Earthquakes today in the U.S. could be left over from seismic activity in the 1800s, scientists have found.
In a new study, published in the Journal of Geophysical Research: Solid Earth, researchers from Wuhan University and the University of Missouri found that up to 30 percent of modern-day quakes are actually aftershocks from major earthquakes that happened in the 1800s.
Earthquakes occur when tectonic plates that have been stuck together due to friction slide past each other, releasing large amounts of energy in the process. These are often followed by many smaller earthquakes called aftershocks, which result from the changing stress distribution along the fault line and surrounding rock. While these aftershocks are smaller than mainshock, they can still damage infrastructure and disrupt recovery.
The 1800s were marked by some of the strongest earthquakes in recorded U.S. history, and many scientists believe that many modern earthquakes are aftershocks leftover from these major quakes. However, others believe that they may instead be foreshocks preceding a larger future quake, or just background seismic activity.
"We wanted to view this from another angle using a statistical method," Yuxuan Chen, a geoscientist at Wuhan University and lead author of the study, said in a statement.
Discerning the identity of these quakes will be important for understanding the future disaster risk in various states across the U.S., as well as gaining a better understanding of the geological underpinnings of this seismic activity.
"To come up with a hazard assessment for the future, we really need to understand what happened 150 or 200 years ago," Susan Hough, a geophysicist at the U.S. Geological Survey who was not involved in the study, said in a statement. "So bringing modern methods to bear on the problem is important."
The team focuses their efforts on three historic earthquake events in North America: a 1663 quake near southeastern Quebec; a trio of quakes near the Missouri-Kentucky border that occurred in 1811 and 1812; and an earthquake that occurred in Charleston, South Caroline, in 1886. They then compared these with modern earthquakes in similar areas by analyzing data from the U.S. Geological Survey.
"You use the time, distance and the magnitude of event pairs, and 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, then one earthquake is likely the aftershock of the other."
In Quebec, recent seismic activity appeared to be unrelated to the 1663 mainshock. But the quakes in Missouri and South Carolina were more closely linked with their historical predecessors. At the Missouri-Kentucky border, the team found that roughly 30 percent of all the earthquakes that occurred in the area between 1980 and 2016 were likely aftershocks from the major earthquakes that occurred in the area between 1811 and 1812.
Likewise, in Charleston, South Carolina, they estimated that roughly 16 percent of the modern-day quakes were likely aftershocks from the main quake in 1886. In other words, earthquakes in these areas are likely a mix of aftershocks from ancient earthquakes and modern background seismicity.
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