A new study suggests the southeastern United States will be seismically active for some time. Pictured, police tape ropes off the Washington National Cathedral after an earthquake hit the area on August 23, 2011 in Washington, D.C. Three pinnacle spires on the central tower (top left) fell off. A 5.9 magnitude earthquake centered in Virginia forced evacuations in the nation's Capital. File photo by Pat Benic/UPI | License Photo
CHAPEL HILL, N.C., May 3 (UPI) -- The southeastern United States isn't known for its seismic activity, but the region does experience the odd earthquake.
A new study -- published in the Journal of Geophysical Research: Solid Earth -- offers an explanation for the phenomenon.
Most earthquakes occur near plate boundaries and major fault lines. The southeastern corner of the United States lies in the middle of the North American Plate, far from the seismically active margins.
However, new analysis suggests the plate's interior continues to lose bits and pieces of its mantle. As fragments break off and sink toward Earth's interior, the mantle is left thinner and more brittle -- and more susceptible to the fault slipping that triggers earthquakes.
Geologists at the University of North Carolina developed their explanation of earthquakes in the Southeast after creating a detailed map of the mantle that lies beneath the region. Researchers built the 3D mantle map by tracing the path of underground seismic waves.
The waves revealed a mantle of varying thickness, a surprise.
"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 UNC and lead author of the new study, said in a news release.
The unevenness of the mantle suggests the plate -- scarred by ancient rifting and accretion -- continues to lose fragments to gravity. Old, dense portions of the mantle get pulled down into the underlying asthenosphere. Newer rock forms in their place as molten rock from the asthenosphere rises up to fill the void and cools.
The addition and subtraction leaves the mantle brittle and makes ancient, deep-lying fault lines within the interior of of the North American Plate more susceptible to slips.
"Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate," added Biryol. "The [seismic] zones that are active will continue to be active for some time."
The authors say the new research will help geologists better understand interior earthquakes and the dangers they present.