Asperities on the Sagaing Fault and 2025 Mw 7.7 Mandalay, Myanmar Earthquake

ABSTRACT An earthquake of Mw 7.7 struck the second largest city of Mandalay in central Myanmar on 28 March 2025 at 11:50:52 am. The focal mechanism showed that the earthquake occurred on the N-S striking, steeply dipping right-lateral Sagaing fault at a shallow depth of ~10 km. Fifty-eight aftershocks of 6.7≤M≥3.0 occurred in three clusters over a ~430 km long segment of the Sagaing fault till 7 June 2025. Based on broadband and strong motion stations data, the USGS suggested a ~460 km long rupture length at supershear speed. This agrees with strong shaking and severe damage to infrastructure and residential buildings in the capital city of Naypyitaw, 250 km south of the epicentre and beyond. In this paper, we propose a geological model to show that the mainshock slip nucleated in a geometrically complex asperity associated with the intersection of a sharp 90° westerly turn of the N-S flowing Irrawaddy River with the Sagaing fault at Mandalay city, where several transpressional features dominate the topography. Though the initial rupture was bidirectional, it propagated southwards at supershear speeds and jumped over a fault step-over width at the restraining bend on the Sagaing fault south of Mandalay and continued further through the releasing bend between Naypyitaw and Pyu. The supershear earthquakes have the potential to transfer large stresses and generate strong ground motion at far-field distances, as seen in Bangkok at >1000 km. Eleven aftershocks of 5.3≤M≥3.4 that occurred in Cluster 2 south of Mandalay suggest seismic activation of the dormant Meiktila segment of the ~1200 km highly segmented Sagaing fault.