© Johan Swanepoel/ShutterstockScientists suspect that chunks from the bottom of the North American tectonic plate, which is the upper portion of the mantle, are peeling off and sinking. Replacing the resulting void is gooey material from the asthenosphere.
This seismicity — or relatively frequent earthquakes — may be the result of areas along the bottom of the North American tectonic plate peeling off, the researchers said. And this peeling motion is likely to continue, leading to more earthquakes in the future, like the 2011 magnitude-5.8 temblor that shook the nation's capital.
To figure out the cause of these earthquakes, Berk Biryol, a seismologist at UNC Chapel Hill, and colleagues created 3D images of the uppermost part of Earth's mantle, which is just below the crust and comprises the bottom of a tectonic plate. These tectonic plates scoot around atop a layer of warm, viscous fluid called the asthenosphere.
The resulting X-ray images revealed that the plate's thickness in the southeast United States was uneven, with thick regions of dense, old rock combined with thinner areas composed of younger rocks that were also less dense.
