‘Dusty’ archives inspire new story about 1886 Charleston earthquake

Late in the evening of August 31, 1886, while many people were sleeping, a large earthquake struck Charleston, South Carolina, and the surrounding region. Buildings collapsed, railroad tracks buckled, and the sand began to “boil,” or bubble due to liquefaction. When the quake stopped, about 2,000 buildings had been damaged and at least 60 people had lost their lives.

The Charleston earthquake of 1886 was one of the strongest earthquakes to hit the eastern United States. The tremors were felt as far away as Boston, Chicago and New Orleans. From the first European settlement in 1670 until this point, the region experienced only occasional minor seismic activity.

As aftershocks shook the region, geologists and engineers rushed into the field, taking detailed notes and photographing the damage. Their observations captured ground disturbances in impressive detail, but scientists did not yet fully understand the connection between earthquakes and faults, so they were unable to piece together the full story.

“The timing of the Charleston earthquake was unique,” ​​said Susan Hough, a seismologist with the United States Geological Survey (USGS). “If it had happened 75 years earlier, we would have had fewer trained and ready scientists. If it had happened ten years later, seismograms probably would have recorded the tremors.”

More than a century after the quake, Hough and CIRES fellow Roger Bilham, a research scientist at CU Boulder, have picked up the trail, building on the original records and more recent attempts to piece together the story of the deadly quake.

“Although a dozen possible faults had been identified beneath the swamps surrounding Charleston, the actual fault that ruptured in the earthquake remained a mystery,” Bilham said.

The team’s search through historical documents led to exciting new discoveries about the Charleston earthquake – from the fault that may be responsible to the strength and deformation on the ground.

Their work, published in a series of four papers in 2023 and 2024, is an example of how scientists can use historical documents to uncover the layers of other geological mysteries. And inside continental plates, where seismic activity is less frequent, the work could help communities better assess their risk for future earthquakes.

Field evidence shows faults

Hough and Bilham began their investigation of the Charleston earthquake by delving into the written accounts of the event, including those of Earle Sloan, a mining engineer who took meticulous notes and measurements of damage to three railroad lines originating from Charleston. They suspected that there were clues hidden in Sloan’s notes that could help them identify the fault responsible for the earthquake.

But first some hurdles had to be overcome.

“Transforming the numbers into a compelling story turned out to be a nightmare,” Bilham explained. “The 1886 notes contained inadvertent input errors and typographical errors that shifted the positions of the buckles back and forth randomly.”

In 2022, the team traveled to Charleston in hopes of untangling the mess. They focused on a section of the railroad in Summerville where serious track disruptions had been reported in 1886. Bilham suggested using GPS methods to determine the observation locations, which Sloan had identified using railroad mileposts.

Much to their surprise, the scientists found an offset of 4.5 meters (14.8 feet) to the right, which should correspond to a straight mile of track. At first, scientists couldn’t believe the size of the offset, but when they read Sloan’s notes more closely, they discovered that he, too, had described an offset in the same location. The offset likely indicated that a fault beneath the tracks had shifted. Modern geologists had identified the Summerville Fault at this location, but no one had linked it to the 1886 earthquake.

“It was a happy coincidence that took the project to a whole new level,” Hough said.

Looking at historical maps of the area, Bilham and Hough also noted that Summerville appeared to have risen by a meter (3.3 feet) after the earthquake, while the docks at nearby Fort Dorchester had remained undisturbed since their construction in the 17th century were. The results confirmed that something significant had occurred near Summerville in 1886.

A new model for identifying the perpetrator

To identify the fault responsible for the 1886 Charleston earthquake, scientists created a mathematical rupture model of movement on the Summerville Fault that could explain both the archaeological and geological evidence, including the proper offset of the Railroad tracks and the uplift in Summerville.

Bilham and Hough found that movements along a west-dipping Summerville fault could explain why the town is higher than the surrounding swamps. The model indicated a magnitude of 7.3, consistent with the large “perceived” area of ​​the earthquake and previous estimates. They published their results in The seismic record in 2023.

“It turns out you can put the pieces together to identify the fault that caused the earthquake and create a detailed model of how the fault ruptured,” Hough said. “It was the first time anyone had done that for the Charleston earthquake.”

After identifying the potential culprit, Hough and Bilham turned their attention back to the impacts on the ground. Based on the location of the fault, they simulated what the tremors might have been like at different locations and compared the results with observations from the ancient records. The comparison, which was published in Bulletin of the Seismological Society of America in January 2024, supports its proposed magnitude of 7.3.

Deformed tracks preserve seismic waves

Bilham continued researching historical documents to find out why the railroad tracks had been damaged and torn apart 20 miles from Summerville.

“It was a monumental undertaking,” Hough said. “It was as if Sloan had passed the torch to Roger over the centuries.”

An old photo taken the day after the Charleston earthquake showed what appeared to be an offshoot of a railroad crossing a low-lying swamp. Many scientists used the photo to infer faults in the area.

Scientists constructed a virtual 3D view of the deformed railroad track using precise measurements of a thousand points in the original photograph preserved in the Charleston Museum archives. The work led to another startling discovery: The jagged tracks around Charleston had collectively recorded the contraction and compression of seismic waves emanating from the earthquake’s epicenter.

“We were able to demonstrate that there were kinks wherever the pipe was compressed more than the expansion joints would allow, and that the pipe was split where the expansion bolts were broken,” Bilham said.

The work also appeared in Bulletin of the Seismological Society of America.

The bigger picture

Hough and Bilham’s efforts demonstrate that even after 137 years, scientists can still gain new insights into the Charleston earthquake and contribute to a more comprehensive understanding of seismic activity in the region.

“Charleston is a brick in the wall,” Hough said. “Now we understand an event in one place, but there is still a lot of work to be done to piece together the bigger picture.”

Intraplate earthquakes like the one in Charleston are different from their counterparts, which occur where large portions of the Earth’s crust rub together. There is no consistent pattern that explains why they occur, and often each event requires individual investigation. But Hough hopes her work will inspire scientists to look deeper — into the past and into the future.

“There is a tendency to assume that all knowledge is on the Internet and is easily available,” Hough said. “Our efforts confirm how valuable it can be to consider the dusty original data sources.”