Mainshock Rupture Properties, Aftershock Activities and Remotely Triggered Seismicity Associated with the 2025 Mw7.7 Sagaing Fault earthquake in Myanmar

A devastating Mw 7.7 earthquake struck near Mandalay, Myanmar, on March 28, 2025, causing extensive damage and casualties across Myanmar and neighboring regions. The 2025 event occurred in a well-recognized seismic gap along the Sagaing Fault. Here we focus on the mainshock rupture properties based on back-projection of teleseismic P waves and early aftershock locations, analysis of near-field seismic recordings for the mainshock initiation, and remotely triggered seismicity following the Mw7.7 mainshock. We find that the ~500 km mainshock rupture can be revealed by both rapid back-projection of teleseismic P waves from multiple broadband arrays and early aftershock locations within about 3 hours from the Thai Meteorological Department (TMD) catalog. The rupture speed went supershear in the southward propagation after the initial bilateral subshear ruptures, as expected for large strike-slip earthquakes of such sizes. Clear fault zone head waves that are reflected along a bilateral fault interface are observed at the only near-fault station GE.NPW on the slower side about 2.6 km away from the Sagaing fault, consistent with the preferred direction of a supershear rupture propagating to the south. In addition, aftershocks from the regional TMD catalog appear to be located mostly to the east of the mainshock rupture. While we cannot completely rule out mis-locations from the one-sided station distribution, these off-fault seismicity could also be explained by reactivations of subsidiary faults within the Shan Plateau, or an eastward dipping of the mainshock rupture plane. Although no immediate foreshocks were found from several nearby stations, we identify one sub-event with magnitude ~6 at the beginning of the mainshock with a slightly different focal mechanism about 20-30 km south of the hypocenter determined by the United States Geological Survey (USGS). The mainshock also occurred when the tidal stresses reached its maximum on the right-lateral strike-slip fault, likely indicating that the timing of the mainshock is modulated by the solid earth tides. We find a significant increase of seismic activity near the Thailand/Myanmar border, in multiple (geothermally active) regions of Yunnan province in Southwest China, as well as the Xingfengjian reservoir in the Guangdong province in South China. Because static stress changes from the mainshock are small but negative near the Thailand/Myanmar border, the occurrence of microseismicity in this and other regions can be mainly explained by remote triggering from dynamic stress changes of the mainshock rupture. Our analyses demonstrate the importance of rapid analysis on openly available seismic data and catalog to better understand the rupture properties and triggered seismicity following large earthquakes.