The North American Plate is shedding some layers.
If you understand the basics of earthquakes, you wouldn’t expect to hear about any activity in the southeastern United States. Why? Because it is located within the interior of the North American Plate, far away from any plate boundaries where earthquakes usually happen.
However, this area has recently experienced some substantial seismic events, with the most recent occurring in 2011 when a magnitude-5.8 earthquake struck near Mineral, Virginia. The cause of this earthquake has remained a pretty big mystery, but according to a new study, scientists have come up with a likely explanation for this strange activity.
It turns out that pieces of the mantle under this region have been periodically breaking off and sinking down into the Earth, thinning and weakening the remaining plate. This makes the region more prone to the slipping that causes earthquakes. In fact, the authors conclude that this process is ongoing and will likely 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, in a university news release. "The [seismic] zones that are active will continue to be active for some time."
Generally, earthquakes occur at the boundaries of tectonic plates — composed of Earth’s crust and the uppermost portion of the mantle — where one plate dips below another (normal fault), is pushed upward (reverse fault), or where plates scrape alongside each other (strike-slip fault). However, earthquakes can also occur in the middle of plates, which scientists previously attributed to the reactivation of ancient faults far below the surface.
"[The center of the North American Plate] has not been active for a long time," Biryol said. "We were intrigued by what was going on and how we can link these activities to structures in deeper parts of the Earth."
To figure out this mystery, the researchers created 3D images of the mantle portion of the North American Plate by tracing the path of seismic waves created by earthquakes as they move through the ground. These waves travel faster through colder, stiffer, and denser rocks, and slower through warmer, more elastic rocks. Since rocks cool and harden as they age, faster traveling seismic waves indicate that the rocks are older.
What the researchers discovered is that the plate thickness in the southeast US is fairly uneven — with thick areas of dense, older rock stretching downward, and then thin areas of less dense, younger rocks.
"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, the researchers thought the thick, old rocks could be remnants of ancient tectonic plates. However, their shapes and locations prompted them to consider a different explanation.
Through past rifting (splitting apart) and accretion (adding of new material), areas of the North American Plate had become more dense and were being pulled downward into the mantle through gravity. Every once and awhile, these dense sections break off from the plate and sink into the warm asthenosphere — what the mass of the continents rest on — below. But since the asthenosphere is lighter and more buoyant, it surges in to fill the void, eventually cooling to become thin, young rock. However, this thinning process is likely what causes earthquakes in an otherwise stable region.
According to Biryol, pieces of the mantle have likely been breaking off from underneath the plate since at least 65 million years ago, and this process could continue. However, the new findings, published in the Journal of Geophysical Research — Solid Earth, could help scientists better understand the phenomenon and the dangers these earthquakes present.
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