Modern earthquakes have been called aftershocks of the tremors of the century before last

After major earthquakes, aftershocks are expected to occur in the hours and days that follow, but aftershocks from some of the strongest earthquakes in recorded United States history may still be occurring—nearly 200 years later, a new study has found.

Frequent aftershocks caused by three earthquakes near the Missouri-Kentucky border between 1811 and 1812 and a separate Charleston, South Carolina, earthquake in 1886 are likely continuing today, according to CNN, according to a study published recently in the Journal of Geophysical Research: Solid Earth.

One of the regions the researchers focused on, called the New Madrid Seismic Zone, covers present-day Memphis and the surrounding Middle Mississippi River Valley region, while another includes Charleston and the adjacent coastal plain. Seismic activity in these relatively stable regions of North America is not well understood, and its nature is debated among scientists, the study authors write.

“You use time, distance and magnitude of pairs of events and try to find a connection between two events—that’s the idea,” says lead study author Yuxuan Chen, a geoscientist at Wuhan University in China. “If the distance between a pair of earthquakes is closer than expected due to background events, then one earthquake is likely an aftershock of the other.”

Background events, also known as background seismicity, basically refer to the current level of seismic activity that is considered normal for a particular region.

The researchers found that approximately 30% of all earthquakes from 1980 to 2016 near the Missouri-Kentucky border with a magnitude of 2.5 or greater were likely aftershocks from three large earthquakes that struck the area in 1811 and 1812, which were recorded at magnitude from 7.3 to 7.5. In the Charleston area, the results showed that approximately 16% of the region’s modern earthquakes were likely aftershocks from the 1886 magnitude 7.0 earthquake.

Determining whether the current earthquakes are actually aftershocks of previous large earthquakes or are new, unrelated earthquakes is important for understanding future disaster risk in these regions—even if the new seismic activity causes little or no damage, the researchers say.

The modern seismic activity the researchers studied is likely a mixture of aftershocks from large earthquakes in the 1800s and background seismicity, Chen said.

“In some respects, earthquakes look like aftershocks when you look at their spatial distribution, but earthquakes can be closely clustered for several reasons,” said Susan Hough, a geophysicist with the US Geological Survey. “Firstly, there are aftershocks, but there can also be a creep process that is not part of the aftershock process. What exactly their results mean is still an open question.”

Another thing to consider when trying to determine whether an earthquake is an aftershock is how seismically active (or inactive) the region typically is, Hugh emphasizes.

“In an area where small earthquakes are common, it doesn’t take that long for the aftershock frequency to drop below normal seismic activity,” says Susan Hu. “Sequences of aftershocks in relatively quiet areas may last longer simply because there is less background seismic activity.”

Hough co-authored a similar 2014 study that used extensive computer modeling to understand activity in the New Madrid seismic zone and came to a different conclusion.

“Are the small earthquakes in the New Madrid seismic zone aftershocks of 1811-1812 or not? – Hugh notes. “We’ve looked into it and it doesn’t seem consistent with a long sequence of aftershocks.”

She and co-author Morgan Page, a geophysicist at the USGS Seismology Science Center, concluded that the recent tremors were new, unrelated earthquakes caused by rising stress along the New Madrid zone.

Because there were no seismographs in the area in the early 1800s, these earthquakes are not officially recorded. Existing data on magnitude and impact were obtained from newspaper reports and personal diaries. Using these reports, the USGS has a pretty good idea of ​​where the earthquakes were concentrated and how widely their effects were felt.

If the 1811-1812 earthquake sequence were actually still causing aftershocks, the area would have experienced a number of small to moderate earthquakes during the 19th and 20th centuries, Hugh explains.

“The new study looks at the issue from a different angle, considering how closely the earthquakes are clustered, and concludes that some of the events are ongoing aftershocks,” says Susan Hu.

The big difficulty in confirming or disproving the results of these studies, or long-lived aftershocks more broadly, is that there is no generally accepted definition among seismologists of what an earthquake aftershock is, admits John Ebel, a professor of geophysics at Boston College.