Strange earthquakes in South Carolina trace back to a man-made lake

Geologists say a series of small earthquakes northwest of Columbia, South Carolina, was caused by a man-made lake that was built more than 40 years ago.

According to The State, the small tremors – with a magnitude of 2.0 or less – are nerve-racking near Lake Monticello in South Carolina, but the tremors are not unprecedented. The reservoir caused a series of small earthquakes when it was first filled in the late 1970s. Another small swarm occurred between 1996 and 1999. Since October 25, seven earthquakes have been detected near the lake, according to the South Carolina Department of Natural Resources.

These earthquakes are so small that someone standing on the surface may not notice them unless they are directly above the epicenter and there is no heavy traffic nearby.

“No major earthquakes have occurred at Monticello,” said Pradeep Telwani, a professor emeritus of geology at the University of South Carolina who has spent his career studying earthquakes caused by man-made lakes. Back in 1977, he said, all earthquakes in the region were less than 3 on the Richter scale. Earthquakes of less than 3 degrees are seldom felt.

Related Topics: The 10 biggest earthquakes in history

Earthquakes caused by reservoir

What happens in Lake Monticello is called “reservoir-induced earthquakes”. This phenomenon occurs in relatively few reservoirs around the world, Talwani told Live Science. Regardless of the location, the physics is always the same: a reservoir is built over rocks containing fluid-filled fractures. When more water is loaded on top, it seeps into the fractures, causing the fluid to migrate and increasing pressure. Ultimately, the pressure causes the rocks to slide and shake the surrounding ground. This is the same reason why pumping fluids into oil wells for the purpose of fracking can cause earthquakes.

Mostly, these man-made earthquakes are small. Only three earthquakes globally have occurred in reservoirs with a magnitude of 6 or higher, Talwani said. (Damage caused by earthquakes can vary based on local conditions and building materials, but magnitude 6 is usually the line where significant damage occurs.) These devastating earthquakes occurred in deep reservoirs with more than 328 feet (100 meters) of water, Talwani said. By comparison, Lake Monticello is 89 feet (27 meters) deep at its deepest point.

“Compared to everything globally, Monticello is a small pond,” Altalwani said.

Watch for earthquakes

However, it was a well-controlled pond of water. Researchers first learned about reservoir-induced earthquakes in the 1960s in Denver, Colorado. Operators at a chemical weapons facility called Rocky Mountain Arsenal drilled a deep well and began injecting waste fluids into what turned out to be highly fractured rock, causing more than 700 earthquakes in five years, according to a 1966 article in The Mountain Geologist.

Thus, scientists learned of the reservoirs’ potential to cause earthquakes by the time Monticello was built. Al-Talwani and his team were already monitoring and studying small swarms in reservoirs like the Jocassee near the North Carolina and South Carolina border.

Lake Monticello was created in the 1970s as a water source for the Virgil C summer nuclear power plant. Since scientists already know that reservoirs can cause earthquakes, the Nuclear Regulatory Authority has ordered careful monitoring of earthquakes in the region. Most of this monitoring was done by Talwani’s research group, which gave them a stunning detailed view of small earthquakes that the US Geological Survey usually doesn’t pick up.

The lake has been the source of thousands of small earthquakes over the years, most of which were too subtle to be felt. The initial swarm of earthquakes after the reservoir was full was not surprising. But the earthquakes in the 1990s, 20 years after Lake Monticello was built, were a bit of a mystery. Thanks to detailed seismic monitoring, al-Talwani and his colleagues were able to figure out what had happened. Over time, they discovered that the lake’s water had dissolved mineral “caps” that were sealing fractures in the rocks. As these new fractures opened, water was able to move into them, increasing the pressure again and causing the rocks to slide.

Talwani said something similar might be happening at Monticello now to cause the new earthquakes. However, it is impossible to say because the accurate earthquake monitoring system no longer exists. This means that researchers can only see the largest earthquakes, not the small ones that help them determine the origin of the earthquakes.

“Now we have no idea what’s going on, because we only have one [seismic] in that region.”

Scott Howard, a state geologist with the South Carolina Department of Natural Resources, said researchers at the University of South Carolina may soon deploy more seismometers to the area. Earthquakes are likely to either fade or persist at current levels of magnitude, Howard told Live Science.

Originally published on Live Science.

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