Mature fault mechanics revealed by the highly efficient 2025 Mandalay earthquake.

Understanding the causes of variability in earthquake rupture characteristics, particularly the distribution of shallow slip and the extent of off-fault deformation, is crucial for refining seismic hazard models and understanding fundamental rupture mechanics. The 28 March 2025 magnitude 7.7 earthquake on Myanmar's Sagaing fault, the largest instrumentally recorded event in the region, is an extreme example of rupture behavior on mature, geometrically simple strike-slip systems. Here, we use Interferometric Synthetic Aperture Radar (InSAR) and optical satellite imagery to characterize the coseismic surface deformation field and infer the slip distribution along its ~500-km rupture. Our results reveal a remarkably sharp surface rupture with effectively zero shallow slip deficit along its entire length; peak slip consistently occurred at or near the surface, contrasting sharply with recent large earthquakes on less mature faults with significant off-fault deformation and shallow slip deficit. Furthermore, while the rupture smoothly propagated through seismic gaps, we show that it also re-ruptured segments previously broken in 1929, 1930 and 1956, but with reduced slip in these zones - consistent with a slip-predictable model. These observations highlight that rupture processes on mature plate boundary faults can be highly efficient, with implications for seismic hazard assessment on similar strike-slip fault systems worldwide.