WASHINGTON – In a study that is sure to shake up the geological world, parts of the United States are still dealing with the aftershocks of earthquakes that struck in the 1800s. Two centuries ago, some of the most powerful earthquakes ever recorded in the US they shook the central and eastern regions of the country, such as Missouri, Kentucky and South Carolina. These earthquakes are believed to have had a magnitude between 6.5 and 8.0.
The replicas are smaller seismic events that can occur as a result of a major earthquake, persisting for days or years after the initial earthquake. They are a natural part of the Earth’s fault adjustment process after a major seismic event. Although aftershocks are usually smaller in magnitude than the main earthquake, they can still cause damage to infrastructure and hamper the recovery process.
“Some scientists assume that contemporary seismicity in stable parts of North America is aftershocks, and other scientists think it is mostly background seismicity,” says lead study author Yuxuan Chen, a geoscientist at Wuhan University. in one media release. “We wanted to look at this from another angle using a statistical method.”
Regions near the epicenters of these historical earthquakes they still experience seismic activity today, leading to the possibility that some of the modern earthquakes may be persistent aftershocks of the past. They might as well be foreshocksthat precede larger earthquakes, or simply background seismic activity, which is the usual level of seismic activity in a given region.
According to the US Geological Survey (USGS), distinguishing between foreshocks and background seismic activity is challenging until a larger earthquake occurs. However, scientists can identify replicas. Understanding the cause of modern earthquakes is crucial to assessing future disaster risks in these regions, even if current seismic activity is not causing significant damage.
To determine whether some of today’s earthquakes are indeed long-lived aftershocks, the researchers needed to identify which modern earthquakes to focus on. Aftershocks tend to cluster around the epicenter of the original earthquake, so the team considered earthquakes within a 155-mile radius of historical epicenters. They focused on earthquakes with a magnitude equal to or greater than 2.5since smaller earthquakes are difficult to record reliably.
The researchers used a statistical approach called the nearest neighbor method with earthquake data from the USGS to assess whether the recent earthquakes were likely aftershocks or unrelated background seismic activity. Aftershocks usually occur near the epicenter of the original earthquake and before background seismic activity returns to normal levels, according to the USGS. By examining factors such as the timing, distance and magnitude of pairs of earthquakes, scientists can link one earthquake to another.
“In some ways, the earthquakes look like aftershocks if you look at the spatial distribution, but the earthquakes could be clustered closely for a couple of reasons,” says Susan Hough, a USGS geophysicist who was not involved in the study. “One is that they are replicas, but you could also have a replica process that is not part of a replica process. What exactly their results mean is still open to question.”
The study revealed that the sequence of replicas of the 1663 earthquake near southeastern Quebec, Canada, he concluded, and modern seismic activity in the area is unrelated to the historic earthquake. In the case of the other two historic earthquakes in the Missouri-Kentucky border region (1811-1812) and Charleston, South Carolina (1886), aftershocks of these events may still be occurring centuries later.
In the Missouri-Kentucky border region, about 30 percent of the earthquakes recorded between 1980 and 2016 were likely aftershocks of the large earthquakes that occurred in 1811 and 1812. In Charleston, South Carolina, around 16 percent of today’s earthquakes were probably aftershocks of the 1886 earthquake. Consequently, the modern seismic activity in these areas it is influenced by a combination of aftershocks and background seismic activity.
“It’s kind of a mix,” Chen notes.
To assess a region’s current seismic risk, scientists monitor not only aftershocks, but also fault lines and background seismic activity. The study revealed that background seismic activity is the main cause of earthquakes in the three regions studied. This suggests that stress continues to build up, which may lead to larger earthquakes in the future. Some faults, however, can experience fluidity without significant stress build-up.
“To get to one hazard assessment for the futurewe really need to understand what happened 150 or 200 years ago,” concludes Hough. “So it’s important to apply modern methods to address the problem.”
The study is published in Journal of Geophysical Research: Solid Earth.
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