Earthquakes Can Continue to Have an Impact for Centuries

In the United States, some of today’s earthquakes could actually be the aftershocks of severe intraplate earthquakes that occurred 150 to 200 years ago, as researchers have discovered. Consequently, the aftershocks of such intraplate earthquakes can occur not only a few weeks and years later, as is usually the case, but even centuries later. Understanding the cause of seismic activity can help assess the future earthquake risk in these regions.

Following an earthquake, the earth’s crust realigns along a fault, leading to a release of stress in the rock. This can result in smaller tremors occurring in the affected region around the original epicenter, ranging from days to years later. Sometimes, these aftershocks even occur on the other side of the world. Over time, they become less frequent and weaker until the seismic activity returns to normal for that region. Overall, the stronger the initial earthquake, the more aftershocks and delayed damage it triggers.

The Puzzle of Intraplate Earthquakes

More than 6,000 earth tremors were recorded in the New Madrid quake zone between 1974 and 2011 alone.
More than 6,000 earth tremors were recorded in the New Madrid quake zone between 1974 and 2011 alone. Image: Daten USGS, Grafik: Kbh3rd, CC-by-sa 3.0.

Typically, earthquakes and their aftershocks occur mainly along the boundaries of continental plates, but there are exceptions, such as in North America. Despite being far from the coasts and plate boundaries, the central USA and parts of Canada continue to experience seismic activity. Yuxuan Chen from the University of Wuhan and her colleague Mian Liu from the University of Missouri note that the cause of these intraplate earthquakes remains mysterious and debated.

Some scientists believe these tremors represent harmless background activity that is normal for the region. However, they could also be precursors to new, strong intraplate earthquakes. Distinguishing such foreshocks from seismic background activity is challenging. There is also the theory that earthquakes in the central North American region could be a kind of delayed aftershock of historically strong earthquakes.

Three Historical Strong Earthquakes in Focus

Chen and Liu have explored this hypothesis, focusing on the three historically largest earthquakes and earthquake sequences in recent North American history. All three were intraplate earthquakes with magnitudes between 6.5 and 8.0. The first occurred in 1663 in southeastern Quebec around Charlevoix, and the last in 1886 near Charleston, South Carolina. Between 1811 and 1812, three catastrophic earthquakes shook the New Madrid region on the border between Missouri and Kentucky.

To determine if these strong earthquakes could still have aftershocks, the researchers analyzed the epicenters of modern earthquakes occurring within 250 kilometers of the epicenters of the historical strong earthquakes, with a magnitude of at least 2.5. They used the statistical “nearest neighbor method” to compare the spatial distribution of epicenters and the strength of the earthquakes.

One uses the time, distance, and magnitude of event pairs and tries to find the connection between two events,” describes Chen. “If the distance between two earthquakes is less than expected from background events, then one earthquake is likely the aftershock of the other.”

Two Strong Earthquakes Still Affecting Today

Our results show that the answer to the question of aftershocks is not a simple yes or no,” report Chen and Liu. The study provided different answers for the three intraplate earthquake zones. The current tremors in Canadian Quebec seem to result from normal background seismicity, and no local connection to the 1663 Charlevoix earthquake is apparent, according to the researchers.

However, for the other two earthquake zones, the situation is different. The two other historical earthquakes probably still trigger aftershocks today, the scientists report. About 23 to 30 percent of all earthquakes occurring between 1980 and 2016 in the New Madrid zone were likely aftershocks of the 1811–1812 strong earthquakes, Chen and Liu report, with a range from 10.7 to 65 percent. “The proportion of long-lasting aftershocks in the New Madrid earthquake zone is significant, if not dominant,” they write.

In the earthquake zone of South Carolina, the percentage of late aftershocks could be even higher. “Up to 72.4 percent of modern seismicity in the Charleston earthquake zone could be attributed to the long-lasting aftershocks of the 1886 earthquake,” the team reports.

Late Intraplate Aftershocks Elsewhere?

According to the two seismologists, their results emphasize that aftershocks from intraplate earthquakes can occur much later than commonly thought. “Within stable continents, aftershock sequences can last for decades to centuries,” they write. “The current seismicity in these regions may therefore include both background earthquakes and late aftershocks.” This may not only apply to North America but also to intraplate earthquakes on other continents.

Knowing the proportion of such late aftershocks in today’s seismicity is crucial for assessing earthquake risk, as Chen and Liu explain: “Long-lasting aftershocks can be an indication of post-seismic processes in the subsurface, and these must be considered when determining how quickly stress in the subsurface is building up and when the next earthquake could occur.

Or Could It Be Other Causes?

Geophysicist Susan Hough from the United States Geological Survey (USGS), not involved in the study, views Chen and Liu’s results somewhat critically. The spatial proximity of earthquakes could indicate aftershocks, but there could be other reasons. “Another creeping process could also be occurring,” she says.

The exact implications of the results are therefore still questionable and need further investigation. “To create a hazard assessment for the future, we really need to understand what happened 150 or 200 years ago,” says Hough.

Source: Journal of Geophysical Research Solid Earth, 2023; doi: 10.1029/2023JB026482