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After large earthquakes, there is an expectation that aftershocks could occur in the hours and days that follow, but aftershocks of some of the strongest earthquakes in recorded U.S. history may still be occurring – nearly 200 years later, according to a new investigation.
According to a study recently published in the Journal of Geophysical Research: Solid Earth.
One of the regions the researchers focused on, called the New Madrid seismic zone, encompasses present-day Memphis and the surrounding Mid-Mississippi River Valley area, and 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 the subject of debate among scientists, the study authors wrote.
“You use the time, distance and magnitude of pairs of events, and try to find the link between two events, that’s the idea,” the study’s lead author, Yuxuan Chen, a geoscientist at Wuhan University in China. “If the distance between a pair of earthquakes is smaller than expected from the background events, then it is likely that one earthquake is 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 specific region.
The researchers found that about 30% of all earthquakes that occurred between 1980 and 2016 near the Missouri-Kentucky border, all of magnitude 2.5 or greater, were likely aftershocks of the three large earthquakes that hit the area in 1811 and 1812, which recorded magnitudes between 7.3 and 7.5. In the Charleston area, findings showed that about 16% of the region’s current earthquakes were likely aftershocks of the 1886 magnitude 7.0 earthquake.
Identifying whether modern earthquakes are actually aftershocks of previous large earthquakes, or are new, unrelated earthquakes, is important for understanding the future disaster risk of these regions, even if the most recent seismic activity causes little or no damage, the researchers said. researchers.
Earthquakes versus aftershocks
The modern seismic activity the researchers studied is likely a mix of aftershocks of the great earthquakes of the 19th century and background seismicity, Chen said.
“In some respects, the earthquakes look like aftershocks if you look at the spatial distribution, but the earthquakes could be closely clustered for a couple of reasons,” said Susan Hough, a geophysicist at the US Geological Survey who was not involved in the study. “One is that they are replicas, but there could also be a propagation process that is not part of a replica process. What exactly their results mean is still debatable.”
Another thing to consider when trying to determine whether an earthquake is an aftershock is how seismically active (or inactive) the region typically is, Hough said.
“In an area where small earthquakes are common, it doesn’t take that long for aftershock rates to drop below the normal seismic rate,” Hough said. “Aftershock sequences in relatively quiet areas might appear to last longer simply because there is less background seismic activity.”
Debate over long-lasting replicas
Hough co-authored a similar 2014 study that used numerous computer models 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?” Hough said in an email. “We’ve looked into it and it doesn’t seem consistent with a long-lived aftershock sequence.”
She and co-author Morgan Page, a geophysicist at the USGS Earthquake Science Center, concluded that the recent tremors were instead new, unrelated earthquakes caused by the buildup of stress along the New Earth zone. Madrid.
Because there were no seismographs in this area in the early 19th century, there is no official recorded data on these earthquakes. Existing magnitude and impact data were estimated through newspaper reports and personal diaries. Using those reports, the USGS has a pretty good record of where the earthquakes were centered and how widespread the impacts were felt.
If the 1811 to 1812 sequence was still causing aftershocks, the area would have experienced a certain number of small and moderate earthquakes during the 19th and 20th centuries, Hough explained.
“The new study looks at the question from a different angle, considering how closely clustered the earthquakes are and concluding that some of the 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 great difficulty in confirming or denying the results of these studies or the long-duration aftershock more generally is that among seismologists there is no universally accepted definition of what an earthquake aftershock is, said John Ebel, professor of geophysics. at Boston College, which was not involved in the latest study.
“Every seismologist who studies such phenomena 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 somewhat different ways, and that makes comparison of studies by different researchers subject to uncertainty and disagreement.”
For Hough’s 2014 study, researchers considered an aftershock sequence to have ended when the earthquake rate fell below the rate before the main shock. Aftershocks may still continue, but once the normal seismic rate for the area returns, she said, they will no longer be identifiable as aftershocks.
Defining a replica
In areas of frequent seismic activity like California, aftershocks from a large earthquake last less than a decade, Ebel said. He added that this is particularly the case for earthquakes of magnitude 6.5 or greater that have occurred in the last 50 years or so.
“More importantly, there are no earthquakes that any seismologist would call aftershocks of those currently occurring on the San Andreas Fault in Southern California, which last had a large earthquake in 1857, or on the San Andreas Fault. San Andreas in northern California, where the last earthquake occurred. The big earthquake happened in 1906,” Ebel told CNN. “The San Andreas faults and other faults in California appear to behave differently with respect to aftershocks than faults in central and eastern North America.”
However, in places far from plate boundaries, such as in central or eastern North America, the rate of background earthquakes is very low. Other studies have also speculated that aftershocks in areas far from plate boundaries could last for many centuries. According to Ebel, the new study simply applies another statistical method to reach a similar conclusion.
“Because all of these studies are based on statistical analyses, which inherently have some variability, these studies cannot answer the questions they address with complete certainty,” Ebel said.
It would be easier to distinguish this, he explained, if we had thousands of years of data on earthquakes in both California and eastern North America.
“For this reason, we seismologists sometimes disagree about which earthquakes are precursors or aftershocks,” Ebel said, “and I think those disagreements are inherently unresolvable.”
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