What caused all of the earthquakes in the Midlands? Scientists have a theory

Just a few miles from a small, earthquake-shaken community, a large river winds past Interstate 20 on its way to South Carolina’s coastal plain.

The Wateree River is a major natural feature of the state’s mid-section, a mighty stream known for good fishing and hydroelectric power.

But the Wateree River also is a suspect in a geologic mystery that has gripped South Carolina for months.

A swarm of earthquakes – small by national standards but noticeable in the Southeast – has jolted the Lugoff-Elgin area regularly since December, leaving people nervous and perplexed about a phenomenon more commonly associated with the western United States.

Now, some of the state’s leading seismic experts are discussing a possible link between the Wateree River and the earthquakes northeast of Columbia.

They want to know if the combination of a single moderate earthquake in December and high water levels in the Wateree River during parts of this year have contributed to the earthquakes.

The question is whether the 3.3 magnitude quake in late December and higher river levels changed some of the pressure below ground, causing the earth to move.

“We think it probably ought to be looked into,’’ said Scott Howard, the state’s chief geologist with the S.C. Department of Natural Resources.

His agency is expected to release a joint statement with the University of South Carolina and the College of Charleston soon, examining the possible connection.

Scientists concede it’s only a theory and may not pan out. But Howard and earthquake scientist Pradeep Talwani are looking at water levels in the Wateree River to learn more.

Howard said he also wants to gather data about any changes in drinking water wells. Discoloration or depletion of well water could signal changes below ground that could then indicate a relationship to earthquakes, he said.

“It would be important to know if people experienced dirty water, or cloudy water or the well went dry,’’ Howard said.

The issue centers on a concept called hydroseismicity, the effect of water impacting earthquake faults.

The idea is that water moving above ground, such as from melting snow or higher rainfall, can change the pressure on water below ground. In turn, that can cause the earth to move.

In this case, the Wateree River is affected by releases of water from a dam that forms Lake Wateree upstream.

Stars lined up

While the dam has operated for decades with few problems, scientists question whether the quake in December altered small cracks in the earth below where the Wateree River snakes through the area.

Scott White, who heads the Seismic Network at USC, said more water in the river could “push down on’’ groundwater that already exists in cracks beneath the earth’s surface.

“What I’m thinking is contributing to all these earthquakes was the month, to month-and-a-half of very high water levels we had,’’ said Talwani, a retired University of South Carolina geologist and leading authority on the impact of water on seismic activity. “Those pressure pulses are going into these fractures.’’

Federal data analyzed by Talwani indicate the flow of the Wateree River was higher in parts of March, April and early May than in previous months. The flow wasn’t unprecedented, but Talwani said it may have been high enough to have contributed to recent earthquakes. That’s because the Dec. 27 quake had already changed cracks in rock below the surface, making it easier for the additional water to cause the earth to move, scientists say.

“All these stars sort of lined up,’’ Talwani said.

Since December, at least 56 earthquakes have shaken the Interstate 20 corridor northeast of Columbia, many in the area between Elgin and Lugoff. Five earthquakes with a magnitude of more than 3 have been recorded in the area, the DNR says. The largest quake was 3.6 in magnitude last month, an event that was felt across the region.

The rumbling has sparked a barrage of questions about what’s causing it. Scientists have so far discounted mining and underground injection as possible reasons. But they are still looking for answers, and the possibility of high water has recently been a point of discussion.

If the December quake and higher water levels contributed to the swarm of earthquakes, the effect was not likely an immediate one, scientists say. Earthquakes would not necessarily occur right away because it takes time for the earth to shift, which could explain the more recent flurry of earthquakes this summer.

The Lugoff-Elgin area has had more earthquakes in June and July than in other months this year, Howard said.

One 2017 research paper, conducted by scientists in Europe, found that hydroseismicity can contribute to earthquakes. The study indicated that some of 121 earthquake swarms documented over a 24-year period in a section of the Czech Republic were caused by hydroseismicity.

Scientific debate

Duke Energy, which operates the Lake Wateree dam, doesn’t believe releases of water through the dam contributed to earthquakes, the company said in a statement. Although human-made lakes can trigger earthquakes when they are being built, Lake Wateree has existed since 1919, the company said.

The company said that while some heavier precipitation in March and April hit the area, it does not believe that was enough to have contributed to earthquakes.

The theory that seasonal or short-term changes in water levels can change pressure enough to cause an earthquake is controversial. A key question is whether more water flow near Lugoff-Elgin contributed to a swarm of earthquakes that may have eventually happened anyway.

“Hydroseismicity is kind of a nice way of assigning a trigger to an event that was already going to happen sooner or later,’’ White said. “That’s really where a lot of the uncertainty comes from with this hypothesis.’’

Steven Jaume, an earthquake scientist at the College of Charleston, said the scientists don’t have any data to support the theory in South Carolina.

“People always want to find a reason for what happened,’’ Jaume said. “But for some earthquakes, it’s just their time to let go. It isn’t anything specific.’’

Earthquake swarms are not unusual, and one occurred last fall near Lake Monticello north of Columbia in Fairfield County. But that swarm involved seven quakes, and none of them were as big as the largest ones recorded more recently in Elgin northeast of Columbia.

“The current earthquake swarm is unusual because of its location (near Elgin), and that it contains earthquake magnitudes larger than in previous swarms observed in the state of South Carolina,’’ Howard’s office said in a statement.

So far, no damage has been reported because the quakes’ magnitudes have not been at a level that tends to cause that. Buildings begin to suffer damage in earthquakes that exceed 5 in magnitude. But the DNR says the current swarm of earthquakes could weaken materials from repeated shaking.

The earthquake fault zone where the swarms have occurred runs in a northeast-southwest direction and are not connected to bigger faults in the Charleston area, his office said.