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After major earthquakes, there is an expectation that aftershocks may occur in the hours and days that follow, but new research has found that nearly 200 years later, aftershocks from some of the strongest earthquakes in recorded United States history can still occur.
Frequent aftershock activity from a trio of earthquakes near the Missouri-Kentucky border between 1811 and 1812 and a separate earthquake in Charleston, South Carolina, in 1886 likely continues today, according to a study recently published in the Journal of . Geophysical Research: Solid Earth.
One of the areas the researchers focused on, the New Madrid seismic zone, covers present-day Memphis and the surrounding Mid-Mississippi River Valley region, while the other includes Charleston and the surrounding 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 wrote.
“You use the time, the distance, and the magnitude of pairs of events and try to find the connection between the two events — that’s the idea,” lead study author Yuxuan Chen, a geoscientist at Wuhan University in China, said in a press release. “If the distance between a pair of earthquakes is closer than expected based on background events, then one earthquake is likely an aftershock of the other.”
Background events, also known as background seismicity, basically refer to the current rate of seismic activity that is considered normal for a particular region.
Researchers found that nearly 30% of all earthquakes near the Missouri-Kentucky border from 1980 to 2016 were magnitude 2.5 or greater, compared to three earthquakes that hit the area in 1811 and 1812, ranging in magnitude from 7.3 to 1812. He found that there were aftershocks from the large earthquake. 7.5. The findings showed that in the Charleston area, roughly 16% of today’s earthquakes are likely aftershocks of the 1886 magnitude 7.0 earthquake.
Determining whether modern earthquakes are actually aftershocks of previous large earthquakes or new, unrelated earthquakes is important for understanding the future disaster risk of these regions, even if new seismic activity causes little or no damage, the researchers said.
Earthquakes and aftershocks
The modern seismic activity the researchers studied is likely a mix of aftershocks from large earthquakes in the 1800s and background seismicity, Chen said.
“When you look at the spatial distribution, in some ways earthquakes look like aftershocks, but the earthquakes can be tightly clustered for a number of reasons,” said Susan Hough, a geophysicist with the U.S. Geological Survey who was not involved in the study. “First, it is possible that these are aftershocks, but it is also possible that a creep process is going on that is not part of the aftershock process. “Exactly what their results mean is still open to debate.”
Hough said another thing to consider when trying to determine whether an earthquake is an aftershock is how seismically active (or passive) the area is.
“In an area where small earthquakes are common, it doesn’t take very long for aftershock rates to drop below the normal seismic rate,” Hough said. “Aftershock trains in relatively quiet areas may appear to last longer due to less background seismic activity.”
Debate over long-lasting aftershocks
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 minor earthquakes in the New Madrid seismic zone aftershocks of 1811-1812 or not?” Hough said in an email. “We looked into it and it doesn’t seem consistent with a long-lived aftershock sequence.”
He and co-author Morgan Page, a geophysicist at the USGS Earthquake Science Center, concluded that the recent tremors were new, unrelated earthquakes caused by the buildup of stress along the New Madrid area.
Since there were no seismographs in this region in the early 1800s, no official data was recorded regarding these earthquakes. Available size and impact data were estimated through newspaper reports and personal diaries. Using these reports, the USGS has a pretty good record of where earthquakes are centered and how widely the effects are felt.
Hough explained that if the series of earthquakes from 1811 to 1812 were, in fact, still causing aftershocks, the region would have seen a number of small to moderate earthquakes in the 19th and 20th centuries.
“The new study looks at the question from a different angle by considering how tightly clustered earthquakes are and concludes that some events are ongoing aftershocks,” Hough said. “The question remains: If the New Madrid earthquakes are aftershocks, why don’t they follow the rules that aftershocks are known to follow?”
The biggest difficulty in confirming or rejecting the results of these studies, or long-lived aftershocks more generally, is that there is no universally accepted definition among seismologists of what an earthquake aftershock is, said geophysics professor John Ebel. Boston College, which was not included in the latest study.
“Any seismologist who studies such events has no choice but to make assumptions about how to define foreshocks, mainshocks, and aftershocks,” Ebel, who is also a senior research scientist at Boston College’s Weston Observatory, said in an email. “Therefore, different seismologists will define foreshocks, mainshocks, and aftershocks in slightly different ways, making comparing the work of different researchers subject to uncertainty and disagreement.”
For Hough’s 2014 study, researchers thought the series of aftershocks ended when the earthquake rate dropped below the rate before the main shock. Aftershocks may still be occurring, but once normal seismic velocity returns for the area, you can no longer describe them as aftershocks, he said.
Identifying an aftershock
In areas with frequent seismic activity, such as California, the aftershocks of a major earthquake last less than a decade, Ebel said. This is especially true for earthquakes of magnitude 6.5 or greater that have occurred in the last 50 years, he added.
“More importantly, there is currently no earthquake that any seismologist would call an aftershock on the San Andreas Fault in Southern California, where the last major earthquake occurred in 1857, or on the San Andreas Fault in Northern California, where the last earthquake occurred. Major earthquake.” That was in 1906,” Ebel told CNN. “The San Andreas and other faults in California appear to respond to aftershocks differently than faults in central and eastern North America.”
However, in places far from plate boundaries, such as Central or Eastern North America, the rate of background earthquakes is very low. Other studies also suggest that aftershocks in regions far from plate boundaries could last for centuries. The new study applies another statistical method to reach a similar conclusion, according to Ebel.
“Since all such studies rely on statistical analyses, which inherently have some variability, these studies cannot answer the questions they address with complete certainty,” Ebel said.
This would be easier to discern if we had thousands of years of earthquake data for both California and eastern North America, he explained.
“So we seismologists sometimes disagree about which earthquakes are foreshocks or aftershocks, and I think those disagreements are inherently irresolvable,” Ebel said.
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