On Aug. 23, 2011, those living in eastern North America, from Ontario to Georgia, felt an unexpected shock as the earth trembled in the wake of a 5.8 magnitude earthquake that struck near the town of Mineral, Virginia, around 2 p.m. local time.
A notable quake with a magnitude of 4.0 or higher east of the Rockies is a rarity, according to USGS reports.
However, a recent study published in the Journal of Geophysical Research - Solid Earth is shedding light on the likely causes behind the event and may indicate that there are more to come.
Unlike earthquakes that occur near plate boundaries in the more seismically active regions of the world, the 2011 quake raised questions among researchers, and stirred alarm among those living in the Washington, D.C., area who felt its full force.
According to the American Geophysical Union, the journal's parent organization, researchers have discovered pieces of the mantle have been breaking off below the North American Plate in this region and sinking deeper into the earth.
"Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate," Berk Biryol told the AGU.
Biryol is a seismologist at the University of North Carolina Chapel Hill and lead author of the recently published study. According to Biryol's research, the geological processes the researchers found revealed thinning and weakening of the plate.
While most earthquakes tend to occur at subduction zones, or near plate boundaries, the processes causing the earthquakes in the middle of plates have often remained a mystery to scientists.
In order to figure out what was happening deep below the earth's surface, Biryol and the study's team had to use seismic waves generated by earthquakes as far away as 2,200 miles away to create a 3D map of the region by tracing the paths of the waves as they moved through the ground.
Virginia along with the rest of the North American continent, Greenland and portions of the Atlantic and Arctic oceans are all located on the vast North American Plate. The plate rides on thin, heated layer of viscous rock called the asthenosphere.
Biryol's study, which found varying, uneven plate thickness, and a mix of older rock and younger rock, may now help solve many of the mysteries behind tectonic plates' interactions with the asthenosphere.
"At certain times, the densest parts broke off from the plate and sank into the warm asthenosphere below," the AGU reported.
"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."
Coupled with the thinning and weakening of the plate, ancient fault lines long considered stable become more susceptible to slipping, which leads to earthquakes, according the AGU.
The new study unveils that there is much more going on deep beneath the earth's surface than scientists originally thought, and that pieces of the mantle have likely been breaking off under the plate for nearly 65 million years.
As the research on these occurrences continues, scientists may gain a better understanding on where these earthquakes will likely occur.
Biryol told the AGU that these seismic zones will remain active over time and will likely cause additional earthquakes in the future.