We were intrigued by what was going on and how we can link these activities to structures in deeper parts of the Earth.'

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

At first, they thought the thick, old rocks could be remnants of ancient tectonic plates.

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Area hit by a series of strange unexplained quakes

Most recent was 2011 magnitude-5.8 earthquake near Mineral, Virginia

Area should be seismically quiet as it is in middle of Earth's tectonic plate

The southeastern United States has been hit by a series of strange unexplained quakes - most recently, the 2011 magnitude-5.8 earthquake near Mineral, Virginia that shook the nation's capital.

Researchers have been baffled, believing the areas should be relatively quiet in terms of seismic activity, as it is located in the interior of the North American Plate, far away from plate boundaries where earthquakes usually occur.

Now, they believe the quakes could be caused by pieces of the Earth's mantle breaking off and sinking into the planet.

A new study found pieces of the mantle under this region have been periodically breaking off and sinking down into the Earth.

This thins and weakens the remaining plate, making it more prone to slipping that causes earthquakes.

The study authors conclude this process is ongoing and likely to produce more earthquakes in the future.

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

'The [seismic] zones that are active will continue to be active for some time.'

The study was published in the Journal of Geophysical Research – Solid Earth, a journal of the American Geophysical Union.

'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 team wrote.

Compared to earthquakes near plate boundaries, earthquakes in the middle of plates are not well understood and the hazards they pose are difficult to quantify.