Sediment Yielding Amplifies Delocalization of Fault Deformation in the 2025Myanmar Earthquake

Global observations reveal faults respond to earthquake ruptures through localized on-fault slip and distributed off-fault deformation (OFD). Deformation becomes increasingly delocalized along faults that are immature or geometrically complex, rupture slowly, or propagate through sediment-rich regions. However, the physical processes by which sediments control this delocalization remain largely unresolved. Here we utilize high-resolution optical imagery to characterize surface rupture and OFD of the 2025 Mw 7.8 Myanmar earthquake, a supershear rupture event on a mature fault topping with thick sediments including the Quaternary alluvium and Irrawaddy Formation. Our results show averaging 32% OFD, far exceeding 13-19% expectation from global observations of such mature faults with simple geometry and supershear rupture speeds. Sediment-rich terrains along this rupture significantly amplify OFD to ~31-42%, nearly double the 19% observed in bedrock, and generate two highly diffused deformation sections lacking clear surface rupture. Dynamic rupture simulations incorporating variations in shear-wave velocity and frictional properties reproduce the observed OFD spectrum (from localized to fully delocalized deformation, revealing that plastic yielding of sediments dramatically delocalizes fault strain in the uppermost few hundreds of meters. We suggest such process should be integrated into models of shallow faulting and seismic hazard assessment in sediment-rich regions worldwide.