How many times can you get your hands on an active seismic fault? In the eastern United States, no one was doing this until two years ago.
This is because no recorded earthquake east of the Mississippi has produced surface rupture, not even such devastating shaking as the 1886 earthquake in Charleston, South Carolina. Think of earthquakes, and images of cracked highways showing obvious fault lines come to mind. In the eastern United States, such sites had not occurred until a magnitude 5.1 earthquake in Sparta, North Carolina on August 9, 2020, became the first to reveal a fault line.
That morning, Caroline geologist Kevin Stewart rushed to Sparta. Stewart is a professor in the Department of Earth, Marine and Environmental Sciences at 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 Apennines of Italy where there are active faults, in the Rocky Mountains and in western North Carolina.
Stewart was electrified by the US Geological Survey’s advisory of the Sparta earthquake, which people felt across North Carolina, much of the eastern United States and west of Tennessee and Kentucky. He was on site at noon, along with colleagues from USGS and NC State. One of the most visible cracks was in the parking lot of a landscaping company, which deployed an excavator to carefully dig along the fault line.
Stewart’s research in North Carolina focused on ancient faults that occurred in some cases hundreds of millions of years ago. He is particularly interested in long linear valleys or prominent state lineaments, which in other parts of the world are commonly associated with fault lines. In North Carolina, however, Appalachia was considered a dead mountain range with little chance of discovering an active fault that could cause earthquakes.
By the time Stewart arrived in Sparta, the damage was evident: collapsed chimneys, cracked and shifting foundations of homes and businesses, ruptures in roads and parking lots. The crack in the landscaping company looked like a step, a straight slice of about 10 inches. He could put his hand down the step and deeper into the gap once the excavator had finished digging. Some of his graduate and undergraduate students joined him for fieldwork along the fault line, getting their hands on this unique geological phenomenon.
Master of Science graduate student Ashley Lynn mapped the trace of the Little River Fault as part of her thesis work.
“We realized that the earthquake fault had actually come to the surface. It was the largest earthquake in 100 years in North Carolina. Overall it is a moderate earthquake. Typically, only large earthquakes starting in the upper 5 through the lower 6 cause surface rupture.
Stewart said large earthquakes in 1811 and 1812 occurred on the line of the Mississippi River. After the 1886 earthquake around Charleston, South Carolina, no one found a surface break.
That the Sparta quake was the first East Coast earthquake with a fault rupture is significant, Stewart said. Most earthquakes occur where two tectonic plates rub against each other. The areas where they come into contact are the boundaries of the plates. The San Andreas Fault in California is a well-known boundary between the Pacific Plate and the North American Plate.
“When there’s a known plate boundary, you know what your danger is,” he said.
Some earthquakes, however, like Charleston and Sparta, occur inside a plate, in these cases the North American plate. Intraplate earthquakes, usually of lower magnitude, are difficult for scientists to understand because they do not occur along a clearly defined plate boundary.
In Sparta, scientists are learning more about intraplate earthquakes, which can cause enormous damage. “They’re hard to predict, but with this one breaking up the surface, we can see the fault,” Stewart said. “Without this flaw, earthquake hazard analysis and earthquake hazard prediction are much trickier.”
Better forecasts can help communities avoid the kind of damage seen in the Sparta region. To address residential damage caused by the earthquake, the state paid out $7.2 million on 525 claims, 60 of which are considered major. Damage to government infrastructure, which totals $7 million and is growing, includes two Alleghany County schools and the county courthouse, Sparta’s water and sewer systems, and utility Elkin fire. The total business damage is unknown, as business owners seek insurance or other assistance themselves.
Stewart and others, including his students and researchers from USGS, NC State and the North Carolina Geological Survey, mapped the fault, starting with the parking lot crack.
“We walked a lot in the woods,” he said. “You find a spot where a block of terrain has moved up and another down. You keep walking in the direction you think the fault broke to look for other places. The displacement in some places is sometimes a step of almost 15 inches.
They also relied on aircraft-deployed lidar (short for Light Detection and Ranging), a detection method that uses laser pulses to determine the presence, shape and distance of objects, for precise topographic measurement. Researchers can digitally remove all vegetation from the images to clearly show the fault line. Stewart compared pre-earthquake images of Sparta to post-earthquake images. “Subtract the old lidar from the new lidar, and it shows us where the ground has moved. It’s spectacular.
They mapped a segment of the fault, a 2.2-mile curved arc northwest of Sparta and southeast toward Glade Valley. Where the surface rupture dies out, Stewart carefully searched for evidence of previous earthquakes.
“We believe we have evidence of further faulty episodes prior to August 2020. It likely takes the fault an additional 3 or 4 kilometers (2.5 additional miles),” he said. Stewart’s mapping efforts provide information that the USGS and the state Geological Survey use for earthquake planning.