Aftershocks happen after big earthquakes, lasting weeks or even decades. In the U.S., certain places might be feeling shocks from events that occurred centuries ago.
Back in the 1800s, some of the most powerful earthquakes in U.S. history hit the middle part of North America. Now, nearly 200 years later, a recent study suggests that the central and eastern parts of the United States could still be having aftershocks from those old earthquakes.
Scientists have different ideas about the earthquakes happening now in stable parts of North America.
Some think they’re aftershocks, while others believe it’s just regular seismic activity, explained Yuxuan Chen, a geoscientist at Wuhan University and the main person behind the study.
Yuxuan Chen wanted to take a look at this from another angle using statistical methods.
The research was shared on November 7 in the Journal of Geophysical Research: Solid Earth.
Seismic activity could be a sign of old or upcoming earthquakes
Areas close to where these old earthquakes happened still have a lot of seismic activity today. So, the earthquakes we’re having now might be long-lasting aftershocks from those old ones. But, they could also be signs of upcoming bigger earthquakes or just the regular amount of shaking a place usually has, as reported by SciTechDaily.
The U.S. Geological Survey (USGS) says we can’t tell the difference between signs of bigger earthquakes and normal shaking until a big earthquake actually happens. However, scientists can figure out if something is an aftershock.
Figuring out why these modern earthquakes are happening is crucial to knowing how much risk these areas have for future disasters, even if the current shaking isn’t causing much harm.
The team looked closely at three big earthquake events that were thought to be between magnitude 6.5 and 8.0. One happened near southeastern Quebec, Canada, in 1663. The second was a group of three earthquakes near the Missouri-Kentucky border from 1811 to 1812.
The third was an earthquake in Charleston, South Carolina, in 1886. These three events are the most significant earthquakes that have happened recently in stable North America, and bigger earthquakes usually cause more aftershocks, according to research.
Considering earthquakes greater than or equal to a magnitude of 2.5
To see if today’s earthquakes are lasting aftershocks, the team had to pick which recent quakes to study.
Aftershocks usually happen close to where the first earthquake was, so they looked at earthquakes within 250 kilometers (155 miles) of where the old ones were.
They paid attention to earthquakes with a magnitude of 2.5 or higher because it’s hard to accurately record anything smaller than that.
The team used a statistical method called the nearest neighbor method on earthquake data from the U.S. Geological Survey (USGS). This helped them figure out if recent earthquakes were probably aftershocks or just regular shaking.
Aftershocks happen near the original earthquake’s center and before the normal shaking starts again. So, by looking at where an earthquake is and the usual shaking in that area, scientists can connect it to the main earthquake, according to the USGS.