A series of small earthquakes has been rumbling underneath the South Carolina midlands over the last two weeks, and geologists are puzzled. The state has never seen a series of small earthquakes like this before.
WFAE's Nick de la Canal spoke with some of those scientists and joined WFAE's Gwendolyn Glenn to talk about what's going on.
GWENDOLYN GLENN: Nick, can you first catch us up and bring us up to speed.
NICK DE LA CANAL: Sure. So this started back on December 27. It was the Monday after Christmas, and at about 2:00 in the afternoon, a 3.3 magnitude earthquake hit about 30 miles northeast of Columbia. Thousands of people felt the shaking across the state, even as far north as Rock Hill, which is on the North and South Carolina state border. The earthquake wasn't strong enough to cause any major damage that we know about, but it certainly would have shook the ornaments on some Christmas trees.
Now the smaller earthquakes that followed appear to be aftershocks, but what's unusual is the number of them and that they've continued for so long. There have been nine so far, the most recent was a 2.6 magnitude on Wednesday, and that's kind of baffling to Steven Jaume', associate professor of geology at the College of Charleston.
STEVEN JAUME': This is probably one of the longest-running aftershock sequences for a pretty small mainshock. Like I would expect a few days, not well over a week.
GLENN: So Nick, do geologists have any idea why the aftershocks have continued for so long?
DE LA CANAL: You know, I asked, and there doesn’t seem to be a good answer right now. I talked a little with Susan Hough of the U.S. Geological Survey in Pasadena. She said these continuous rumblings kind of resemble another phenomenon that seismologists call an earthquake swarm. They can be triggered by fluids moving through the earth’s crust, causing a series of earthquakes. Here she is.
SUSAN HOUGH: So a swarm typically produces a number of earthquakes with the largest events around the same size, as opposed to a large mainshock followed by aftershocks.
DE LA CANAL: What’s happening in South Carolina doesn’t match her swarm theory because the tremors have been smaller than the first quake. But on the other hand, they’re acting like a swarm.
HOUGH: So I’m kind of hemming and hawing because this is a case where it’s really not clear cut. Is this a swarm, or is this an especially energetic aftershock sequence?
GLENN: And Nick, as South Carolina scientists are just scratching their heads over this, is there any kind of precedent for something like this?
DE LA CANAL: Yes and no. Minor earthquakes are a regular occurrence in the Carolinas. And sometimes we get big ones too.
You might remember in 2020, there was a 5.1 magnitude earthquake that struck the town of Sparta, on the North Carolina - Virginia border. That caused a series of aftershocks that continued for weeks. It’s important to note that a large fault system stretches from Georgia through the Carolinas into Virginia. And all of these quakes are along that system.
But the smaller quakes have never kept shaking for this long for an earthquake this small.
The College of Charleston’s Steven Jaume’ says a lot of geologists are excited and as you can hear, they’re getting busy.
JAUME: Lots of emails have been bouncing back and forth and ‘Hey, did you see this? Did you read this paper?’ There was some gold mining in that area back in the day and there was some detailed maps of the geology based on where the gold mining was being done. So they’re looking that up. Other people are looking at magnetic field data, you know. So we’re investigating it.
DE LA CANAL: Now he says we may never really know why these aftershocks have carried on for so long, but they may help us learn more about where the fault lines are in South Carolina and what they’re doing, and that could help the state prepare for future earthquakes.
JAUME: If we have a really good idea of exactly where the fault is, and also from the smaller earthquakes how the local geology influences how strong the ground shakes at different locations, we can produce detailed seismic hazard maps.
DE LA CANAL: And he says that could help inform things like building codes and emergency management plans.
GLENN: And Nick, could these aftershocks — are they going to continue, or has the ground settled?
DE LA CANAL: Well, unfortunately, there’s no way to know. The thing about these earthquakes is there’s no warning. However, it’s likely that any more aftershocks will be on the smaller side, maybe even too small to feel, and researchers will be ready to track those tremors and use that data for their research.
GLENN: That’s WFAE’s Nick de la Canal. Nick, thank you.
DE LA CANAL: You’re very welcome.
Copyright 2022 WFAE