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A sizable earthquake hit NC, but Charleston is vulnerable, too

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Earthquake-North Carolina

Various items litter the floor of the 4 Brothers Store in Sparta, N.C., after an earthquake shook much of North Carolina early Sunday, Aug. 9, 2020. Michael Hull via AP

The magnitude 5.1 earthquake that shook western North Carolina on Sunday was felt all the way into South Carolina. 

While it wasn't severe enough to cause damage here, the Palmetto State is no stranger to seismic activity and was home to the worst-known earthquake on the East Coast over a century ago.

The North Carolina rattle was the largest in the immediate region since 1916, according to the U.S. Geological Survey. Its epicenter was near the rural town of Sparta, N.C., close to the Virginia border. 

The USGS issued a yellow alert for economic losses, meaning "some damage is possible and the impact should be relatively localized." Photos shared on social media showed inches-wide fissures through roads and at least one home with a tilting front wall after the shake. 

Reports from those who felt the earthquake stretched into the Upstate. In the Palmetto State, the quake most resembled a 1913 Union County rumble estimated near magnitude 5.5, said Steven Jaume, a geologist at the College of Charleston. Because seismometers weren't broadly deployed then, the recent event in Sparta may help scientists verify exactly how strong the Union County shaking was, Jaume said.

Quakes are possible all along the Appalachian Mountains, which were formed by faults in the Earth's crust squeezing together. Those faults are still there, Jaume said, though often inactive. 

pc-081120-ne-earthquake

Rubble litters Cumberland and East Bay streets on Sept. 1, 1886, after a devastating earthquake struck Charleston. A fault line runs through Summerville, and seismologists say the Charleston area is at high risk for a damaging temblor within the next half-century. File

But the most devastating quake known in South Carolina actually occurred in the Charleston area, likely with its epicenter in Summerville. The 1886 quake turned solid ground into liquid, killed dozens and flattened most brick buildings in downtown Charleston. It is estimated at a magnitude of 7.3. 

The ground is usually shaking at low levels around Summerville, but it is mostly imperceptible except to sensitive equipment. Sometimes the rumbling does register: just this spring, a minor temblor in Summerville awoke residents and more than 400 reports of shaking were sent to the USGS.

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Scientists estimate that a major event on the scale of the 1886 quake happens every 300 to 800 years, Jaume said, meaning it's not terribly likely one will strike tomorrow. 

Earthquakes of that size are detectable in layers of soil because of liquefaction. Looser soils, particularly if they are wet, are prone to literally turn to liquid in a large enough shake. This can spit sand up from below the surface, and it's those sand deposits that help geologists date when shaking occurred. 

The Charleston peninsula, built by sediments carried in on the region's rivers and artificial fill used to build out land, is particularly prone to liquefaction in a major quake. Recent research has shown this effect could get worse as sea levels rise.

But what soil records cannot tell, Jaume said, is how many earthquakes fall just short of liquefying the ground. There are many more people and buildings in the Lowcountry than in 1886, he said, meaning the risk of destruction in a moderate quake is much higher, especially for buildings that are still standing after 1886. 

He said people often ask him when Charleston might see it's next "big one" — but that question misses the point. 

"If a medium one happens tomorrow, people will think it's a big one," Jaume said.

Reach Chloe Johnson at 843-735-9985. Follow her on Twitter @_ChloeAJ.

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