On August 23, 2011, a magnitude 5.8 earthquake hit Mineral, Virginia, about 90 miles southwest of Washington D.C. The temblor was the strongest east of the Mississippi since 1944, and was felt by more people than any other quake in U.S. history, reaching 12 states and several Canadian provinces. It shook the nation’s capital so strongly it cracked the Washington Monument and caused 20 million dollars in damage to the National Cathedral.

The earthquake was also a bit of a mystery. D.C. lies over 1,000 miles from the edge of the North American Plate, away from any fault zones or seismic activity. Earthquakes in the area are rare if not non-existent. But a new study in the Journal of Geophysical Research has a new theory of why the capital shook: Bits of the Earth’s mantle are breaking off underneath the southeast U.S. and sinking. This thins the remaining plate, making it more prone to slip and cause seismic activity.

“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 lead author of study tells Lauren Lipuma at the American Geophysical Union blog.

Biryol and his colleagues used seismic waves from earthquakes in other parts of the world to study the ground below the eastern United States. Seismic waves act almost like an X-ray, Lipuma explains, with the waves moving quickly through dense, cold rock and speeding up in magma and warmer rocks. Measuring these waves give researchers insight into what is below the Earth's surface.

The 3-D map produced by the study shows that the North American Plate east of the Mississippi and south of the Ohio River is very uneven, caused by chunks of the plate breaking off and sinking into the athenosphere, a viscous layer of the upper mantle.

“They learned that the southeastern US is a patchwork of thicker and thinner areas of crust and mantle, like a weatherbeaten road that’s been fractured and filled in over and over,” writes Maddie Stone at Gizmodo. “When a new chunk of the plate breaks off the bottom, it can trigger an earthquake.”

It’s not great news for D.C. or the southern U.S. in general. “Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate,” Biryol tells Lipuma. “The [seismic] zones that are active will continue to be active for some time.”