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.

The new findings could help scientists better understand the dangers these earthquakes present.

Today, the southeastern U.S. is more than 1,056 miles from the nearest edge of the North American Plate, which covers all of North America, Greenland and parts of the Atlantic and Arctic oceans.

But the region was built over the past billion years by periods of accretion, when new material is added to a plate, and rifting, when plates split apart.

Researchers initially suspected ancient fault lines or pieces of old plates extending deep in the mantle following episodes of accretion and rifting could be responsible for earthquakes in the area.

'This region has not been active for a long time,' Biryol said.

WHAT ARE TECTONIC PLATES?

Tectonic plates are composed of Earth's crust and the uppermost portion of the mantle.

Below is the asthenosphere: the warm, viscous conveyor belt of rock on which tectonic plates ride.

Earthquakes typically occur at the boundaries of tectonic plates, where one plate dips below another, thrusts another upward, or where plate edges scrape alongside each other.

The world's tectonic plates: Earthquakes typically occur at the boundaries of tectonic plates, where one plate dips below another, thrusts another upward, or where plate edges scrape alongside each other.

Earthquakes rarely occur in the middle of plates, but they can happen when ancient faults or rifts far below the surface reactivate.

These areas are relatively weak compared to the surrounding plate, and can easily slip and cause an earthquake.

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.

Shaking from the magnitude 5.8 earthquake near Mineral, Virginia on August 23, 2011 was felt by more people than any other earthquake in U.S. history, according to the U.S. Geological Survey. Researchers now believe the quakes could be caused by pieces of the Earth's mantle breaking off and sinking into the planet.

But the shapes and locations of the thick and thin regions suggested a different explanation: through past rifting and accretion, areas of the North American Plate have become more dense and were pulled downward into the mantle through gravity.

At certain times, the densest parts broke off from the plate and sank into the warm asthenosphere below.

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 in the images.

Volcanoes were once active in the southeastern. U.S. Mole Hill, pictured here, is a mound of volcanic rock in the Shenandoah Valley in Virginia that formed from an active volcano 48 million years ago (a relatively recent event, in geological time scales).

The researchers concluded this process is likely what causes earthquakes in this otherwise stable region: when the pieces of the mantle break off, the plate above them becomes thinner and more prone to slip along ancient fault lines.

Typically, the thicker the plate, the stronger it is, and the less likely to produce earthquakes.

According to Biryol, pieces of the mantle have most likely been breaking off from underneath the plate since at least 65 million years ago.

Because the researchers found fragments of hard rocks at shallow depths, this process is still ongoing and likely to continue into the future, potentially leading to more earthquakes in the region, he said.

HOW THEY LOOKED INSIDE THE EARTH

To find out what was happening deep below the surface, the researchers created 3D images of the mantle portion of the North American Plate.

Just as doctors image internal organs by tracing the paths of x-rays through human bodies, seismologists image the interior of the Earth by tracing the paths of seismic waves created by earthquakes as they move through the ground.

These waves travel faster through colder, stiffer, denser rocks and slower through warmer, more elastic rocks.

Rocks cool and harden as they age, so the faster seismic waves travel, the older the rocks.

The researchers used tremors caused by earthquakes more than 2,200 miles away to create a 3D map of the mantle underlying the U.S. east of the Mississippi River and south of the Ohio River.

They found plate thickness in the southeast U.S. to be fairly uneven – they saw thick areas of dense, older rock stretching downward and thin areas of less dense, younger rock.

Read more: http://www.dailymail.co.uk/sciencetech/article-3573759/Giant-chunks-Earth-s-mantle-FALLING-causing-quakes-southeastern-coming-warn-researchers.html#ixzz47nsKUnEB
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Giant wildfire turns Canadian oil country into a post-apocalyptic nightmare

A massive wildfire is destroying everything in its path in a remote town located in the region of Canada that holds the third-largest oil reserves in the world. Nearly 90,000 residents of Fort McMurray have fled the flames, which have already consumed entire neighborhoods.

As 19-year-old resident Cassie White fled, she saw the gas station explode. “It almost looks like a zombie apocalypse,” she told The Globe and Mail. “At the time, I didn’t know if I was going to make it out … I felt like I was in a vacuum bag and all the air was being sucked out.”

While homes burn, the fire has left the nearby Athabasca oil sands unscathed. That could change; Fort McMurray Fire Chief Darby Allen called the fire a “moving animal.”

The massive fire in Fort McMurray comes unusually early in the year. It’s an increasingly common occurrence thanks to climate change, which is lengthening the wildfire season and increasing fires’ intensity. Last year’s wildfires were among the most damaging in history. If this early May fire is any indication, that is sure to continue this year. Perhaps it is fitting that Shell, a company directly responsible for a great degree of climate change, has been forced to shut down operations in the area. More oil companies are expected to follow.

See scenes from the conflagration below.

rabbleca Hace 22 horas

Coral reefs are straight-up dissolving now

Florida’s coral reefs are disintegrating much faster than expected. And who’s to blame? Oh, you know, just the ENTIRE OCEAN.

Ocean water is growing increasingly acidic as it absorbs the extra CO2 we’re pumping into the atmosphere, and now that water is eating away at the limestone foundations of coral reefs. A new study found that in the northern section of the Florida Keys’ reef — the third largest barrier reef ecosystem in the world — 6 million tons of limestone have disappeared over the past six years.

This wasn’t unexpected. It’s just that scientists had predicted the reef’s “tipping point,” where coral development is so severely limited by ocean acidification that reefs erode, was a good 40 years off — not today.

Ocean acidification is different from coral bleaching, another threat to reefs, though both have a common cause (climate change) and a common effect (dying corals). We’re looking to our most resilient corals to survive the challenges of living in today’s oceans.

We hope we never have to hold a farewell party for Florida’s coral reef, but ocean acidification is spurring along that unsavory outcome. Here’s to hoping we can usher this uninvited guest out of our coral reefs before it’s too late.

Watch our video on ocean acidification to learn more: