Earthquakes in the U.S. today might be echoes of those from the 19th century, according to scientists
Three earthquakes near the border between Missouri and Kentucky between 1811 and 1812, along with one in Charleston, South Carolina, in 1886, have been linked to frequent aftershocks, some of which may still be ongoing, according to a recent study published in Geophysical Research Letters.
Two areas of concern for researchers are the New Madrid Seismic Zone, encompassing Memphis and the central Mississippi River region, and Charleston and its surrounding coastal area. The seismic activity in these relatively stable areas of North America is unclear, with scientists debating their nature, write the study's authors.
"We use the timing, distance, and magnitude of event pairs and try to find a connection between the two events – that's the idea," said study lead author Yuxuan Chen, a geoscientist at Wuhan University in China, in a press release. "If two earthquakes are closer together than we would expect based on background seismicity, then the second earthquake is likely a aftershock of the first."
Background seismicity, or ongoing seismic activity, refers primarily to the current seismic event rate that is considered normal for a particular region.
Researchers found that about 30% of earthquakes with a magnitude of 2.5 or higher in the border region between Missouri and Kentucky between 1980 and 2016 were likely aftershocks of the three large earthquakes that struck the region in 1811 and 1812. The three earthquakes had magnitudes of 7.3, 7.3, and 7.5. In the Charleston region, results suggest that about 16% of modern earthquakes could be aftershocks of the 1886 earthquake, which had a magnitude of 7.0.
Determining whether modern earthquakes are indeed aftershocks of previous large earthquakes or new, independent events will be crucial for understanding future catastrophic risk in these regions, even if new seismic activity causes little or no damage, scientists say.
Earthquakes and Aftershocks
Chen said the modern seismicity the researchers studied is likely a mix of aftershocks of 19th-century earthquakes and ongoing background seismicity.
"When you look at the spatial distribution, these earthquakes in a way look like aftershocks, but for several reasons they can be quite close together," Chen, who was not involved in the study, said in a press release. "They are aftershocks, but it could also be a slow process that is not an aftershock process." What precisely their results mean is still a matter of debate.
When determining whether an earthquake is an aftershock, it's also important to consider the background seismic activity (or inactivity) of the area, said study co-author Lori Dengler, a geophysicist at the University of Missouri-Rolla.
"In areas with frequent small earthquakes, the aftershock rates will decrease quickly below the background earthquake rates, but aftershock sequences in more quiescent areas can keep going for a longer time because there's less ongoing seismic activity," Dengler said.
Debate Over Long-Term Aftershocks
Hough was a co-author of a similar study published in 2014 that used extensive computer modeling to understand seismic activity in the New Madrid Seismic Zone, but reached different conclusions.
"Are the small earthquakes in the New Madrid Seismic Zone a aftershock of 1811-1812?", Hough wrote in an email. "We studied that and found it not to be consistent with long-term aftershock sequences."
Historical records from the 19th and 20th centuries do not include official data on these earthquakes, as there were no seismographs in the region at the time. Data on their size and impact had to be estimated from newspaper accounts and personal diaries. The U.S. Geological Survey now has detailed records of where and how widely the earthquakes were felt.
Hough explained that the region would have experienced a series of smaller and moderate earthquakes if the earthquake sequence of 1811-1812 had in fact continued to generate aftershocks.
"This new study looks at the problem from a different perspective, considers the intensity of earthquakes and concludes that some events are prolonged aftershocks," Hough said. "The question remains, though: If the 1812 earthquake in New Madrid was an aftershock, why isn't it behaving like an aftershock?"
The greatest challenge in confirming or rejecting these findings or the idea of long-term aftershocks in general is that seismologists have no widely-accepted definition of what an earthquake aftershock is, said John Ebel, a seismologist at Boston College, who was not involved in the study.
"Every seismologist studying these types of phenomena has no other choice but to make assumptions about how to define foreshocks, mainshocks, and aftershocks," Ebel said in a press release. "As a result, different seismologists define foreshocks, mainshocks, and aftershocks in different ways, so comparisons of studies from different researchers are clouded with uncertainties and differing opinions."
In Hough's 2014 study, the authors suggested that an aftershock sequence ends when the earthquake rate falls below the pre-mainshock rate. Aftershocks might still continue, but they would no longer be distinguishable as such once seismic activity in the region normalizes, Hough said.
Definition of Aftershocks
In areas with high seismic activity, such as California, aftershocks of major earthquakes generally last less than a decade, particularly for earthquakes with magnitudes of 6.5 or stronger that happened in the past 50 years, Ebel added.
"Even more important, though, are the earthquakes that occurred in California during the 20th century, but were not big enough to be considered any sort of aftershock," Ebel told CNN. "1906 had a major earthquake, but the San Andreas Fault and other faults in California show different behavior when it comes to aftershocks compared to faults in Middle and Eastern North America."
Far from plate boundaries, such as in Middle or Eastern North America, the frequency of background seismicity is extremely low. Other studies have speculated that aftershocks in remote areas could continue for centuries.
Ebel said the new study has simply applied a different statistical method to reach similar conclusions.
"Since all such studies rely on statistical analyses that are inherently variable, they can't definitively answer the questions they address," Ebel said.
He explained that it would be easier to distinguish aftershocks from mainshocks with decades of seismic data from California and the eastern United States.
"So why are seismologists sometimes uneasy about defining 'aftershocks'?", Ebel asked. "I think these differences of opinion are inevitable."
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The study published in Geophysical Research Letters shows that many modern earthquakes in the Missouri-Kentucky border region and Charleston, South Carolina, are likely aftershocks of 19th-century earthquakes.
Aftershocks from the New Madrid earthquakes, which took place in the New Madrid Seismic Zone, which encompasses Memphis and the central Mississippi River region, could still be influencing seismic activity in the area. Similarly, earthquakes in Charleston and its surrounding coastal area may still be feeling the effects of an 1886 earthquake.
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Enrichment Data:
- New Madrid earthquakes, which occurred from 1811 to 1812, were a series of major earthquakes that shook the New Madrid Seismic Zone, a region of high seismic activity in midwestern United States. The aftershocks from these earthquakes continued for several years, and some researchers believe that they may still be influencing seismic activity in the area.
- The earthquakes in Charleston, South Carolina, which occurred in 1886, were also a significant event in seismic history. The aftershocks from this earthquake also lasted for several years, and some researchers believe that they may still be influencing seismic activity in the area.
- The New Madrid Seismic Zone is a region of high seismic activity in midwestern United States, which has experienced significant seismic activity over the centuries. This zone is characterized by high earthquake risk, and researchers continue to study its seismic activity to better understand the region's geological history.
- The exact duration and intensity of aftershocks can vary significantly from one event to another, depending on the magnitude of the main event and the specific seismic zone involved. In general, larger earthquakes tend to have longer-lasting and more intense aftershock sequences. However, the specific duration and intensity of aftershocks can also be influenced by other factors, such as the underlying tectonic conditions and the specific faults involved.
The data suggests that aftershocks from the New Madrid earthquakes, which took place between 1811 and 1812, could still be influencing seismic activity in the Missouri-Kentucky border region. Similarly, earthquakes in Charleston, South Carolina, might still be feeling the effects of an earthquake that occurred in 1886. The specific duration and intensity of these aftershocks can vary depending on the magnitude of the main event and the specific seismic zone involved. New Madrid Seismic Zone is a region of high seismic activity in midwestern United States, which has experienced significant seismic activity over the centuries. Researchers continue to study its seismic activity to better understand its geological history.
