In a very interesting earthquake-related twist, southeastern United States may expect more ground shaking and tremors in the future as plates become weaker and more brittle.

One late afternoon of August 2011, a less-than-a-minute quake shook Virginia, driving hundreds of workers out of their building. The tremors were also felt across nearby states such as North and South Carolina, as well as Washington DC, whose parts of its national cathedral ended up being damaged.

The National Earthquake Information Center (NEIC) estimates at least 20,000 detected earthquakes each year, according to U.S. Geological Survey (USGS). But that of Virginia was rare: they have never felt something like it for the past 67 years.

This rarity may be attributed to the location of the southeastern U.S. in the North American Plate. It sits in the middle, several thousands of miles away from the edges, where most of the earthquake-causing tectonic activities usually happen.

The Study That Might Change The Pattern

A new geological study looked into the reasons for the odd earthquakes in the region and found out something startling: while the edges are not at fault, the mantle underneath it is.

"Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate," said Berk Biryol, lead author and seismologist at the University of North Carolina, Chapel Hill.

Through a "CAT scan," which included getting a 3D image of the mantle, the researchers discovered that there was a periodic breaking off of pieces of the mantle.

These pieces, on the other hand, break off as they become dense through years of accretion and rifting. The densest of these pieces then sink because of gravity, allowing parts of the asthenosphere, which lies underneath, to rise to fill up the space with molten rocks that eventually cool off.

"This was an interesting finding because everybody thought that this is a stable region, and we would expect regular plate thickness," Biryol said.

As this process continues, the plate in the region becomes thinner, making it susceptible to slipping in its very old fault lines.

Since the researchers continue to find hard rocks in the shallow parts of the mantle, they believe this event will continue, leading to more earthquakes in the future.

The study is now available in the Journal of Geophysical Research, Solid Earth.