An odd phenomenon may explain why the Southeastern United States has experienced recent earthquakes, even though the region sits snugly in the middle of a tectonic plate and not at the edges, where all the ground-shaking action usually happens.

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, likethe 2011 magnitude-5.8 temblor that shook the nation's capital. [Image Gallery: This Millennium's Destructive Earthquakes]

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.

Here's what the researchers think caused the wonkiness: Over time, as new material was added to the plate and parts of the plate were pulled apart, areas of higher density formed. Gravity would have pulled down the denser areas into the mantle, and at some point chunks would have broken off to sink into the gooey asthenosphere below, the researchers speculated.

At the same time, to fill the void left by the chunks peeling off from the bottom of the plate, the lighter material in the asthenosphere would have moved up to fill in the space. That buoyant material then would have cooled to become the thinner, younger section of the plate.

Where parts of the plate broke off or peeled, it became thinner and more prone to slip along fault lines, thus causing seismic activity. Biryol estimates that this activity has occurred for the past 65 million years or so.

"These events, usually, when they occur, they occur over long periods of time. The geological time scale is millions of years," Biryol said.

While the research looks only at what has occurred in the past, and not what seismic activity there may be in the future, Biryol said people living in the Southeast don't need earthquake preparedness kits just yet.

"I don't think things will be changing in the future, at least not in our lifetimes or our grandchildren's grandchildren's life," Biryol told Live Science. "Geological processes take place over long periods of time and nothing will change dramatically overnight."

Follow Kacey Deamer @KaceyDeamer. Follow Live Science @livescience, on Facebook & Google+. Original article on Live Science.

By Noah Barkin

BERLIN (Reuters) - German authorities are investigating 40 cases in which Islamic militants are suspected of having entered the country with the recent flood of refugees from the Middle East, the federal police said on Wednesday.

That represents a doubling of such cases since January and is likely to deepen concerns about the threat level in Germany, which has not suffered a large-scale Islamist attack like those that have rocked neighbours France and Belgium in recent months.

In the past, the German government has played down the risks of Islamic State fighters entering Europe with the tide of migrants, in part to avoid exacerbating public concerns about the influx, which hit a record 1.1 million last year.

But the head of Germany's domestic intelligence agency Hans-Georg Maassen told a conference last week that although there were more efficient ways to smuggle in fighters, Islamic State appeared to have sent some via the Balkan route from Greece in order to fan fears about refugees and "send a political signal".

"I am not telling you a secret when I say that I am concerned about the high number of migrants whose identities we don't know because they had no papers when they entered the country," Maassen said.

PARIS ATTACKS

The number of migrants entering Germany reached peaks of more than 10,000 a day last autumn, but has fallen dramatically in recent months due to the closing of the Greek border with Macedonia and a deal between the European Union and Turkey that has discouraged refugees from crossing the Aegean Sea.

The reduction in the numbers has eased pressure on German Chancellor Angela Merkel, who came under fierce criticism last year for welcoming hundreds of thousands of migrants fleeing war in the Middle East with the optimistic slogan "We can do this".

A spokeswoman for the Bundeskriminalamt (BKA), or federal police, said there had been 369 warnings about possible extremists entering the country since the influx of refugees accelerated last year, of which 40 merited further investigation by federal and state authorities.

That represents a sharp increase from the 213 warnings and 18 investigations that the police had recorded in early January.

"German security officials have indications that members and supporters of terrorist organisations are being smuggled in with refugees in a targeted, organised way in order to launch attacks in Germany," the BKA spokeswoman said, noting however that there was no definitive confirmation of this.

Two of the suicide bombers from the Nov. 13 attacks in Paris that killed 130 people came into Europe through the Balkan route and so did two men who authorities believe were meant to participate in those attacks but were delayed and arrested in a refugee centre in Salzburg in December.

There is also evidence that Saleh Abdeslam, believed to be the lone surviving suspect from the attacks, picked up three unidentified militants who entered Europe with the refugees in the southern German city of Ulm in October of last year.

In early February, German authorities arrested a 35-year-old Algerian man and his wife at a refugee centre in the town of Attendorn. The man, a suspected Islamic State member, reportedly posed as a Syrian when he entered Germany in the autumn of 2015.

Days later, a 32-year-old man was arrested in the city of Mainz who is suspected of having fought with the militant group in eastern Syria before travelling to Germany via Turkey.

(Editing by Ralph Boulton)