‘Dusty’ archives inspire new story about the 1886 Charleston earthquake

Late on August 31, 1886, while many people slept, a major earthquake shook Charleston, South Carolina, and the surrounding region, toppling buildings, buckling railroad tracks, and “boiling” or bubbling sand as it liquefied. By the time the shaking stopped, about 2,000 structures had been damaged and at least 60 people had lost their lives.

The 1886 Charleston earthquake was one of the most powerful earthquakes in the eastern United States, with tremors felt as far away as Boston, Chicago and New Orleans. From 1670, when Europeans first settled in Charleston, until that time, the region experienced only occasional minor seismic activity.

As aftershocks rocked the region, geologists and engineers rushed into the field, taking detailed notes and taking photos of the damage. Their observations captured ground disturbances in impressive detail, but scientists did not yet fully understand the relationship between earthquakes and faults, leaving them 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, fewer scientists would have been trained and able to take action. If it had happened 10 years later, seismograms would likely have captured the shaking.”

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

“Although a dozen possible faults had been identified beneath the marshes around Charleston, the actual fault that ruptured during 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The team’s search through historical documents led to exciting new discoveries about the Charleston earthquake – from the rupture that may be responsible to its size and deformation on the ground.

Their work, published in a series of four papers in 2023 and 2024, provides an example of how scientists can use historical documents to uncover the layers of other geological mysteries.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

Field research shows that there are errors

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

But there were a few hurdles they had to overcome first.

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

In 2022, the team traveled to Charleston in hopes of untangling the muddle. They targeted a section of the railroad in Summerville where serious track disruptions had been reported in 1886. Bilham proposed using GPS methods to determine the locations of sightings, which Sloan had counted using railroad milestones.

Much to their surprise, the scientists identified a 15-foot distance to the right in what should have been a straight line of a mile. At first the scientists couldn’t believe the magnitude of the shift, but when they looked more closely at Sloan’s notes, they discovered that he too had described a shift at the same location. The offset probably meant that a fault had moved under the tracks. Modern geologists had identified the Summerville Fault at that location, but no one had linked it to the 1886 earthquake.

“It was a moment of serendipity that opened up a whole new dimension to the project,” Hough said.

Looking at historical maps of the area, Bilham and Hough also found that Summerville appeared to have risen 1 meter (3.3 feet) after the earthquake, while the docks of nearby Fort Dorchester had been undisturbed since their construction in the 17th century stayed. The findings confirmed that something momentous had occurred near Summerville in 1886.

A new model to identify the perpetrator

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

Bilham and Hough found that movement along a western Summerville fault could explain why the city is higher than the surrounding swamps. The model indicated a magnitude of 7.3, which is consistent with the large “felt” 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 come up with a detailed model for how the fault broke down,” Hough said. “It was the first time anyone had done that before the Charleston earthquake.”

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

Distorted traces retain seismic waves

Bilham continued to delve into historical records to find out why the railroad tracks twenty miles from Summerville were bent and torn.

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

An old photo taken the day after the Charleston earthquake showed what appeared to be a divergence of the railroad track where it crossed a low-lying swamp. Many scientists used the photo to deduce faults in the area.

The scientists constructed a virtual 3D representation of the distorted railroad using precise measurements of a thousand points on the original photo, which was preserved in the archives of the Charleston Museum. The work led to another astonishing realization: the buckled tracks around Charleston had collectively recorded the contraction and compression of seismic waves rushing from the earthquake’s epicenter.

“We were able to demonstrate that buckling occurred throughout, that the line was compressed more than allowed by the expansion joints, and that the line had separated where the expansion bolts had broken,” Bilham said.

The work was also published in the Bulletin of the Seismological Society of America.

The bigger picture

Hough and Bilham’s efforts show that even after 137 years, scientists can still learn new things about the Charleston earthquake and contribute to the broader understanding of seismic activity in the region.

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

Intraplate earthquakes like the one in Charleston differ from their counterparts, which occur where large pieces of the Earth’s crust rub against each other. There is no clear pattern that can explain why they occur, and often each event requires unique investigation. But Hough hopes their work will inspire scientists to look deeper – into the past and the future.

“There is a tendency to assume that all knowledge is on the Internet and readily available,” says Hough. “Our efforts confirm how much value there can be in considering the dusty original data sources.”