Aftershocks follow large earthquakes — sometimes for weeks, sometimes for decades. But in the United States, some areas may be experiencing shocks from centuries-old events.
In the 19th century, some of the strongest earthquakes in recorded United States history struck the continental interior of North America. Nearly two centuries later, the central and eastern United States may still be reeling from those events, a new study finds.
When an earthquake strikes, smaller earthquakes known as aftershocks can continue to shake the area for days to years after the original earthquake. These small earthquakes decrease over time and are part of the fault reset process after the original earthquake. While aftershocks are smaller in size than the main shock, they can still damage infrastructure and hinder recovery from the original earthquake.
New perspectives on seismic activity
“Some scientists assume that contemporary earthquakes in parts of stable North America are aftershocks, and other scientists believe they are mostly background seismicity,” said Yuxuan Chen, a geologist at Wuhan University and lead author of the study. “We wanted to look at this from another angle using a statistical method.”
The study was published on November 7 in Journal of Geophysical Research: Solid Earth, The AGU journal is dedicated to research into the structure, evolution and deformation of the interior of our planet.
Historical earthquakes and their legacy
Areas near the epicenters of these historic earthquakes remain seismically active to this day, so it is possible that some modern earthquakes are long-lived aftershocks of past earthquakes. However, they can also be foreshocks that precede larger earthquakes or a background earthquake, which is the normal amount of seismic activity in a given area.
According to the United States Geological Survey (USGS)There is no way to distinguish between foreshocks and aftershocks until a larger earthquake strikes, but scientists can still distinguish aftershocks. Thus, determining the cause of recent earthquakes is important for understanding future disaster risks in these areas, even if current seismic activity causes little or no damage.
The team focused on three historical seismic events with estimated magnitudes of 6.5-8.0: an earthquake near southeastern Quebec, Canada, in 1663; and three earthquakes near the border of Missouri and Kentucky in 1811-1812; and the Charleston, South Carolina, earthquake in 1886. These three events are the largest earthquakes in the modern history of settled North America — and larger earthquakes trigger more aftershocks.
The stable continental interior of North America lies farther from plate boundaries and has less tectonic activity than areas closer to plate boundaries, such as the west coast of North America. As a result, the three study areas do not experience earthquakes often, raising further questions about the origins of modern earthquakes.
To find out whether some of the earthquakes occurring today are long-lasting aftershocks, the team first needed to identify recent earthquakes on which to focus their efforts. Aftershocks cluster around the original epicenter, so they include earthquakes within a 250 km (155 mi) radius of the historic epicenters. They focused on earthquakes that were magnitude greater than or equal to 2.5 because it is difficult to reliably record anything smaller than that.
The team applied a statistical approach called the nearest neighbor method to USGS seismic data to determine whether recent quakes were likely aftershocks or unrelated background seismic activity. Aftershocks occur near the original epicenter and before the background level of seismicity resumes, according to the USGS. Thus, scientists can use the seismic background of the area and the location of the earthquake to link the earthquake to the main shock.
“You use time, distance, and size of pairs of events, and you try to find the link between two events — that’s the idea,” Chen said. “If the distance between a pair of earthquakes is closer than expected from background events, one earthquake is more likely to be an aftershock of the other.”
The distance between epicenters is only one piece of the puzzle, says Susan Hough, a geophysicist at the USGS who was not involved in the study.
“In some ways, earthquakes look like aftershocks if you look at the spatial distribution, but earthquakes can be tightly clustered for several reasons,” Hogue said. “One is that they are aftershocks, but it is also possible to have a creep process that is not part of the aftershock process. Exactly what their results mean is still in question.”
Results and implications
Looking at the spatial distribution, the study found that the 1663 aftershock sequence near southeastern Quebec, Canada, has ended and that modern seismic activity in the region is unrelated to the ancient earthquake. However, the other two historical events may still trigger aftershocks centuries later.
Near the Missouri-Kentucky border, researchers found that about 30% of all earthquakes from 1980 to 2016 were likely aftershocks from major quakes that struck the region between 1811 and 1812. In Charleston, South Carolina, the team found about 16% of modern earthquakes are likely to be aftershocks from the 1886 quake. Thus, modern earthquakes in these areas are likely attributable to both aftershocks and aftershocks.
“It’s kind of a mixture,” Chen said.
Seismic risk assessment
To assess recent seismic hazards in the region, scientists monitor creep and background earthquakes as well as any aftershocks. The study found that backseismic activity is the main cause of earthquakes in all three study areas, which could be a sign of continued pressure buildup. Aftershock sequences weaken over time, but the buildup of pressure could lead to larger earthquakes in the future. However, some defects can creep in without resulting in increased pressure.
“To come up with a future risk assessment, we really need to understand what happened 150 or 200 years ago,” Hogg said. “So using modern methods to deal with the problem is important.”
Reference: “Long-duration aftershocks in the New Madrid Seismic Zone and the rest of stable North America” by Yuexuan Chen and Mian Liu, November 7, 2023, Journal of Geophysical Research: Solid Earth.
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