 Solution

Interferometric synthetic-aperture radar compares the phase of coherent radar returns acquired before and after the earthquake from closely repeated viewing geometry. After image registration and removal of orbital and topographic phase, the residual interferometric phase constrains relative ground displacement along the radar line of sight. Defining $u_{\mathrm{LOS}}$ positive toward the satellite and choosing the interferogram's phase order accordingly,One complete colour cycle therefore represents $\lambda /2$ of relative line-of-sight displacement. Reversing the image order reverses the displayed phase sign. This observes a projection of both horizontal and vertical motion, not uplift alone.^[ref]

If the main reverse rupture reaches the surface, coherent areas on opposite sides can show a displacement step: fringes become offset, truncated or abruptly juxtaposed along a trace. A narrow belt can lose interferometric coherence where cracking, intense gradients or changed scatterers prevent reliable phase comparison. If the rupture remains blind, a simple elastic model instead predicts a continuous surface displacement field, often a broad asymmetric lobe with continuous curved or closed fringes and a smooth displacement gradient across the projected tip.

These are diagnostic tendencies, not a unique test. An ordinary wrapped-phase boundary is just a $2\pi$ colour reset and is not itself a rupture. A step of an integer number of cycles can be hidden in wrapped phase; loss of interferometric coherence can hide either kind of near-field deformation. Vegetation, atmosphere and viewing direction also affect the pattern, and a displacement perpendicular to the radar line of sight is poorly sensed. Inspect coherence and unwrapped displacement, compare ascending and descending views where possible, and test a fault model against field observations. A sharp fold scarp above a blind rupture or a secondary shallow fault can also create localized gradients, so smooth fringes do not prove a buried tip and broken fringes alone do not prove a surface-breaking main rupture.