For centuries, southeastern United States has been regarded as relatively safe from earthquakes since it is far from plate boundaries where seismic activities occur.

Researchers, however, recently found out that this particular region in the U.S. is no longer as safe as it was previously thought. In fact, it has become more vulnerable to powerful quakes because the ground below it is reportedly sinking into the ground.

In a study published in the Journal of Geophysical Research—Solid Earth, the researchers said they found out that giant chunks of the Earth's mantle, or the middle layer of our planet, under the U.S. southeastern region have periodically been breaking off.

"Based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere, we propose that piecemeal delamination has occurred beneath the region throughout the Cenozoic, removing a significant amount of reworked/deformed mantle lithosphere," the study read, as quoted by The Daily Mail.

Berk Biryol, a seismologist at the University of North Carolina at Chapel Hill and lead author of the new study, explained that because of this, the remaining plate becomes weak and thinner, and therefore more vulnerable to slipping, which in turn causes earthquakes.

"Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate," Biryol also told The Daily Mail.

Worse, this process is still ongoing, according to the lead researcher.

"The [seismic] zones that are active will continue to be active for some time," Biryol added.

To complicate things even further, earthquakes in the middle of plates, which are likely to happen in southeastern U.S., have not yet been fully explored by geologists and are harder to quantify compared to those that occur near plate boundaries.

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