What causes earthquakes near Lake Lanier?

BUFORD, Ga. —

What is usually a calm and peaceful place at the start of a summer night was shaken by tremors from the ground. A series of three earthquakes occurred over the past week near Lake Lanier, and many residents felt the shaking.

The first earthquake occurred last Thursday around 11:15 p.m. It was 2.5 magnitude at a depth of only 2.5 miles below the earth's surface.

A little over two hours later, first response. A magnitude 2.1 at 5 miles depth at 1:30 a.m. last Friday.

Then, at the end of the weekend, another aftershock. A magnitude 2.3 earthquake at a very shallow depth of 2.3 km below the surface around 8:37 p.m. Sunday evening.

Dr. Andrew Newman is a geophysicist and professor at Georgia Tech. He said they captured the earthquakes with seismograph instruments here in Atlanta – and several hundred miles north in the Appalachian Mountains.

While showing us the wide array of seismometer graphs, which show the series of primary and secondary waves that earthquakes send through the Earth's crust, he explained that there are four main factors that likely helped explain why so many people felt these earthquakes.

Newman said proximity and depth are key factors in why many people were able to feel the earthquakes.

“The first thing is proximity. If you're close to it, you'll feel it. Another thing is depth. They were all very shallow,” he said.

Most earthquakes occur more than 10 kilometers underground. Seismic waves gradually weaken as they pass through the Earth's outer layer. But when a very shallow earthquake occurs, seismic waves reach the surface faster and with more energy.

The third factor concerns our East Coast rocks. We experience earthquakes differently here than on the West Coast. In the West, earthquakes are more frequent.

“On the West Coast, we have a lot more earthquakes. And so when we have a lot more earthquakes, we have a lot more fragmentation of the Earth. And wherever the Earth is more fragmented, the interior part of Earth is the seismic zone. “The wave passes through… Some of that energy is lost because the rock slides on itself,” Newman said. “And that causes some thermal energy. inside the Earth. But the seismic signal itself gets smaller and smaller, much faster the further away you go. Where in the eastern United States, the rock is much stronger, much more crystalline, not as many observable or weak defects. And so these signals will flow very freely.

“Here in the east, seismic waves propagate more efficiently. They go farther and are bigger. So you have a larger area where you could potentially shake and feel it.”

And the fourth factor: time of day.

“This happened in the evening and at night. Most people are not out in their cars busy. They are stationary and at home. So they are more likely to notice a difference when earthquakes occur only during the day.

Dr Newman said last Thursday night's tremor was probably the main shock, and the next two were probably aftershocks. A larger earthquake is very unlikely, but not impossible either.

Scientists don't know exactly what causes earthquakes. They have hypotheses or educated guesses, but they really need more data to draw conclusions.

These earthquakes are located near, but not on, the Brevard fault line, which extends into Virginia. But this fault is not active, and has not been for more than 100 million years. So Newman doesn't think it's associated with that.

Sometimes changes in reservoir water levels can create tension and stress until the right time.

“It’s possible that something recent was the straw that broke the camel’s back”

Water levels at Lake Lanier and near Buford Dam have not seen any significant change in recent weeks or months.

Dr. Newman also noted that sometimes the warming of the earth can also cause more stress and strain.

In general, earthquakes are just sudden releases of energy after tension builds up in the Earth's crust.

Scientists from Georgia State University, Georgia Tech and the University of Georgia will collaborate to uncover more information about the series of earthquakes. They will install a handful of seismic nodes that will be buried in the ground and several surface seismographs. These will be located very close to the epicenters of the 3 previous earthquakes.

There is no guarantee that further aftershocks will occur, but if they do, they will be able to collect data.

“Then they can start to map the individual faults where these earthquakes are occurring,” Newman explained.