Our seismically chill nation’s capitol. Image: Nicolas Raymond/Flickr

Californians may scoff at the idea of a mid-sized earthquake, but when our nation’s capitol started shaking on August 23, 2011, people freaked out. Having grown up in the DC metro area, I can tell you why: we don’t get noticeable earthquakes. We are seismically boring and perfectly cool with it.

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But now, geologists have figured out a likely mechanism for the 2011 shakeup, which registered a magnitude of 5.8. And if they’re correct, it’ll mean more seismic events in the future. Basically, chunks of the North Atlantic plate are peeling off the bottom, sinking deep into the Earth and creating instabilities in the upper crust.

“Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate,” seismologist Berk Biryol, lead author on a new study describing the mechanism told AGU News. “The [seismic] zones that are active will continue to be active for some time.”

Epicenter of the 2011 East Coast earthquake. Image: USGS

Earthquakes normally occur at tectonic boundaries, where one plate thrusts beneath or slides alongside another. The East Coast is considered a “passive margin” because it’s in the center of the North American plate, far away from any dangerous points of contact that could prompt geologists to drop phrases like “locked and loaded.”

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So, when a magnitude 5.8 quake—the largest earthquake east of the Rockies since 1944—struck the town of Mineral, Virginia in 2011, it caused quite a stir. Biryol and his colleagues jumped on the opportunity to study the forces behind the unusual event.

In their new paper, the researchers used seismic activity from thousands of miles away to construct 3D maps of the bottom of the North American plate. 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. When a new chunk of the plate breaks off the bottom, it can trigger an earthquake.

This mechanism would not only explain the 2011 DC quake, but 19th century quakes in Charleston, South Carolina and New Madrid, Missouri, which had estimated magnitudes of 7 and 8.1, respectively. So, reality check, East Coasters: as long as we’re living on a 200 million-year-old scab of slow-peeling rock, occasional shakeups are going to be a part of life.

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[AGU News]

Correction 5/5/16: An earlier version of this post stated that the 2011 quake “registered a 5.8 on the Richter scale.” Silly blogger, geologists don’t use the Richter scale anymore. The moment magnitude of the quake was, in fact, 5.8. Learn more about the two scales here.