A new study from scientists at the University of North Carolina suggests that what caused the 2011 earthquake that rocked Mineral—and shook much of the East Coast—is likely to produce more tremors in the future.
But before anyone worries that central Virginia is ready to rumble along the lines of Southern California, take heart.
The report also says the activity has been going on since the days of dinosaurs.
For about 65 million years, pieces of the Earth’s mantle under the southeastern United States have broken off and sunk farther underground. That’s caused the remaining tectonic plate to weaken, shift and produce earthquakes—or so believes Berk Biryol, seismologist at the University of North Carolina at Chapel Hill and lead author of the study.
“This has been going on a long time, based on what I observed from the region,” said Biryol, who studied earthquakes from 1900 to 2015. “I don’t think there will be a change in pattern or in activity.”
In August 2011, a magnitude-5.8 earthquake hit the town of Mineral in Louisa County. It surprised that community, as well as the Fredericksburg region—and the East Coast at large.
While tremors were reported as far north as Canada, the epicenter in Mineral bore the brunt of the damage. Two schools—Louisa County High School and Thomas Jefferson Elementary School—had to be rebuilt.
The Federal Emergency Management Agency awarded the county $19 million to help with school construction and another $13 million in disaster aid to homeowners in eight surrounding counties and the city of Fredericksburg.
Biryol and other scientists undertook the study because Virginia “should be relatively quiet in terms of seismic activity,” according to a news release from the university. It’s in the interior of the North American plate, far away from plate boundaries where earthquakes usually occur.
To find out what was happening beneath the surface, the researchers created 3D images of the Earth’s mantle—the part between the core and the crust. In the southeastern part of America, the plate thickness was uneven, Biryol said.
Some areas of older rock were dense and stretched downward toward thinner and younger stretches of rock.
“This was an interesting finding because everybody thought that this is a stable region, and we would expect regular plate thickness,” Biryol said.
The researchers concluded that when pieces of the mantle break off, the plate above them becomes thinner and more prone to slipping along ancient fault lines.
“I think that’s an interesting preliminary model,” said Grant Woodwell, a geology professor at the University of Mary Washington.
He said it’s believed that the same process has caused earthquakes in other parts of the world.
But as far as what this assumption means for increased seismic activity in Virginia and throughout the East Coast, Woodwell takes a step back.
He pointed out that 90 percent of earthquakes take place near the boundary of active tectonic plates. Virginia is nowhere near one of those.
On the other hand, the San Andreas fault is atop one. It stretches about 800 miles through California and forms a tectonic boundary between two major plates of the Earth—the Pacific Plate and the North American Plate.
“There’s a dramatic difference in activity” here and there, Woodwell said.
To put it in perspective, the U.S. Geological Survey has recorded fewer than 300 earthquakes in Virginia since 1973.
“You can have a thousand in a single month in Southern California,” Woodwell said.
They’re not all movers and shakers, and in that vein, Virginia also has had quakes that didn’t register beyond the Richter scale. The state has had more than 160 earthquakes since 1977, but only 16 percent of them were felt, according to the Virginia Tech Seismological University.
“This equates to an average of one earthquake every month with two felt each year,” the website states.
Louisa alone has had two earthquakes, of magnitude 2.0 and 2.3, in the last month, according to the earthquaketrack.com website.
”Very few people even felt them,” said Keith Green, chief of Louisa County Fire and Emergency Medical Services.
In the wake of the 2011 quake, he said residents have reinforced buildings that were damaged or added earthquake insurance to their policies.
His department also has adopted a response plan, should another magnitude quake hit.
While Green said his department is as prepared as it can be, he admitted the big quake made him a little gun shy. He assumes that every bump or loud noise is another earthquake in the offing.
“An empty tractor–trailer going down the road will attract my attention,” he said. “There’s really not a whole lot that will spook me, but with Mother Nature, there’s no warning.”