Nearly five years ago, a 5.8-magnitude earthquake struck Mineral, Virginia, jolting Washington D.C. and much of the eastern third of the United States. Scientists were baffled by the quake, which was the strongest in the region in 67 years and one of only a handful of notable earthquakes inside the North American Plate since 1737.

Most earthquakes take place at the boundaries of tectonic plates, not the middle; so-called intraplate earthquakes like the 2011 one are rarer, and their causes are not well understood. Now, the American Geophysical Union (AGU) reports that seismologists believe they’ve discovered the reason for the relatively recent tremors in the southeastern U.S.

According to a study published earlier this week in Journal of Geophysical Research – Solid Earth, the quakes that have taken place in the region over the years were likely the result of chunks of the mantle under the region descending into the Earth, which left the remaining plate more vulnerable to the slipping that causes earthquakes. The areas the researchers studied, in the southeastern U.S., are roughly 1056 miles from the nearest edges of the North American Plate.

Initially, the scientists expected to find evidence of ancient fault lines. Instead, they discovered a disparity in the thickness of the plates in the southeast U.S.; some parts of the plate were made of dense, older rock that stretched downward into the Earth, while others were made of lighter, younger rock. From the AGU:

" ... through past rifting and accretion, areas of the North American Plate have become more dense and were pulled downward into the mantle through gravity. At certain times, the densest parts broke off from the plate and sank into the warm asthenosphere below. The asthenosphere, being lighter and more buoyant, surged in to fill the void created by the missing pieces of mantle, eventually cooling to become the thin, young rock in the images."

The thinner and younger the plate, the more likely it is to slide along ancient fault lines, triggering an earthquake, they say.

“This was an interesting finding because everybody thought that this is a stable region, and we would expect regular plate thickness,” Berk Biryol, a seismologist at the University of North Carolina Chapel Hill and the lead author of the new study, told the AGU.

Biryol added that mantle pieces are likely still breaking off from the plate, a process which began an estimated 65 million years ago. The on-going geological phenomenon means that quakes and other seismic activity will probably continue in the region.

“Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate,” Biryol said. “The [seismic] zones that are active will continue to be active for some time.”