Little movements. Thunder. Artillery. A vehicular crash. Railroad cars coupling and uncoupling. A sensation in the bottom of one’s feet. Things that go boom in the night.
An unprecedented wave of minor earthquakes focused near Elgin, a small town in Kershaw County, have local residents struggling to describe what they’re experiencing. For a big chunk of 2022, “Did you feel that?” became almost as common a greeting as “How are you?” across the Midlands.
“I’ve felt a lot of them,” says Dean Wells, assistant manager at the IGA grocery store on Main Street in Elgin. “It’s like a big boom of thunder or a gun going off. The ones that have been the most sensitive have been the ones at night where you’re sleeping and then boom, you start moving.”
The U.S. Geological Survey refers to the Elgin phenomenon as a “swarm.” It began Dec. 27, 2021, with a magnitude 3.3 earthquake. Since then, upward of 80 earthquakes have been recorded.
“There have probably been a lot more than that,” says Scott Howard, South Carolina’s state geologist. “It takes at least three stations to report an earthquake for it to be confirmed. Lots of small, low magnitude earthquakes go unreported.”
A magnitude 3.6 earthquake occurred on June 29, 2022. Pat Mantilla, co-owner of the Elgin Barber Shop, describes that one as “pretty serious.”
“I was surprised how loud it was,” he says. “You could literally hear it coming from one direction and then through you.”
Others have been widely felt as well, based on the “Did you feel it?” websites where non-scientists can report seismic sensations. The USGS says several Elgin tremors resulted in more than 3,000 online reports. Meanwhile, many others are never noticed.
“We’re in a pretty earthquake-prone state,” says Scott White, professor at USC and director of the S.C. Seismic Network, which operates seismic stations throughout the state. “Most people don’t realize it because they won’t feel an earthquake that’s below about magnitude 3. We’ll have five to 10 earthquakes a year that aren’t felt but are recorded by the seismometers.”
The latest swarm is a lot more than that, however. The USGS describes a swarm as a prolonged sequence of earthquakes that lacks any clear primary event, such as a larger-magnitude earthquake. A large earthquake can often trigger an aftershock sequence of mostly smaller earthquakes. No rules exist for how long a swarm can last or why one stops.
“Typically, we don’t see them go on for years,” White says. “This one has already overstayed its welcome. We should have already felt its last earthquake.”
While a series of smaller earthquakes can be an aftershock from a bigger one, at times “foreshocks” are a prelude to a big quake. Thomas Pratt, eastern region coordinator of the USGS earthquake program, says a series of foreshocks occurred in the Summerville area prior to the 1886 Charleston earthquake. That famous event, a magnitude 7 temblor, killed 60, damaged practically every building in the city, knocked trains off their tracks, and left visible cracks in the earth.
“This earthquake sequence we’re seeing in Elgin is not related to any of the faults associated with the great 1886 Charleston earthquake,” White says. However, one rumor says that the swarm might be lingering aftershocks from the 1886 quake. The Palmetto State’s institutional memory of that event has sparked worry.
The concerns were enough that the USGS crunched some numbers and in August issued a report on the likelihood whether the Elgin swarm is a precursor to a “big one” like in 1886. At the time, it estimated a less than one percent chance of an earthquake of magnitude 5 or higher in the area.
“Based on statistics, it is unlikely there would be a big one,” Thomas says. “We will never say there is no chance of a larger earthquake. But there have been thousands of small earthquakes without there being a larger earthquake.”
Along with worries about a large quake, amateur speculation has been going on as to why the swarm started. Elgin sits along U.S. Highway 1 in the rolling sandhills northeast of Columbia on the way to Lugoff and Camden, just north of Interstate 20.
Fort Jackson is just south of the interstate, and some people think munitions are exploding when they’re actually hearing an earthquake. A sand quarry practically straddles the interstate east of town, and others assume all that digging might have something to do with the earth shaking.
“It’s not the quarry, period,” White says. “First, you’d have to establish that they’re doing something new at the quarry. These earthquakes are 2 miles down into the earth. At the quarry, they’re really just scooping sand off the surface.”
So, if it’s not the quarry, what’s causing the swarm? “I get asked that a lot,” he says, then admits, “I really don’t know. This is the first seismic swarm of that many earthquakes that we’ve seen on the Eastern Seaboard.”
Yellowstone National Park is one of the most seismically active places in the U.S. It experiences 1,500 to 2,500 earthquakes per year, according to the USGS, with swarms accounting for half of those temblors. The Yellowstone swarms typically last one or two days and total 10 to 20 quakes, although some have lasted several weeks and included thousands of quakes.
