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NC dams were unfazed by Sunday's earthquake, thanks to an earthquake in 1886

The (Raleigh) News & Observer logo The (Raleigh) News & Observer 8/14/2020 By Matthew Diasio, The News & Observer (Raleigh, N.C.)

On Sunday morning, Doreen Peros was getting ready to make breakfast when she felt her house start to shake. “Sometimes, when trucks come by on Highway 21, the windows rattle,” she said.

But Peros had never felt such strong rattling for so long before. “I thought two semi trucks hit the building.”

“My sister called me on FaceTime and asked, ‘Are you alright? You look like you’re crying.’ I said ‘I think the earth just shook.’”

Peros and her husband own the High Meadows Inn and Nikola’s Restaurant in Roaring Gap, about 10 miles southeast of Sparta and the epicenter of Sunday’s 5.1 magnitude earthquake. Their property is also just down the river from the Lake Louise reservoir, which is held in place by one of North Carolina’s more than 3,000 dams.

The Lake Louise dam was undisturbed by the rattling. In North Carolina, dealing with an earthquake is just part of a dam’s job.

After Sunday’s earthquake, the Division of Energy, Mineral, & Land Resources said it contacted owners of “high hazard” dams in Alleghany, Surry, Wilkes, Ashe, and Yadkin counties to make them aware of the earthquake and the possibility that their dams may have been affected.

Dams are classified as high hazard if their failure would likely result in death — it is not a reflection of the quality of the dam. According to the Department of Environmental Quality dam inventory, there are four high hazard dams in Alleghany County, seven in Ashe County, 17 in Surry County, six in Wilkes County, and 12 in Yadkin County.

DEQ staff from the Winston-Salem Regional Office inspected dams in Ashe and Alleghany Counties on Tuesday. Robert Johnson, a spokesman for the Division of Energy, Mineral, and Land Resources that is responsible for dam safety, said there were no reports of damage to the dams.

Duke Energy has 49 dams with its hydroelectric power plants in North and South Carolina. Kim Crawford, a spokeswoman for Duke, said company policy is to “inspect our dams any time there is a felt seismic event” and no issues were identified at any of those plants.

Mohamed Gabr, a professor of civil engineering at NC State, said it is rare for modern dams to fail because of an earthquake. In recent history, he only knew of dam failures after very large, magnitude 8 earthquakes in Sichuan, China, in 2008 and in central Chile in 2010.

The magnitude scale used to measure the energy released by an earthquake represents exponential increases in energy, so those earthquakes were over 30,000 times more powerful than the North Carolina temblor. DelWayne Bohnenstiehl, a geophysicist at NC State, said that for a magnitude 5 earthquake, any structural damage would happen close to the epicenter.

Most of the movement during an earthquake is side-to-side, not up-and-down. “Dams, by their nature, are designed to resist lateral movement and forces,” Gabr said, since they are holding water behind their walls. Buildings are primarily designed to withstand the vertical force of their weight, so they are more susceptible to damage by the lateral forces generated by an earthquake.

“For seismically active areas, dams are designed for events that occur once in every 10,000 years,” Gabr said. “That could explain why we have not had a catastrophic dam failure due to earthquakes in the U.S.”

North Carolina isn’t by an active fault line like California, but western parts of the state are on the edge of the East Tennessee Seismic Zone with a history of earthquakes. Other seismic zones in South Carolina and central Virginia have also caused earthquakes felt in North Carolina.

Earthquakes in these zones tend to be smaller than those along fault lines. But a large earthquake in Charleston, South Carolina, in 1886 damaged most of that city and was felt across the Southeast. That event and smaller earthquakes in the historical record have influenced building regulations in the region.

Floods are more likely to damage or destroy dams than earthquakes. Gabr said dams are typically designed to withstand flood levels that are predicted to occur once in every 100 or 500 years. The dam that failed in Midland, Michigan, this May was due to such a 500-year flood.

“The risk is defined as the probability of failure multiplied by the consequences of failure,” Gabr said.

Due to climate change, extreme weather events like the Midland flood are becoming more common.

Last year, the Associated Press identified almost 1,700 dams across the country that are high hazard and were rated to be in poor or unsatisfactory condition.

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©2020 The News & Observer (Raleigh, N.C.)

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