about $62^{\circ }$ from ${\sigma }_1$, which would give $\varphi =90^{\circ }-2(62^{\circ })=-34^{\circ }$ in the optimal-formation formula. A negative friction angle is inadmissible for ordinary frictional strength. Thus $0.675$ is the formal magnitude inferred from unoriented angles, while the full map does not support a self-consistent positive-friction, optimally formed conjugate pair. The common-stress assumption can still describe inherited or anisotropically controlled faults, but then their orientations no longer determine a unique coefficient.

Use compression-positive principal stresses, ${\sigma }_1\ge {\sigma }_2\ge {\sigma }_3$. For the intended thrust fault regime during approximately north–south tectonic shortening, the likely orientations areA locally horizontal traction-free surface motivates taking one principal direction as vertical, while the strong north–south compression selects the maximum horizontal principal stress. The expected fault strike is then approximately east–west.

This ordering assumes the two horizontal compressions exceed the vertical principal stress. North–south shortening by itself does not establish the magnitudes of all three: if the vertical principal stress were intermediate, strike-slip faulting would instead be possible. The answer above is the simple compressional end member used in the following parts.

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

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.