Supershear rupture sustained through a thick fault zone in the 2025 M w 7.8 Mandalay earthquake

On 28 March 2025, a moment magnitude ( M w ) 7.8 earthquake ruptured a seismic gap along the Sagaing fault in Myanmar, generating a surface rupture 480 km in length and causing widespread damage. With holistic geodetic and seismic techniques, we resolved its rupture dynamics and fault zone structure. The rupture initiated as bilateral subshear and transitioned to supershear (~5.3 km/s) about 100 km south of the epicenter, sustaining this velocity for more than 200 km. The supershear segment aligns with a ~2-km–thick low-velocity fault zone exhibiting ~45% shear wave speed reduction. We suggest that the thick fault zone, aided by fault geometry and basin structure, enabled prolonged supershear propagation. Our findings emphasize that observations and models encompassing the entire fault zone are crucial for advancing both specific-event and earthquake-cycle simulations.