Aftershocks occur after major earthquakes, sometimes lasting for weeks or even decades. But in some parts of the United States, the impact may be due to events from centuries ago.
In the 1800s, the strongest earthquake in U.S. history struck the interior of the North American continent. Almost two centuries later, aftershocks from these earthquakes may still be occurring in the central and eastern United States, a new study finds.
understanding aftershocks
When an earthquake occurs, small earthquakes known as aftershocks can continue to shake the area for days to years after the initial quake. These small earthquakes diminish over time and are part of the fault realignment process following the initial earthquake. Although aftershocks are smaller in magnitude than the main shock, they can still damage infrastructure and hinder recovery from the initial quake.
A new perspective on seismic activity
“Some scientists believe that the current seismic activity in parts of stable North America is aftershocks,” said Yushuan Chen, a geoscientist at Wuhan University and lead author of the study. “But other scientists think it’s mainly background seismic activity.” “I wanted to look at this from a different angle using statistical methods.”
The study was published in the journal Nov. 7. Geophysical Research Journal: Solid Earth; AGU journal dedicated to the study of the structure, evolution, and deformation of Earth’s interior.
Historic earthquakes and their legacy
Some modern earthquakes may be long-term aftershocks of past earthquakes, as areas near the epicenters of these historic earthquakes still experience high seismic activity. However, these can also be foreshocks that precede larger earthquakes or background earthquakes that are the normal amount of seismic activity in a given region.
According to the United States Geological Survey (USGS), there is no way to distinguish between foreshocks and background seismic activity until a larger earthquake occurs, but scientists can still identify aftershocks. Therefore, identifying the causes of modern earthquakes is important for understanding future disaster risks in these regions, even if current seismic activity causes little damage.
The research team focused on three historical seismic events estimated to range in magnitude from 6.5 to 8.0. One was the 1663 earthquake near southeastern Quebec, Canada. Three earthquakes occurred near the Missouri-Kentucky border between 1811 and 1812. These three earthquakes are the largest in recent stable North American history, and larger earthquakes produce more aftershocks.
The stable continental interior of North America is located far 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, earthquakes do not occur frequently in the three study areas, raising further questions about the origins of modern seismicity.
Research methodology
To find out whether some of today’s earthquakes are long-lasting aftershocks, the research team first needed to decide which modern earthquakes to focus on. Because aftershocks are concentrated around the epicenter of the original earthquake, earthquakes within a radius of 250 kilometers (155 miles) of the historical epicenter were also included. They focused on earthquakes with a magnitude of 2.5 or greater. This is because it is difficult to reliably record smaller earthquakes.
The research team applied a statistical approach called the nearest neighbor method to USGS seismic data to determine whether recent earthquakes were likely aftershocks or unrelated background seismic activity. According to the USGS, aftershocks occur near the epicenter of the original earthquake and before the level of background seismic activity resumes. Scientists can therefore use the regional background seismic activity and the earthquake’s location to link the earthquake to the main shock.
“The idea is to use time, distance, and the magnitude of a pair of events to try to find a connection between two events,” Chen says. “If the distance between a pair of earthquakes is closer than expected from background events, one earthquake is likely an aftershock of the other.”
USGS geophysicist Susan Huff, who was not involved in the study, said the distance between the epicenters is only one piece of the puzzle.
“When you look at the spatial distribution, in some ways the earthquakes look like aftershocks, but they may be clustered together for several reasons,” Hough said. “One is that it’s an aftershock, but it’s also possible that there’s a creep process going on that’s not part of the aftershock process. What their results mean is still open to question.”
Findings and implications
Examining the spatial distribution, the study found that the 1663 aftershock near southeastern Quebec, Canada, has ended and that modern seismic activity in the region is unrelated to older earthquakes. But two other historical events can still cause aftershocks centuries later.
Near the Missouri-Kentucky border, researchers found that about 30% of all earthquakes from 1980 to 2016 may have been aftershocks from a major earthquake that struck the region in 1811-1812. was found to be high. And in Charleston, South Carolina, the researchers found about 16%. Therefore, modern seismicity in these regions can be attributed to both aftershocks and background seismicity.
“It’s kind of a mixture,” Chen said.
Earthquake risk assessment
To assess a region’s modern seismic risk, scientists monitor creep earthquakes and background seismic activity in addition to aftershocks. This study found that background seismicity was the main cause of earthquakes in all three study areas, which could be a sign of continuous strain development. Aftershocks weaken over time, but the strain that develops can lead to larger earthquakes in the future. However, some faults can progress without accumulating strain.
“We need to really understand what happened 150 or 200 years ago to come up with risk assessments for the future,” Hough said. “Therefore, it is important to implement modern techniques to address this issue.”
References: “New Madrid Seismic Zone and Stable Long-Lasting Aftershocks in the Rest of North America,” by Yuxuan Chen and Mian Liu, November 7, 2023. Geophysical Research Journal: Solid Earth.
DOI: 10.1029/2023JB026482