“Normally earthquake swarms occur in volcanic or geothermal regions, and scientists found that in those cases either magma or fluid migration can drive swarms,” says Zhigang Peng, a professor of geophysics at Georgia Institute of Technology. “But in the Elgin-Lugoff swarm sequence, scientists have not found any evidence for such driving forces yet.”
Researchers at Georgia Tech have reported on a 1993 swarm in the Norris Lake community outside Atlanta. Over six months, more than 10,000 earthquakes were detected, with approximately 500 being felt. The largest was magnitude 2.7.
Norris Lake is a man-made lake, and the neighborhood sits near a quarry. Many residents at first thought the earthquakes were explosions at the quarry. Experts say man-made reservoirs in South Carolina such as Keowee, Jocassee, and Monticello experience earthquakes due to the pressure exerted by the water. The USGS says there were seven earthquakes in October 2021 near Jenkinsville and Lake Monticello, but the Elgin swarm doesn’t appear to be related to those earthquakes 40 miles away.
While it’s not known what triggered the Elgin swarm, scientists do know a lot about why earthquakes occur in general. The Eastern Piedmont Fault System runs through the Midlands. According to the USGS, a fault is basically a crack, or area of cracks, that allows large sections of rock to move. However, a recent report suggested that the swarm is in between these fault systems, rather than along them.
“An earthquake is the release of energy on a fault,” Scott Howard says. “The energy is released because the fault has reached critical failure. The rocks break. They can’t take it anymore.”
White says this part of the U.S. has many deep, old fracture systems from millions of years ago when Pangea split up to form the continents as we know them today. It’s easy for stress to release where the cracks are so old and weak.
“When you get down by Charleston, you find additional factors,” Thomas says. “You have a sediment groundcover that amplifies ground shaking. Closer to the Atlantic Ocean are a whole series of fault zones. The Charleston area has more of these younger faults.”
The magnitude 5.8 earthquake that rocked Washington, D.C., in 2011 was felt so widely because it sits on the same type of sediment as Charleston. However, South Carolina is not like California with its constant concern of a big one along the San Andreas Fault. In California, the faults and rocks are younger and larger.
Meghan Semino, part owner of the Jane Keels gift boutique in Elgin, says she got her fill of earthquakes when she lived in the Los Angeles area. Now she’s experiencing them at home and in her shop, which is packed with the happy clutter of decorative items and gifts.
“A couple of them did shake some pictures off the wall,” she says of the swarm. “We had some little ones where some stuff would start rattling. Sometimes I feel little movements, but maybe it’s just in my head.”
Meghan says they took some items off the wall at the store for a while before hanging them back up. She also took some items down at her home. Dean says only on one occasion a tremor shook items on the shelves at the IGA, although one was so loud, he thought a delivery truck had hit the side of the store.
Like the proverbial tree that falls in the forest, if nobody hears, feels, or measures an earthquake, did it really happen? In October, Zhigang brought in 86 seismic nodes, and his students teamed with University of South Carolina researchers and students to place them amid the Elgin swarm area. He hopes the additional sensors will be able to detect even the smallest, negative-magnitude earthquakes. They plan to keep the sensors in Elgin for several months before retrieving them in early 2023.
One definite outcome of the Elgin swarm is a heightened awareness of earthquake safety. Gov. Henry McMaster declared Oct. 16-20 Earthquake Preparedness Week, and the state Emergency Management Division unveiled a new website, Earthquake.sc. The site provides safety tips and background on Palmetto State quakes while also dispelling some myths, such as the belief that fracking causes earthquakes in the state.
“There is no fracking in South Carolina,” says division spokesperson Derrec Becker. “We have no natural gas reserves, so there is nothing to frack.”
The current safety mantra for earthquakes is “drop, cover, and hold on.” Drop to your hands and knees, cover your head and neck with your arms, and, if possible, find a sturdy piece of furniture to climb under and hold onto.
“It needs to be almost an instinctive reaction,” Derrec says. “Earthquakes cannot be predicted, despite all of our technology. That’s why we want everyone to be as prepared as possible for earthquakes.”
At some point, the swarm will end. Howard says the tremors will become “out of sight, out of mind.” People will get back to saying, “How are you?” instead of, “Did you feel that?” and those who were around for it will be able to pass stories of the Great Elgin Swarm to future generations.