Aftershocks can often occur hours or days after the main earthquake and create more shaking. But new research suggests that some modern earthquakes may be aftershocks of some of the strongest earthquakes recorded in the 1800s.
A new study published in the Journal of Geophysical Research: Solid Earth says frequent aftershocks may have occurred after a trio of earthquakes near the Missouri and Kentucky border in the United States between 1811 and 1812. Additionally, the effects of a separate earthquake felt in Charleston, South Carolina, in 1886 may continue today.
Now that the nature of seismic activity in these areas is better understood, these particular areas stand out as being particularly problematic. Scientists often measure the time, distance, and magnitude of pairs of events to see if there is a connection between two earthquakes. If the distance is close enough, one earthquake is likely to be an aftershock of another.
While examining most of the modern earthquakes felt near the Missouri-Kentucky border between 1980 and 2016, researchers found that 30 percent of them may have been aftershocks of three major earthquakes that struck the region nearly 200 years ago. The old earthquakes had magnitudes between 7.3 and 7.5, and all of the new earthquakes range from 2.5 to larger earthquakes.
Additionally, researchers say that about 16 percent of modern earthquakes felt in the Charleston area may be aftershocks of the 1886 magnitude 7.0 earthquake. Of course, it is not easy to determine whether these are just aftershocks or new earthquakes. But these possibilities also raise some additional questions about how we measure seismic data.
Scientists say the theory that modern earthquakes are aftershocks is largely due to the overall spatial distribution. However, they note that even these seemingly tightly connected earthquakes may be entirely new earthquakes, as there may be other reasons why they are clustered closely together.
If these are not aftershocks, then it is possible that these earthquakes are part of an ongoing creep process, and scientists are not yet sure how to explain it.
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