South Carolina has a very diverse environment, and that includes its geology. The Palmetto State has been earthquake country for many years. In 1886, an earthquake estimated at magnitude seven devastated Charleston in one of the worst quakes in American history. While nothing of that degree has happened since – knock on wood - many smaller earthquakes occur virtually every year, most not even detected.
However, a recent rash of smaller earthquakes has been felt since October, chiefly around the Jenkinsville and Elgin areas. Jenkinsville is the site of Lake Monticello, a fairly new artificial lake completed in 1978, and according to College of Charleston geology professor and earthquake specialist Dr. Steven Jaume’, building new lakes sometimes produces earthquakes.
“When you put a reservoir, an artificial lake, there’s this big body of water where there wasn’t one before. The sheer weight of that water will put force on the earth’s crust. In addition, you’re allowing some of that water to leak into the ground…which will raise the local water table. The combination of those effects is sometimes known to set off earthquakes,” said Jaume’.
While most manmade lakes don’t cause earthquakes, enough of them do to have the phenomenon named. It’s called Reservoir-Induced Seismicity, according to Jaume’.
There’s more to it than just building a lake, however, said University of South Carolina Distinguished Emeritus Professor of Geology Dr. Pradeep Talwani, who has studied earthquakes for decades. “Most of the lakes that cause earthquakes, it’s the initial filling which is the culprit,” he said. “Or extremely quick rise in water levels. We saw that in Clark’s Hill. One year we were working… and it rained very heavily and the water level went up 10 feet. And 46 hours later, the earthquake started. So sudden filling up of water is a trigger. Large scale fluctuation of lake levels is a trigger.”
Jaume’ said there are 10 to 20 earthquakes per year in South Carolina that scientists can actually record and locate, but if they had more seismometers to better cover the state, they would probably be able to record several hundred.
Talwani explained the system of rating the power of earthquakes. “The energy released goes up by a factor of 32 for every one unit of magnitude. So the difference between a magnitude 1 and a magnitude 3 is 32 times 32, so about a thousand. Between a 3 and a 5 is another thousand. So between a 1 and a 5, which can do serious damage, it’s a million times greater.”
He said the 1886 Charleston quake was an intraplate quake, meaning it was in the middle of a tectonic plate, rather than on its edge, where most large earthquakes occur. Talwani added it’s unlikely the state will see another quake of the power of 1886 for a while.
“Now we think a major earthquake like Charleston occurs every 500 years, on an average. So we are 130 years into that cycle. So when they built the bridge over the Cooper River, they used our numbers of 500 years to design the Cooper River Bridge. That’s the best that we’ve got, is that earthquakes like that occur on an average of 500 years, which is on par with most intraplate earthquakes.”
Jaume’ offered some advice for anyone who may find himself in an earthquake. First: unless you’re in the doorway, don’t run outside.
That may sound counterintuitive, but Jaume’ pointed out the obvious: coming from the middle of the house, one would have to clear not only the door, but the height of the building itself, in the seconds before the walls might fall.
“Chimneys fall off, the brick walls come off and they go out, away from the building. That’s what you do, is you stay inside,” the professor advised, adding “You drop to the ground. You want to go down first before you get knocked down, because you get hurt that way. You cover – under a table, under a desk, maybe just against a wall, away from a window in case the glass breaks. Cover yourself as best you can, hold on until the shaking stops to go outside.”
He added that while it’s unlikely that a big quake like Charleston’s will happen anytime soon, there’s still potential for a medium one. “It’s only been about 135 years, so it’s a little too soon for another 7,” he said.
“What I do tell people, though, is that in most places, you get smaller earthquakes more often than bigger earthquakes. So while I don’t expect a 7 anytime soon, there’s still potential for what I’d call a medium one. The magnitude 6 would be a medium one. An example of that would be up in Virginia, 2011. It damaged the Washington Monument. That was 5.8, so that kinda (would be) medium size. So that earthquake in the near future is potentially more likely than having a repeat of 1886.”
Jaume’ said he was surprised there were as many aftershock quakes as have followed a magnitude 3.3 December quake recently, but that they will die away eventually.
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