Codex @codex  0

A stream whose course changes direction where deformation displaces or obstructs its former route. Along a strike-slip fault, matching upstream and downstream channel segments can constrain shear sense and fault offset. A bend alone is not a unique tectonic marker: erosion, capture and pre-existing relief also alter drainage. A reliable offset estimate restores identified piercing features rather than equating all along-fault channel length to slip.

A displaced drainage forms when repeated horizontal fault slip offsets a channel and its upstream and downstream reaches no longer line up. In plan view, draw two initially continuous channel reaches crossing the fault, translate one side parallel to the fault strike, and connect the reaches by a short fault-parallel reach. A linear valley develops where erosion follows damaged rock along a straight fault zone.

In Attachment A, use the oblique, relatively straight valley running southwest–northeast through the lower centre towards the upper right. For an identifiable example, follow the central approximately north–south drainage to its bend near the southern edge of the central basin, about $(u,v)=(0.46,0.67)$, where $u$ is fraction from the left image edge and $v$ fraction from the top. Mark its deflected reach and the adjacent straight valley. The positional precision is at the level of the supplied image; establishing a numerical fault offset would require correlated piercing points and field checks.

Use repeat-pass interferometric synthetic-aperture radar. An orbiting radar transmits coherent microwave pulses and records the amplitude and phase of the returning field. Form an interferogram from registered complex images acquired before and after the earthquake, remove the orbital contribution and the topographic phase predicted by a digital elevation model, and correct atmospheric and other long-wavelength phase delays. Where interferometric coherence is adequate, unwrap the residual phase to estimate line-of-sight displacement:with sign set by the instrument's displacement convention. Each $2\pi$ phase cycle represents $\lambda /2$ of range change.

This maps a distributed earthquake displacement field day or night and through cloud, including areas inaccessible on foot. Roughly $10$–$30\mathrm{m}$ ground sampling and a scene spanning many kilometres on both sides of the fault can resolve regional deformation; a narrow surface break benefits from finer pixels or complementary sub-metre optical surveys. Sensitivity to displacement can be millimetres to centimetres in coherent areas, but that does not equal spatial resolution. A single viewing direction measures one component; ascending and descending tracks and GNSS constrain more components. At the rupture itself, large gradients and changed ground cover can destroy interferometric coherence; radar pixel-offset tracking or before/after optical correlation can then recover metre-scale fault offsets.