A series of small earthquakes northwest of Columbia, South Carolina, are caused by a man-made lake built more than 40 years ago, according to geologists.

The tiny temblors — magnitude 2.0 and less — are jangling nerves near South Carolina's Lake Monticello, according to The State newspaper, but the tremors are not unprecedented. The reservoir set off a series of minor earthquakes when it was first filled in the late 1970s. Another small swarm occurred between 1996 and 1999. Since Oct. 25, there have been seven earthquakes detected near the lake, according to the South Carolina Department of Natural Resources.

These quakes are so small that someone standing on the surface might only notice them if they were right over the epicenter and there was no rumbling traffic nearby.

"We haven't had any large earthquakes at Monticello," said Pradeep Talwani, a professor emeritus of geology at the University of South Carolina who spent his career studying earthquakes caused by man-made lakes. Going all the way back to 1977, all of the quakes in the area have been less than a magnitude 3, he said. Quakes under magnitude 3 are rarely felt.

Related: The 10 biggest earthquakes in history

Reservoir-induced seismicity

What's happening at Lake Monticello is called "reservoir-induced seismicity." This phenomenon happens at relatively few reservoirs around the world, Talwani told Live Science. Regardless of location, the physics are always the same: A reservoir is built over rocks that contain fluid-filled fractures. When more water is loaded on top, it seeps into the fractures, causing the fluids to migrate and build up pressure. Ultimately, the pressure causes the rocks to slip and rattle the surrounding earth. This is the same reason that pumping fluids into oil wells for the purpose of fracking can cause earthquakes.

Mostly, these man-made earthquakes are small. Globally, only three reservoir-induced quakes with a magnitude of 6 or higher have ever occurred, Talwani said. (Earthquake damage can vary based on the local conditions and building materials, but magnitude 6 is typically the line at which serious damage occurs.) These damaging quakes occurred at deep reservoirs with more than 328 feet (100 meters) of water, Talwani said. In comparison, Lake Monticello is 89 feet (27 m) deep at its deepest.

"Compared to everything globally, Monticello is a little puddle," Talwani said.

Watching for quakes

It has been, however, a very well-monitored puddle. Researchers first learned about reservoir-induced seismicity in the 1960s in Denver, Colorado. Operators at a chemical weapons facility called the Rocky Mountain Arsenal drilled a deep well and began injecting waste fluid into what turned out to be highly fractured rock, triggering more than 700 earthquakes in five years, according to a 1966 article in the journal The Mountain Geologist.

Thus, scientists knew about the possibility of reservoirs triggering earthquakes by the time Monticello was constructed. Talwani and his team were already monitoring and studying small swarms at reservoirs such as Jocassee near the North Carolina-South Carolina border.

Lake Monticello was constructed in the 1970s as a water source for the nearby Virgil C. Summer Nuclear Power plant. Because scientists already knew that reservoirs could produce earthquakes, the Nuclear Regulatory Commission required careful monitoring of seismicity in the area. Talwani's research group conducted most of this monitoring, which gave them a stunningly detailed view of tiny earthquakes that wouldn't normally be picked up by U.S. Geological Survey equipment.

The lake has been the source of thousands of tiny quakes over the years, most far too subtle to be felt. The initial swarm of earthquakes after the reservoir filled wasn't surprising. But the quakes in the 1990s, 20 years after Lake Monticello was constructed, were a little more mysterious. Thanks to their detailed seismic monitoring, Talwani and his colleagues were able to figure out what had happened. Over time, they found, water from the lake had dissolved mineral "caps" that had been sealing off fractures in the rock. With these new fractures opened, water was able to move into them, again building up pressure and causing the rocks to slip.

Related content

—Photo journal: The gorgeous San Andreas Fault

—Gallery: This millennium's destructive earthquakes

—Rainbow Basin: Photos of Yellowstone's colorful grand prismatic hot spring

Something similar is probably happening at Monticello now to cause the new earthquakes, Talwani said. However, it's impossible to say because the fine-grain seismic monitoring system is no longer in place. That means researchers can see only the largest quakes, not the miniscule ones that help them localize the origin of the seismicity.

"Now we have no idea what is going on, because we just have one [seismic] station in that area," Talwani said.

Researchers at the University of South Carolina may soon deploy more seismometers in the area, said Scott Howard, a state geologist at the South Carolina Department of Natural Resources. The quakes are likely to peter out or continue at the current levels of magnitude, Howard told Live Science.

Originally published on Live Science.