The fault(s) in our state: The geological forces that cause SC’s earthquakes, explained

COLUMBIA — You might have missed it, but the Midlands was hit by yet another earthquake this week.

The Magnitude 2.1 earthquake struck just about two miles west of Irmo on March 19, according to a preliminary report by the U.S. Geological Survey. It was a “blink and you’ll miss it” temblor, as earthquakes of that size typically just spur minor disruptions — like causing suspended objects to swing — according to the USGS.

Over the past several years, the Midlands has experienced a higher-than-normal (at least from a human perspective) level of seismic activity, The Post and Courier has previously reported.

The March 19 quake is just one of the many earthquakes the Palmetto State experiences every year.

South Carolina is bisected by a number of fault lines that cause those temblors. Those formations are the lingering scars of seismic activities that slammed continents together, raised the Appalachians from the Earth and created the Atlantic Ocean.

And those wounds are far from healed.

 “Once you form a fault, it never truly disappears,” said Steven Jaume, a professor of geology and seismology at the College of Charleston.

“ If you break anything, you can glue it back together,” Jaume explained. “But unless you happen to have glue that’s stronger than the original material, it’s gonna break in the same place that broke the first time.”

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The coastal plain has historically been the most earthquake-active part of the state, according to the S.C. Department of Natural Resources. Much of that activity — including the 1886 earthquake, one of the deadliest natural disasters to strike South Carolina — has been clustered around the Summerville area.

The faults that run under the coast and Midlands are “inferred” faults. That means they require special equipment and techniques to detect, Jaume said, as the coastal plain’s sediments cover up most of the visual surface indicators.

 “We usually can’t see them directly,” he explained.

“ In 1886, we don’t know exactly what moved because it did not break the Earth’s surface,” he added. “And if it doesn’t break the Earth’s surface, you can’t put your finger directly on it.”

Age is another major difference between coastal faults and their Upstate cousins, Jaume said. Many of the Upstate faults were formed when North America and Africa collided hundreds of millions of years ago, forcing the Appalachians skyward. As the two continents drifted apart, opening the Atlantic Ocean, newer fault lines began to form.

 “We think the (younger faults) are being reactivated now underneath the Charleston area,” Jaume said.

The most recent temblor to strike the coastal area was a Magnitude 2 which occurred a few dozen miles offshore of McClellanville on March 13. It was the first offshore quake to hit the state in at least 20 years, according to a DNR database.

As to the Midlands swarm? It’s actually two distinct clusters — one centered in the Elgin area, and the other by Lake Murray. Jaume said both the Elgin and Lake Murray clusters appear to be the reactivation of a fault line. The March 19 quake was part of the Lake Murray cluster.

He added that it is possible for man-made reservoirs to spur earthquake activity, but that doesn’t appear to be the case with Lake Murray, which was constructed in the late 1920s. But that trend, called “Reservoir-induced Seismicity” has been documented at Lake Monticello, which was built for the VC Summer Nuclear Power Station.

“ When they put that one in, they had thousands of micro-earthquakes following the filling of that lake,” Jaume said. “And they periodically have swarms. There was one in fall of ‘21 and fall of ‘23.”

South Carolina’s fault lines fall into three categories. Strike-slip faults occur when plates move horizontally to one another. A thrust fault occurs when the plate above the fault slides up and over another. A standard fault causes the plate above the fault to slide lower than the opposing one.