The “back-propagating rupture” was detected by underwater seismometers that monitor a fracture zone near the Mid-Atlantic Ridge. It can be difficult to understand the exact mechanics of earthquakes on land, where fault networks are complex. Underwater faults tend to be more straightforward, which is why researchers wanted to study the nearly straight Romanche Fracture Zone, which bisects the Mid-Atlantic Ridge.

In August 2016, just months after the monitors were installed, researchers detected a 7.1-magnitude quake there. The quake traveled upward and eastward along the straight line of the fault, then switched direction.

As it went back westward toward its origin, it generated so many powerful seismic waves that it became the earthquake version of a sonic boom, crossing the threshold of what geologists call a supershear event. At its top speed, the earthquake traveled at about 11,000 mph.

Geologists hope to use the observations to help find similar quakes — and determine if other quakes, like the handful of fast ones thought to have reached supershear status, share those elements.

Although it isn’t clear how often boomerang quakes occur, knowing that they do could help researchers learn how to detect on land and to predict and warn about supershear earthquakes and other temblors. Direction-switching quakes may still be rare — but now that researchers have spotted one, the door is open to a more complete understanding of how Earth shimmies and shakes.