A groundbreaking study has brought to light a remarkable geological phenomenon: certain areas in the United States are still experiencing aftershocks from earthquakes that occurred in the 1800s.
These historic quakes, among the most powerful ever recorded in the U.S., affected the central and eastern regions, notably Missouri, Kentucky, and South Carolina, with magnitudes estimated between 6.5 and 8.0.
Aftershocks are smaller seismic events following a major earthquake, persisting from days to years, and are integral to the Earth’s natural process of fault adjustment. Though typically less powerful than the main quake, aftershocks can still cause infrastructural damage and complicate recovery efforts.
Yuxuan Chen, the study’s lead author and a geoscientist at Wuhan University, aimed to understand whether the current seismic activity in these areas is a continuation of these historic events, using a statistical method. The distinction between aftershocks, foreshocks (which precede larger earthquakes), and background seismicity (the usual level of seismic activity in a region) is crucial for future risk assessment.
The study focused on earthquakes within a 155-mile radius of the historic epicenters, considering those with a magnitude of 2.5 or greater. Researchers applied the nearest neighbor method to USGS earthquake data to ascertain if recent quakes were aftershocks or unrelated seismic activities. Aftershocks typically occur near the original quake’s epicenter and before background seismicity returns to normal levels.
Susan Hough, a USGS geophysicist not involved in the study, pointed out that while the spatial distribution of earthquakes might suggest aftershock activity, factors like fault creep could also be responsible for the clustering. The study’s results indicated that seismic activity near southeastern Quebec, Canada, related to the 1663 earthquake, has ceased to be connected to that event. However, in regions like the Missouri-Kentucky border and Charleston, South Carolina, modern seismic activities may still be aftershocks of the 1811-1812 and 1886 earthquakes, respectively.
In the Missouri-Kentucky area, about 30% of earthquakes recorded between 1980 and 2016 are likely aftershocks of the 1811-1812 events. In Charleston, around 16% of modern earthquakes could be aftershocks of the 1886 quake. This suggests a blend of aftershocks and background seismic activity in these regions.
The study emphasizes that background seismic activity is the main cause of earthquakes in all three areas. This ongoing seismic activity hints at the continuous accumulation of strain, potentially leading to larger future earthquakes. The findings highlight the importance of understanding historical seismic events for accurate future hazard assessments and emphasize the need for continuous monitoring of aftershocks, fault lines, and background seismicity.