Aftershocks can last for centuries after historic earthquakes 

A new study suggests that certain areas of the United States might still be experiencing aftershocks from massive earthquakes that occurred as far back as the 1800s. This new perspective challenges the traditional understanding of aftershocks and their duration.

The central and eastern United States were struck by some of the most powerful earthquakes in recorded U.S. history during the 1800s. 

Historic earthquakes 

These historic quakes ranged in magnitude from 6.5 to 8.0, including events near southeastern Quebec, Canada in 1663, a series near the Missouri-Kentucky border from 1811 to 1812, and an earthquake in Charleston, South Carolina, in 1886.

Now, nearly two centuries later, a study led by Yuxuan Chen, a geoscientist at Wuhan University, suggests that two of these regions might still be feeling the impacts of the historic earthquakes.

Long-lived aftershocks 

Aftershocks, smaller earthquakes following a major quake, typically decrease over time as part of a fault’s readjustment process. They can last from days to years, but this study indicates that in some cases, they might continue for centuries.

“The earthquakes in stable North America (the central and eastern US and part of eastern Canada) remain enigmatic and debated,” wrote the study authors. 

“Some scientists think that present-day earthquakes there are long-lived aftershocks of large historic earthquakes; others believe that they are background seismicity concentrated in weak zones and therefore are indicative of future hazard.”

“We wanted to view this from another angle using a statistical method,” said Chen.

Nearest neighbor approach 

Chen and his team applied a method known as the “nearest neighbor approach” to USGS earthquake data, focusing on seismic events within a 250-kilometer radius of the historic epicenters and with magnitudes above 2.5. 

This method assesses the likelihood of an earthquake being an aftershock based on its proximity in time and space to the original event, compared to background seismicity.

Focus of the study 

Aftershocks occur close to the original quake’s epicenter and before the level of background seismicity has resumed, according to the USGS. This means that scientists can use a region’s background seismicity and an earthquake’s location to link a quake back to a mainshock.

“You use the time, distance and the magnitude of event pairs, and try to find the link between two events – that’s the idea,” said Chen. “If the distance between a pair of earthquakes is closer than expected from background events, then one earthquake is likely the aftershock of the other.”

Tightly clustered earthquakes

Susan Hough, a geophysicist with the USGS who was not involved in the study, noted that the distance between epicenters is only one piece of the puzzle.

“In some respects, the earthquakes look like aftershocks if you look at the spatial distribution, but earthquakes could be tightly clustered for a couple of reasons,” said Hough. 

“One is that they’re aftershocks, but also you could have a process of creep going on that’s not part of an aftershock process. Exactly what their results mean is still open to question.”

Intriguing results

The study revealed that for the 1663 Quebec earthquake, the aftershock sequence appears to have concluded, with modern seismicity unrelated to the historic event. 

However, around 30 percent of earthquakes from 1980 to 2016 near the Missouri-Kentucky border and about 16 percent of recent quakes in Charleston, South Carolina, could be long-lived aftershocks from the 1811-1812 and 1886 earthquakes, respectively.

Background seismicity

The researchers found background seismicity to be the dominant cause of earthquakes in all three of the study regions, which could be a sign of continued strain accrual. 

Aftershock sequences weaken over time, but strain accrual can lead to larger earthquakes in the future, the experts noted. 

“Long-lived aftershocks may be common in stable continents. Improved estimate of the relative levels of long-lived aftershocks and background seismicity could contribute to better hazard assessment,” concluded the researchers.

The study is published in the Journal of Geophysical Research Solid Earth.

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