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Dramatic CCTV video of fault slip during a large earthquake in Myanmar thrilled both scientists and casual observers when it was posted to YouTube. But it was on his fifth or sixth viewing, said geophysicist Jesse Kearse, that he spotted something even more exciting.

When Kearse and his colleague Yoshihiro Kaneko at Kyoto University analyzed the video more carefully, they concluded that it had captured the first direct visual evidence of curved fault slip.

Earthquake geologists often observe curved slickenlines, the scrape marks created by blocks of rock moving past each other during faulting. But until now there has been no visual proof of the curved slip that might create these slickenlines.

The video confirmation of curved fault slip can help researchers create better dynamic models of how faults rupture, Kearse and Kaneko conclude in their paper in The Seismic Record.

The video comes from a CCTV security camera recording along the trace of Myanmar's Sagaing Fault, which ruptured 28 March in a magnitude 7.7 earthquake. The camera was placed about 20 meters to the east of the fault and was 120 kilometers away from the earthquake's hypocenter.

The resulting video is astonishing. A fault in motion as never seen before—shaking followed by a visible northward slide of the land on the western side of the fault.

"I saw this on YouTube an hour or two after it was uploaded, and it sent chills down my spine straight away," Kearse recalled. "It shows something that I think every earthquake scientist has been desperate to see, and it was just right there, so very exciting."

Watching it over and over again, he noticed something else.

"Instead of things moving straight across the video screen, they moved along a curved path that has a convexity downwards, which instantly started bells ringing in my head," Kearse said, "because some of my previous research has been specifically on curvature of fault slip, but from the geological record."

Kearse had studied curved slickenlines associated with other earthquakes, such as the 2016 magnitude 7.8 Kaikoura earthquake in New Zealand, and their implications for understanding how faults rupture.

With the Myanmar video, "we set about to quantify the movement a bit more carefully, to extract objective quantitative information from the video rather than just pointing at it to say, look, it's curved," he said.

The researchers decided to track the movement of objects in the video by pixel cross correlation, frame by frame. The analysis helped them measure the rate and direction of fault motion during the earthquake.

They conclude that the fault slipped 2.5 meters for roughly 1.3 seconds, at a peak velocity of about 3.2 meters per second. This shows that the earthquake was pulse-like, which is a major discovery and confirms previous inferences made from seismic waveforms of . In addition, most of the fault motion is strike-slip, with a brief dip-slip component.

The slip curves rapidly at first, as it accelerates to top velocity, then remains linear as the slip slows down, the researchers found.

The pattern fits with what earthquake scientists had previously proposed about slip curvature, which might occur in part because stresses on the fault near the ground surface are relatively low.

"The dynamic stresses of the earthquake as it's approaching and begins to rupture the fault near the ground surface are able to induce an obliquity to the fault movement," said Kearse.

"These transient stresses push the fault off its intended course initially, and then it catches itself and does what it's supposed to do, after that."

The researchers previously concluded that the type of slip curvature—whether it curves in one direction, or in the other—is dependent on the direction that the rupture travels, and is consistent with the north to south rupture of the Myanmar earthquake.

This means that slickenlines can record the dynamics of past earthquakes, which can be useful for understanding future seismic risks.