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The magnitude 7.7 earthquake in Myanmar on 28 March 2025 caused widespread damage and over 3,800 fatalities, and also resulted in strong shaking and a building collapse in Bangkok, more than 1,000 km away. Preliminary analysis soon after the earthquake pointed to the unusually fast rupture velocity, which is known as a supershear rupture.

Now, a recent study by Felipe Vera from GFZ Helmholtz Center for Geosciences and co-authors confirms this finding and combines multiple methods to shed further light on the rupture process of this earthquake. According to the results, the Myanmar earthquake showed the highest rupture velocity worldwide in more than 20 years. The study is in the journal The Seismic Record.

The comparison of optical satellite images before and after the earthquake allows a reconstruction of how far the ground has moved laterally on both sides of the fault, and similarly, comparison of before and after satellite radar images allows a reconstruction of the vertical shift. These images show that the rupture occurred along the Sagaing fault over a distance of 500 kilometers, with the western side moving to the north, and the eastern side moving south for a maximum offset of 5 meters near the epicenter, which was near the city of Mandalay.

Frederik Tilmann, one of the study's authors and head of GFZ's Seismology section, says, "Whereas satellite image analysis can give us a static view of the earthquake effects on the ground, the team used seismic stations in Europe, Japan, Australia and Alaska as seismic antennas to analyze the dynamics of the rupture propagation. The signals allowed us to track where the tip of the rupture is located at any moment in time."

In the vast majority of earthquakes, the rupture propagates at velocities less than about 3.5 km/s (roughly 12,500 kilometers per hour), which corresponds to velocities a little less than the velocity of seismic shear waves. And indeed, during the initial phase, the rupture propagated at the same time to the north and south, away from the epicenter at usual speeds.

But after about 30 seconds, the rupture to the north arrested, and the rupture to the south accelerated to velocities of at least 5.3 km/s (almost 20,000 km/h). Seismologists call this a supershear rupture, the seismic equivalent of supersonic motion. This makes it probably the fastest recorded earthquake since 2002, when an earthquake in Alaska reached similar speeds. The total duration of the rupture was about 80 seconds.

Supershear propagation gives rise to a phenomenon called the Mach cone; for stations placed within its triangular outline on the surface, seismic waves from the whole 40 to 50 seconds long supershear-phase of the rupture arrive all at once. The study could demonstrate the existence of a Mach cone, which provides further evidence for the supershear propagation. This phenomenon might have enhanced long period ground motion in Bangkok.

The GFZ, in cooperation with Myanmar's Department of Hydrology and Meteorology, operates a seismic station in Nay Pyi Taw, the capital of Myanmar, only 2 km from the fault. This station was active during the Myanmar earthquake and provided a remarkable and extremely rare near-fault recording of a supershear rupture. The recordings have been introduced in a recent data publication by Ssung-Ting Lai and co-authors (Earth System Science Data; in press). The station jumped by 160 cm to the north when the rupture passed nearby; this 'jump' took less than 2 seconds.

It is noteworthy that the Sagaing fault was identified as prone to supershear rupture in earlier studies due to the fact that it is the longest straight strike-slip fault; strike-slip faults are faults where both sides move mostly laterally with respect to each other and the only ones able to sustain supershear rupture. It is also noteworthy that the early slower part of the rupture occurred along portions of the fault that had ruptured in large earthquakes in 1946 and 1956, whereas the acceleration to supershear occurred in the so-called Sagaing gap, which had not seen any very large earthquakes in more than a hundred years and was thus presumably highly stressed.

Given the large size of the earthquake, the number and magnitude of aftershocks was very small, which is another hallmark of supershear ruptures: Due to the smooth propagation, the elastic stress built up over decades is rather evenly relaxed, whereas in typical earthquakes irregularities in the fault slip often leave behind patches of high stress, which then break in aftershocks.