When it comes to earthquakes in the U.S., all eyes tend to be on the San Andreas fault network in California. However, back in 2011, there was an unusually powerful earthquake in Virginia, which is all the way over on the other side of the contiguous United States. The fact that a magnitude 5.8 quake happened at all in this region is quite bizarre; it’s not near any tectonic plate boundaries where tremors are generated.
This seismic conundrum has utterly baffled scientists, but a new study, published in the Journal of Geophysical Research – Solid Earth, may have solved the mystery once and for all. It notes that pieces of the mantle beneath this region are fragmenting and sinking down into the fiery depths of the Earth. Over time, this weakens the overall strength of the plate, making it thinner and prone to bending and slipping, which can generate sizable earthquakes.
“Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate,” Berk Biryol, a seismologist at the University of North Carolina Chapel Hill and lead author of the new study, said in a statement. “The [seismic] zones that are active will continue to be active for some time.”
As you can see, the entire eastern seaboard of the U.S. is nowhere near a plate boundary. Alataristarion/Wikimedia Commons; CC BY-SA 4.0
Earthquakes occur in a range of environments, but they almost always require two things: tectonic plate boundaries and processes that build up stress. Earthquakes do sometimes happen away from plate boundaries, but they tend to be less severe and somewhat harder to explain. The recent seismic activity in the southeastern U.S. is a perfect example of this, so a team of researchers decided to peer beneath the crust and find out what’s going on in the mantle, the partly molten, violently churning layer of the planet.
Tectonic plates, when sliding under one another in subduction zones, can sometimes break into pieces. One of these broken plates has recently been observed impeding the rise of a superheated plume beneath Yellowstone’s caldera. Tectonic plates can also break into pieces far from subduction zones, and this is what the researchers suspected was happening beneath Virginia.
The epicenter of the 2011 Virginia quake. USGS
Scientists use seismic waves, generated by earthquakes, to image what lies below. They travel at different velocities through different materials; the faster they travel, the more solid the material is. Rocks become increasingly solid as they cool, so the faster the seismic waves travel through them, the older the rocks are.
By looking at how seismic waves travel through the world to beneath the U.S., the team produced a 3D map of the mantle in the region. They found that the thickness of the plate beneath the southeastern U.S. is incredibly uneven, with denser older rocks curving downwards and thinner segments remaining relatively stationary.
A slice of the 3D map of the mantle. The blue regions marked with an F are parts of the tectonic plate being dragged downwards. S2 represents upwelling buoyant mantle creating a new plate section. Biryol et al./Journal of Geophysical Research – Solid Earth
It appears that these denser parts are tumbling down into the mantle simply because they’re so heavy. On occasion, the densest parts began to fragment into pieces. The warmer, more buoyant part of the mantle surged upwards to fill in the missing parts of the plate, which is where the thinner, younger bits of rock came from.
These thinner plates, being less stable, are more likely to slip alongside ancient fault lines, or buckle under pressure, causing moderately powerful earthquakes as they do so. This process of plate replacement has been ongoing for the last 65 million years, and is likely to continue into the future.