UC San Diego

The 2025 M7.8 Mandalay earthquake ruptured  ~480 km of the Sagaing Fault, producing one of the longest strike-slip events documented and impacting a vulnerable region. Here we combine remote sensing observations and numerical modeling to show that the subsurface fault geometry and slip distribution could be anticipated from pre-event observations. Along-strike variations in coseismic and interseismic deformation reveal a helix-shaped fault geometry. The average coseismic slip equals or exceeds the interseismic slip deficit, consistent with a minimally slip-predictable model. We generate an ensemble of 3D dynamic rupture models, constrained by the best-fit static slip model, then discriminate among them using independent space-geodetic, seismic, and near-field video observations. The preferred scenario begins bilaterally, then transitions to unilateral southward supershear propagation at depth, while shallow rupture remains subshear. These dynamics are shaped by localized stress heterogeneity and low fracture energy, characteristic of a structurally mature fault zone. Our approach can inform rupture scenarios in understudied regions.