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It appears that some aftershocks were felt more than a hundred years after the earthquake

It appears that some aftershocks were felt more than a hundred years after the earthquake

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Historic earthquakes leave traces that can still be felt centuries later. The study examines current earthquake activity in North America.

Wuhan – Aftershocks from an earthquake can continue to shake an area for a long period of time. Although small in size compared to the original earthquake, they can damage infrastructure and complicate rescue efforts after a large earthquake. A recent study suggests that the effects of some powerful earthquakes that occurred in North America in the 19th century may still be felt in the United States today.

“Some scientists suspect that the current earthquake activity in parts of stable North America are aftershocks, while other scientists believe they are mainly the work of background seismic activity,” explains Yuxuan Chen, a geologist at Wuhan University and lead author of the study. He also adds: “We wanted to look at this from a different perspective using a statistical method.”

Study of earthquakes: Aftershocks can last for centuries

Study that In the Journal of Geophysical Research: Solid Earth published Focus on three historic earthquakes that are among the strongest in inland North America. Chen’s research team specifically excluded the northwest of the continent because its proximity to the edges of tectonic plates causes earthquakes there more often. In contrast, earthquakes are relatively rare in “stable” inland areas.

Earthquakes selected for study:

  • Southeastern Quebec, Canada, earthquake of 1663.
  • Three earthquakes near the Missouri-Kentucky border in 1811 and 1812.
  • Earthquake near Charleston, South Carolina in 1886.
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Earthquakes located within a radius of 250 km from the epicenter were analyzed

To identify or rule out more recent earthquakes as possible aftershocks of previous tremors, the research team created a 250-kilometre radius around the epicenters of historical earthquakes. Only earthquakes in these areas are included because aftershocks usually occur near the epicenter of the mainshock. In addition, only earthquakes with a magnitude of at least 2.5 were included in the study. Data from current earthquakes were analyzed using statistical methods.

During a violent earthquake that struck Turkey, this house in the city of Gaziantep remained standing and was no longer habitable. (Archive photo) © IMAGO/Zakaria Yahya

Chen explains in one notice: “You use time, distance, and force pairs of events and try to find the relationship between two events — that’s the idea.” “If the distance between a pair of earthquakes is less than expected based on background events, one earthquake is more likely to be an aftershock of the other,” he continues.

The spatial distribution of earthquakes reveals that the tremors could be aftershocks from the 19th century

The research team examined, among other things, the spatial distribution of earthquakes and concluded that current earthquake activity in Quebec is unrelated to the 1663 earthquake. However, things could be different for the two American earthquakes examined in the study. Perhaps 30% of all earthquakes recorded on the Missouri-Kentucky border between 1980 and 2016 were aftershocks of the major earthquakes that occurred in 1811 and 1812.

The study shows that the number in Charleston could be even higher: As many as 72 percent of modern earthquakes could be aftershocks from the 1886 quake. Therefore, the current seismic activity in both areas could be caused partly by aftershocks and partly by background activity. “It’s kind of a mix,” Chen explains.

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The earthquake expert isn’t entirely convinced

However, Susan Hough, a geophysicist at the USGS, who was not involved in the study, is not entirely convinced by these findings. “Earthquakes in some ways look like aftershocks when you look at the spatial distribution, but there are several reasons why earthquakes are so close together,” she explains. He adds: “On the one hand, these are aftershocks, but it is also possible that there is a creeping process occurring that is not part of the aftershock process. What exactly their results mean is still unclear.

The editor wrote this article and then used an AI language model to improve at her own discretion. All information has been carefully checked.