COLUMBIA, S.C. — Minor earthquakes that have gently shaken the ground north of Columbia during the past week are occurring in an area where construction of a lake four decades ago is suspected of causing a rash of tremors through the years.
Since engineers built Lake Monticello in 1978, geologists have recorded thousands of earthquakes that, for the most part, are too small for people to notice.
But with seven tremors reported in the past week near Jenkinsville, questions are surfacing about whether the lake is contributing to the latest quakes.
None of the recent earthquakes is considered large, nor has damage been reported, but they are substantial enough to have been picked up by U.S. Geological Survey gauges.
Key questions are whether more earthquakes are on the way for the area near the lake, and if any more do occur, whether they will be big enough to damage property and threaten public safety.
It’s impossible to predict when a major earthquake will hit, but the S.C. Emergency Management Division said in a news release that more quakes are possible “for the next several months or even years in the area near the Monticello reservoir.’’
Overall this year, South Carolina has had 20 earthquakes, about one third of which were in the Jenkinsville area.
Steven Jaume, an earthquake scientist at the College of Charleston who has consulted with the emergency division, said he’s not surprised by the recent activity near Jenkinsville, a crossroads that is a stone’s throw from Lake Monticello.
The Jenkinsville area has had a history of minor earthquakes tied to construction of 6,700-acre Lake Monticello, built to make power and to provide cooling water for the V.C. Summer nuclear power plant.
“They are actually relatively common in that area because of the Monticello reservoir,’’ Jaume said. “When they filled it up, a swarm of earthquakes started under the reservoir.’’
While earthquakes are natural occurrences, they also can be caused by man-made activity, such as drilling or heavy construction work, or pressure from a newly filled lake.
Lake Monticello triggered as many as 10,000 small earthquakes beginning in the late 1970s as changes in pressure from the new reservoir affected the ground below, according to a 2001 study by researchers at the University of South Carolina.
All were under 3 in magnitude, according to the Emergency Management Division. Large earthquakes typically exceed 7 on a scale of 10.
Most of the past earthquakes were discovered because USC researcher Pradeep Talwani had actively looked for them. His research showed how man-made changes in the landscape had caused seismic activity.
Twenty years after the lake was filled, a flurry of small earthquakes again was recorded in the area, that time from what researchers speculated were fractures in the earth dissolving after exposure to water over time.
From 1996-1999, more than 700 small earthquakes occurred around the lake, according to the study. The largest was 2.5 in magnitude, the emergency division reports.
Not only did lake construction likely cause small quakes, but water from the lake continued to have an effect, research shows.
“The fractures were there and surrounded by water for 20 years,’’ said Talwani, a retired USC researcher who co-authored the 2001 study. “The water gradually worked in the fractures and opened them up.’’
Seismologist Don Stevenson, who is writing a book with Talwani on the history of South Carolina earthquakes, said USC’s research showed a clear connection between construction of Lake Monticello and the small earthquakes that Talwani recorded.
“Beyond a shadow of doubt, it pretty much proved that when that lake went in and was filled, it caused a lot of earthquakes, and that effect went on for a number of years,’’ said Stevenson, who formerly ran the seismic monitoring network at the Savannah River Site nuclear weapons complex near Aiken.
But answers about why the recent quakes occurred remain elusive as questions rise in the public.
Columbia resident Bridget Miller Deline asked in a Facebook post this week whether the small quakes at Jenkinsville are preludes to a larger one. She later told The State the number of earthquakes recently is frightening.
“I’m thinking in the same way that a small stroke in an individual can be a warning sign of a major stroke or a heart attack, these small clusters of earthquakes in South Carolina can be a warning sign of larger quakes to come,’’ she said in an interview with the newspaper.
Stevenson understands the feeling.
“People want to know what the hell is going on,’’ Stevenson said. “It’s not really all that clear exactly what is going on.’’
State Geologist Scott Howard, who is with the S.C. Department of Natural Resources, said he wants to know if water trickling from the lake is related to the recent earthquakes near Jenkinsville, which is off S.C. 215 in Fairfield County.
“I still wonder to what degree fluids are playing in it, water seeping out of the reservoir,’’ Howard said, but he also said it’s difficult to know the answer. “What has changed, who knows?’’
Dominion Energy, which runs the V.C. Summer nuclear plant and oversees the lake, said it doubts its operation to make electricity has had anything to do with the small tremors. The company makes some power through a pumped storage project, which moves lake water at different elevations.
A pumped storage operation “normally operates every day, so that would not be a direct cause for an earthquake,’’ Dominion’s Darryl Huger said in an email.
The company said the small tremors had no effect on the nuclear plant, which is built to withstand large earthquakes.
Even so, the emergency management division said the public needs to be on watch for a “large-scale earthquake, however unlikely the possibility may be.’’
South Carolina’s recent earthquakes aren’t unusual on the East Coast. Small tremors sometimes occur in bursts in other areas, said Tom Pratt, eastern region coordinator at the U.S. Geological Survey’s geologic hazards science center in Virginia.
He questioned whether Lake Monticello has caused the recent earthquakes near Jenkinsville because the lake has been in place for decades.
“The fact that there’s a bunch of small earthquakes in an area isn’t very surprising,’’ Pratt said. “There have been some going on underneath Boston over the last few months and there were some under Connecticut a couple of years ago. They just seem to come in little clusters like this.’’
Earthquakes in the eastern United States have generally not proven to be the same threat as those in the west.
But an 1886 Charleston quake blasted parts of South Carolina and, even today, serves as a reminder of what can occur in extreme cases. The 7.3 magnitude quake “devastated the region’’ and was felt in the Midwest, the emergency management division said.
Geologists say small tremors sometimes precede big earthquakes, but oftentimes, they do not. And unlike hurricanes, earthquakes often strike without warning, they said.
“There are too many variables; more variables than there would be for a hurricane or a storm,’’ the DNR’s Howard said of the difficulty in predicting major earthquakes.