A massive March 2025 earthquake in Myanmar tore through nearly 500 kilometers of the Sagaing Fault at extremely high speeds. In a new study – part of a package of four research articles on seismic activity in Myanmar – researchers show that an unusually thick, low-velocity fault zone acted like a high-speed corridor, driving one of the fastest and longest continental ruptures ever recorded. The largest earthquakes that occur within continental crusts can rupture faults extending for hundreds of kilometers and pose significant seismic threats.  Many of these powerful events evolve into supershear ruptures – earthquakes in which the rupture front moves faster than the speed at which shear waves travel through the crust, making them especially powerful. This behavior often occurs on simple, straight faults like the San Andreas, North Anatolian, and Sagaing faults, which let energy concentrate instead of dispersing. During such quakes, parts of the crust around the fault weaken, forming damage zones that can affect how future earthquakes unfold. However, the relationship between fault zones and major strike-slip ruptures, and their influence on long-lasting supershear events, remains poorly understood.

According to Shengji Wei and colleagues, the March 28, 2025, magnitude 7.8 Mandalay earthquake in Myanmar offers new insight into these seismic dynamics. The Mandalay quake struck along the Sagaing Fault, breaking a 250-kilometer section that hadn’t ruptured in over a century, and produced a surface rupture exceeding 450 kilometers that reached supershear speeds. Using an interdisciplinary approach that combined satellite geodesy, broadband seismic data, receiver function imaging, and numerical simulations, Wei et al. reconstructed the earthquake’s 3D surface deformation, slip distribution, and rupture dynamics. They discovered that the rupture started in both directions from the epicenter and transitioned to supershear speeds (~5.3 km/s) about 100 kilometers to the south. This transition occurred along a roughly 2-kilometer-thick low-velocity fault zone, where shear-wave speeds were reduced by about 45%, coinciding with a shift in fault shape and structure. Wei et al. argue that it is these factors likely enabled the initiation and continuation of the supershear rupture and suggest that these events are more likely to occur along wide, simple strike-slip faults that have evolved through repeated seismic activity. Such insights could improve hazard assessments for major continental faults, such as California’s San Andreas and Türkiye’s North Anatolian faults, where similar conditions exist.