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After major earthquakes, aftershocks are expected to occur in the hours and days that follow, but aftershocks may occur from some of the strongest earthquakes in recorded US history – nearly 200 a year later, new research has found.
Frequent aftershock activity caused by three earthquakes near the Missouri-Kentucky border between 1811 and 1812, and an earthquake on born in Charleston, South Carolina, in 1886. Geophysical Research: Solid Earth.
One of the researchers’ focus region, known as the New Madrid seismic zone, includes present-day Memphis and the surrounding area of the Middle Mississippi River Valley, 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, duration and magnitude of the pairs of events, and you try to find the link between two events – that’s the idea,” study lead author Yuxuan Chen, a geoscientist at Wuhan University in China, said in a news release . “If the distance between a pair of earthquakes is closer than expected from background events, one earthquake is likely to be a successor to the other.”
Background events, also known as background seismicity, basically refer to the current rate of seismic activity that is considered normal for a given region.
The researchers found that about 30% of all earthquakes from 1980 to 2016 near the Missouri-Kentucky border, all magnitude 2.5 or greater, were likely aftershocks from the three major earthquakes. land hit the area in 1811 and 1812, which registered between sizes 7.3 and. 7.5. In the Charleston area, the results showed that about 16% of the region’s modern earthquakes were likely aftershocks from a magnitude 7.0 earthquake in 1886.
Recognizing whether modern earthquakes are actually the aftershocks of previous large earthquakes, or whether they are new, unrelated earthquakes, is important to understanding the future disaster risk of these regions – even if the seismic activity does not more recent damage, the researchers said.
Earthquakes vs aftermath
The modern seismic activity studied by the researchers is likely a combination of aftershocks from the great earthquakes of the 1800s and background seismicity, Chen said.
“In some ways, earthquakes look like earthquakes if you look at the spatial distribution, but earthquakes can be tightly clustered for a couple of reasons,” said Susan Hough, a geophysicist with the US Geological Survey who was not involved in the study. “One is that they are ex-students, but also you could have an escape process that is not part of a post-traumatic process. One can question exactly what their results mean.”
Another thing to consider when trying to determine whether a quake is an aftershock is how seismically active (or inactive) the region usually is, Hough said.
“In an area where small earthquakes are common, it doesn’t take long for aftershock rates to drop below the normal seismic rate,” Hough said. “Aftershock sequences in relatively quiet areas may appear to last longer simply because there is less background seismic activity.”
Debate on a post-longevity crowd
Hough conducted a similar study in 2014 using extensive computer modeling to understand activity in the New Madrid seismic zone, and reached a different conclusion.
“Are there small earthquakes in the 1811-1812 New Madrid seismic zone?” Hough said in an email. “We looked into it, and it doesn’t look like a postfix sequence that lasted that long.”
She and co-author Morgan Page, a geophysicist with the USGS Earthquake Science Center, concluded that the recent earthquakes along the New Madrid belt were caused by unrelated new earthquakes.
Because there were no seismographs in this area in the early 1800s, there are no official recorded data from these quakes. Size and impact data were estimated through newspaper reports and personal journals. Using those reports, the USGS has a pretty good record of where the quakes were centered and how widely the impacts were felt.
If the 1811 to 1812 sequence was still the cause, the area would have seen a number of small and moderate earthquakes during the 19th and 20th centuries, Hough explained.
“The new study looks at the issue from a different angle, considering how tightly clustered earthquakes are, and concluding that some of the events are continuous aftershocks,” Hough said. “The question remains: if earthquakes in New Madrid are aftershocks, why don’t they follow the rules known to follow aftershocks?”
The major difficulty in confirming or denying the results of these studies or the wider long-lived aftershock is that there is no universally agreed upon definition among seismologists of what an earthquake aftershock is, said John Ebel , a professor of geophysics at Boston College who was not involved in the latest study.
“All seismologists who study such phenomena have no choice but to make assumptions about how to define foreshocks, aftershocks and aftershocks,” Ebel, who is a senior research scientist at Boston College’s Weston Observatory, said in an email. “Therefore, different seismologists will define overthrusts, overthrusts and aftershocks in slightly different ways, making comparisons between studies by different investigators subject to uncertainty and disagreement.”
For Hough’s 2014 study, researchers considered an aftershock sequence to have ended when the rate of earthquakes fell below the rate before the mainshock. There may still be aftershocks going on, but once the normal seismic rate returns to the area, she said, you can no longer identify them as aftershocks.
Defining aftershock
In areas with frequent seismic activity such as California, the aftermath of a major earthquake lasted less than a decade, Ebel said. He said this is especially true for earthquakes of magnitude 6.5 or greater that have occurred in the last 50 years or so.
“More to the point, there are no earthquakes that any seismologist would call former vessels currently underway on the San Andreas Fault in Southern California that last experienced a major earthquake in 1857 or on the San Andreas Fault in Northern California where the last one was. there was a big earthquake in 1906,” Ebel told CNN. “The behavior of the San Andreas and other faults in California appears to be different in reality than the faults in central and eastern North America.”
However, places away from plate boundaries, such as in Central or Eastern North America, the background rate of earthquakes is very low. Other studies have also speculated that former sites in areas away from plate boundaries may survive for hundreds of years. The new study simply applies a different statistical method to reach a similar conclusion, according to Ebel.
“Because all such studies rely on statistical analyses, which are inherently somewhat variable, these studies cannot answer the questions they address with absolute certainty,” Ebel said.
It would be easier to distinguish this, he explained, if we had thousands of years of earthquake data for California and Eastern North America.
“For this reason, we seismologists sometimes disagree about which earthquakes are foreshocks or aftershocks,” Ebel said, “and I think those disagreements are fundamentally irreconcilable.”
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