Searching for rare earthquake aids in NC, forecasting

How many times can you put your hand on an active earthquake fault? In the eastern United States, no one did this until two years ago.

That’s because there have been no recorded earthquakes east of the Mississippi that ruptured the surface, not even as devastating as the 1886 earthquake in Charleston, South Carolina. Think earthquakes, and think of images of cracked highways that show obvious fault lines. In the eastern United States, these kinds of features didn’t occur until the 5.1-magnitude earthquake in Sparta, North Carolina, on August 9, 2020, became the first earthquake to reveal a fault line.

Professor Kevin Stewart measures vertical displacement in a car park in Sparta.

That morning, Carolina geologist Kevin Stewart rushed to Sparta. Stewart is a professor in the Department of Earth, Marine and Environmental Sciences in the College of Arts and Sciences. His specialty is research in structural geology such as faults and fissures in the Earth’s crust. He has worked in the Apennine Mountains of Italy where there are active faults in the Rocky Mountains and in western North Carolina.

Stewart was electrified by the USGS’s Sparta earthquake notification, which was felt throughout North Carolina, most of the eastern United States, and westward in Tennessee and Kentucky. He was on site by noon, with colleagues from the USGS and NC State. One of the most visible cracks was in the landscaping company’s parking lot, which rolled an excavator to carefully dig along the fault line.

Stewart’s research in North Carolina was on ancient faults that occurred in some cases hundreds of millions of years ago. He is particularly interested in the state’s long linear valleys or salient features, which are commonly associated with fault lines in other parts of the world. However, in North Carolina, the Appalachians were considered a dead mountain range with little chance of detecting an active fault that could cause earthquakes.

Not anymore.

A house on top of the fault line sustained damage to its chimney. The state has paid $7.2 million on 525 housing damage claims, 60 of which are significant. Damage to government infrastructure, totaling $7 million and increasing, includes two schools in Alghanim County, the District Court, Sparta Water and Sewerage Systems, and the Elkin Fire Department.

By the time Stewart reached Sparta, the damage was evident: chimneys had collapsed, the foundations of homes and businesses were cracking and out of order, riven through roads and parking lots. The crack in the landscaping was like a step, a straight slice about 10 inches long. He can put his hand along the stairs and deeper into the error after the excavator has finished digging trenches. Some of his undergraduate and graduate students joined him for fieldwork along the fault line, getting their hands on a unique geological phenomenon.

MSc student Ashley Lynn was mapping the impact of the Little River Fault as part of her thesis.

“We realized that the earthquake fault had already reached the surface. This was the largest earthquake in 100 years in North Carolina. Globally, it is an average earthquake. Usually, only large earthquakes starting in the top five to the lowest sixth cause the surface to rupture.”

Stewart said that major earthquakes occurred in 1811 and 1812 along the Mississippi River line. After the 1886 earthquake around Charleston, South Carolina, no one found a rupture in the surface.

A digital terrain model created from drone photography shows the surface displaced along the Little River fault gap cutting a business near Sparta.

That the Sparta earthquake to be the first East Coast earthquake with a fault is significant, Stewart said. Most earthquakes occur when two tectonic plates interfere with each other. The areas where they touch are the boundaries of the plate. The San Andreas Fault in California is a well-known boundary between the Pacific Plate and the North American Plate.

“When there are known plate boundaries, you know what your risk is,” he said.

Some earthquakes, such as Charleston and Sparta, occur within a plate, in those cases the North American Plate. Internal earthquakes, which are usually low in magnitude, are difficult for scientists to understand because they do not occur along clearly defined plate boundaries.

In Sparta, scientists are learning more about internal earthquakes, which can cause severe damage. “It’s hard to guess, but as this surface ruptures, we can see the rift,” Stewart said. “Without this error, earthquake risk analysis and earthquake risk prediction are much more difficult.”

Better prediction might help communities avoid the kind of damage seen in the Sparta region. To address the residential damage caused by the earthquake, the state paid $7.2 million in 525 claims, 60 of which are considered significant. Damage to government infrastructure, totaling $7 million and increasing, includes two schools in Alghanim County, the District Court, Sparta Water and Sewerage Systems, and the Elkin Fire Department. Total business damage is not known as the business owners file for insurance or other support themselves.

Stewart and others, including his students and researchers from the USGS, North Carolina, and North Carolina Geological Survey, mapped the fault, starting with the parking lot crack.

In a wooded area near Sparta, the fault pushed rock and soil to the right over the rock and soil on the left by about 15 inches.

“We walked a lot in the woods,” he said. “You find a spot where a block of land has moved upward and another piece has gone down. You keep going in the direction where you think the bug has been torn to look for other places.” The offset in spots is sometimes a step of about 15 inches.

They also relied on Airplane Deployed Lidar (short for Light Detection and Ranging), a sensing method that uses laser pulses to determine the presence, shape and distance of objects, for precise topographic measurement. Researchers can digitally remove all plants from images to clearly show the fault line. Stewart compared photos of Sparta before the earthquake with photos after the earthquake. “Subtract the old lidar from the new lidar, and it will show us where the Earth has moved. It’s amazing.”

They mapped part of the rift, an arc 2.2 miles long curved northwest of Sparta and southeast toward an ice valley. As the surface ruptured, Stewart was carefully looking for evidence of previous earthquakes.

The red line shows the 2.2-mile portion of the Little River Fault exposed by the Sparta earthquake. Stewart expects that further mapping will extend the known rupture by another 2.5 miles.

“We feel we have evidence of other faulty accidents before August 2020. This error may take another 3 or 4 kilometers (another 2.5 miles),” he said. Stewart’s mapping efforts provide information that the USGS and the State Geological Service use for earthquake planning.

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