Co-seismic and Post-seismic Seismic Velocity Changes Along the Sagaing Fault Following the 28 March 2025 Mw 7.7 Myanmar Earthquake: Insights from Ambient Noise Interferometry and Volumetric Strain Modelling

The Earth is not a static, unchanging rock; it is a living, breathing system that constantly shifts and adjusts. When a massive earthquake strikes, it does more than just shake the ground and topple buildings. It sends a shockwave through the very fabric of the crust, cracking tiny fractures in the rock and rearranging the pressure of fluids deep underground. Scientists have long known that these violent events change the speed at which seismic waves travel through the ground. Think of the Earth's crust like a sponge: when it is squeezed or cracked, the way sound moves through it changes. By listening to the constant, low-level rumbling of the planet—noise generated by ocean waves and wind hitting the shore—researchers can measure these subtle shifts in speed. This technique allows them to see how the ground is damaged during an earthquake and how slowly it begins to heal afterward, offering a window into the hidden mechanical processes that govern our planet's stability.

On March 28, 2025, a powerful earthquake of magnitude 7.7 struck central Myanmar, tearing along the Sagaing Fault, a massive geological boundary where two tectonic plates grind past one another. The rupture stretched for nearly 500 kilometers, causing widespread devastation in cities like Mandalay. In the wake of this disaster, a team of researchers turned their attention to the ground itself, using a network of four sensitive listening stations to monitor how the earthquake altered the speed of seismic waves. By analyzing continuous recordings of ambient noise from March through April 2025, they were able to track the immediate damage and the subsequent recovery of the crust. Their findings reveal a clear story of destruction and partial healing, showing that the ground closest to the fault suffered the most severe damage, while areas farther away experienced much milder effects.

The researchers observed a dramatic drop in seismic velocity immediately after the mainshock. At the station closest to the fault, the speed of the waves slowed down by nearly 2 percent, a significant change that indicates the rock had been fractured and weakened. As they looked at stations located farther away, the effect became progressively smaller. At a distance of about 140 kilometers, the slowdown was roughly 0.6 percent, and at the farthest station, over 360 kilometers away, the change was barely noticeable. This pattern confirms that the physical damage to the crust is concentrated near the fault line, where the shaking is most intense. The ground essentially became more porous and cracked, allowing the seismic waves to travel more slowly, much like sound traveling through a broken wall versus a solid one.

What makes this study particularly insightful is how the researchers connected these velocity changes to the specific way the ground was stretched or squeezed during the quake. They modeled the static strain, which is the permanent stretching or compressing of the Earth's crust caused by the fault slipping. They found that the station with the largest drop in velocity sat in a zone where the ground was being pulled apart, or dilated. This stretching likely opened up existing cracks and created new ones, amplifying the damage. In contrast, the stations farther away were located in zones where the ground was being slightly compressed. This compression acted like a clamp, holding the rock together and preventing some of the damage that the shaking would have otherwise caused. This suggests that while the violent shaking of the earthquake creates the initial cracks everywhere, the underlying stress field of the fault determines exactly how severe the damage becomes in any given spot.

Following the initial shock, the ground began a slow process of recovery. The seismic velocity started to climb back up as the tiny cracks in the rock began to close and the pressure within the crust stabilized. However, this healing was incomplete. Even a month after the earthquake, the ground had not returned to its pre-quake state, particularly near the fault. The researchers noted that the recovery was not a smooth, steady climb; it was interrupted by small dips whenever strong aftershocks occurred. This indicates that the crust is still adjusting, with new stress being released and old cracks being re-opened. The data also ruled out other potential causes for these changes, such as heavy rainfall or shifts in atmospheric pressure, confirming that the observed effects were purely the result of the earthquake's mechanical impact on the rock.

The study concludes that the Earth's response to a major earthquake is a complex interplay between the immediate violence of the shaking and the slower, permanent changes in stress. The most severe damage occurs where the ground is stretched, and the recovery is a gradual process of healing that can take a long time. By listening to the Earth's constant hum, scientists can now map these invisible injuries and understand how the planet repairs itself. This knowledge is crucial for understanding the long-term risks in earthquake-prone regions, reminding us that even after the shaking stops, the ground beneath our feet continues to change and adapt.