'Dusty' archives inspire new story about the 1886 Charleston earthquake

A photo taken the day after the 1886 Charleston earthquake appears to capture the displacement of the railroad line to the right. Computer analysis of the scene reveals distances (in meters) and offsets in the scene that tell a different story. The track buckled at the bend with about 4 inches of longitudinal stress. The inset shows the same path in 2023. Source: Pelham and Hough

Late on the day of August 31, 1886, while many people were asleep, a large earthquake shook Charleston, South Carolina, and the surrounding area, toppling buildings, buckling railroad tracks, and causing sand to “boil” or bubble from liquefaction. By the time the shaking stopped, nearly 2,000 buildings were damaged and at least 60 people lost their lives.

The Charleston earthquake of 1886 was one of the strongest earthquakes to hit the eastern United States, with shaking felt as far away as Boston, Chicago, and New Orleans. From 1670 when Europeans first settled in Charleston until that time, the area had only occasionally experienced minor seismic activity.

As aftershocks rocked the area, geologists and engineers rushed to the field, recording detailed observations and taking photos of the damage. Their observations recorded ground disturbances in impressive detail, but scientists did not yet fully understand the relationship 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 US Geological Survey (USGS). “If this had happened 75 years ago, we would have had fewer scientists trained and able to get to work. If it had happened ten years later, seismometers would likely have recorded the shaking.”

More than a century after the quake, Hough and CIRES fellow Roger Pelham, a research scientist at the University of Colorado Boulder, retraced the path, drawing on original records and more recent attempts to piece together the story of the deadly quake.

“Although dozens of potential faults have been identified beneath the marshes surrounding Charleston, the actual fault that ruptured in the earthquake has remained a mystery,” Pelham said.

The team's research into historical documents led to exciting new discoveries about the Charleston earthquake, starting with the fault that may be responsible for its size and deformation on the ground.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 layers of other geological mysteries. In the interior of continental plates, where seismic activity is less frequent, this work may help communities better assess their risk of future earthquakes.

Field evidence reveals an error

Hough and Pelham began their investigation of the Charleston earthquake by delving into written accounts of the event, including those written by Earl Sloan, a mining engineer who took careful notes and measurements of damage to three railroad lines radiating from Charleston. They suspected that there was evidence buried within Sloan's notes that could help them pinpoint the fault responsible for the earthquake.

But there were some hurdles they had to overcome first.

“Turning the numbers into a compelling story turned out to be a nightmare,” Pelham explained. “The 1886 notes inadvertently included input errors and typographical errors that caused the positions of the buckles to be randomly changed here and there.”

In 2022, the team traveled to Charleston in hopes of resolving the mess. They focused on a section of railway track at Summerville where severe track disturbances had been reported in 1886. Pelham suggested using GPS methods to locate observations, which Sloan recorded using railway landmarks.

To their surprise, scientists identified a 4.5-meter (14.8-foot) deviation to the right in what should have been a straight mile of track. At first, scientists couldn't believe the scale of the displacement, but after closer reading of Sloan's notes, they discovered that he too had described the displacement at the same location. The displacement likely means that a fault beneath the tracks has moved. Modern geologists have identified the Summerville Fault at that location, but no one has linked it to the 1886 earthquake.

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

When they looked at historical maps of the area, Pelham and Hogue also found that Summerville appeared to have risen by 1 meter (3.3 ft) after the quake, while the docks at nearby Fort Dorchester had remained undisturbed since their construction in the 17th century. The results confirmed that something momentous had happened near Summerville in 1886.

A new model for identifying the perpetrator

To determine which fault was responsible for the 1886 Charleston earthquake, scientists built a mathematical rupture model of motion on the Summerville Fault that could account for both archaeological and geological evidence, including right displacement in railroad tracks and uplift at Summerville.

Pelham and Hogg found that movement along the Somerville Fault that dips westward could explain why the city lies at a higher elevation than the surrounding marshes. The model indicated a magnitude of 7.3, which is consistent with the large “felt” area of ​​the earthquake and previous estimates. They will publish their results in the seismic record in 2023.

“It turns out that you can put the pieces together to determine which fault caused the earthquake and come up with a detailed model of how the fault occurred,” Hogue said. “This was the first time anyone had done this in the Charleston earthquake.”

After identifying the likely culprit, Hogg and Pelham turned their attention back to the effects on the ground. Using the location of the fault, they simulated what shaking might be like at different locations and compared the results to observations from ancient records. The comparison, published in the Bulletin of the Seismological Society of America in January 2024, supports its proposed magnitude of 7.3.

Deformed tracks sustain seismic waves

Pelham continued to research historical documents to find out why the railroad tracks were torn up 20 miles from Summerville.

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

An old photo taken the day after the Charleston earthquake showed what appeared to be a derailment of the railroad track as it crossed a low-lying swamp. Many scientists have used the image to deduce rifting in the area.

Scientists built a virtual 3D rendering of the deformed railway track using precise measurements of a thousand points in the original image, which remains preserved in the Charleston Museum's archives. This work led to another startling realization: that the winding tracks around Charleston collectively recorded the contraction and compression of seismic waves emanating from the epicenter.

“We were able to show that buckles were occurring everywhere, that the line had been compressed more than the expansion joints would allow, and that the line had separated where the expansion bolts had broken,” Pelham said.

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

The bigger picture

Hough and Pelham'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 place, but there is still a lot of work to do to piece together the bigger picture.”

Inland earthquakes like the one in Charleston are different from their counterparts, which occur when large pieces of the Earth's crust rub against each other. There is no single pattern to explain why they occur, and oftentimes, each event requires a unique investigation. But Hough hopes their work will inspire scientists to look deeper into the past and future.

“There is a tendency to assume that all knowledge is online and readily available,” Hogue said. “Our efforts underscore how valuable it can be when looking at dusty, original data sources.”

More information: Roger Pelham et al., Static and Dynamic Strain in the Charleston, South Carolina, Earthquake of 1886, Bulletin of the Seismological Society of America (2024). doi: 10.1785/0120240025

Susan E. Hogue et al., Charleston, South Carolina 1886, Earthquake: Intensity and Ground Motions, Bulletin of the Seismological Society of America (2024). doi: 10.1785/0120230224

Roger Pelham et al., Charleston, South Carolina 1886, Earthquake: Displacement of Remnant Railroad Reveals Rupture, Seismic Record (2023). doi: 10.1785/0320230022

Provided by the University of Colorado at Boulder

Citation: 'Dusty' archives inspire new story about 1886 Charleston earthquake (2024, May 21) Retrieved May 21, 2024 from https://phys.org/news/2024-05-dusty-archives-story-charleston-earthquake .html

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