Parts of the Earth could be shaking from earthquake aftershocks that began 200 years ago!

If you fancied yourself as a grudge-holding expert, think again — the Earth has outdone you in that department, quite splendidly. A recent study indicates that we might still be experiencing minor earthquakes stemming from events that transpired not just a century ago, but two!

Significant earthquakes seldom occur in isolation, typically followed by dozens or even hundreds of aftershocks that persist in unsettling the region. While these smaller tremors diminish over time, their duration can range from days to even years as the Earth's fault gradually readjusts to its new position.

To determine the prolonged impact of these tremors on regions, the study examined three noteworthy historic and devastating earthquakes that hit parts of the US in 1663, 1811-1812, and finally, in 1886. Ranging from 6.5 to 8 in magnitude, these three earthquakes constitute the largest seismic events in recent history for stable North America.

The research team found that the aftershock sequence associated with the 1663 earthquakes has concluded. Still, approximately 30% of earthquakes in the Missouri-Kentucky border region from 1980 to 2016 were likely aftershocks from the 1811-1812 earthquakes. Similarly, around 16% of present-day earthquakes in Charleston, South Carolina, were identified as probable aftershocks from the area's 1886 earthquake.

Statistical methods determined that these recent earthquakes were not merely the result of unrelated seismic activity. However, the study suggests that a complex mix of aftershocks and background seismicity could be influencing current seismic activity in these regions.

While aftershock sequences weaken over time, the continued accumulation of strain in regions with background seismicity could potentially lead to more significant, more catastrophic earthquakes in the future. Therefore, monitoring fault movements, background seismic activity, and aftershocks becomes crucial for a thorough seismic risk assessment.

"To develop a hazard assessment for the future, we truly need to comprehend what occurred 150 or 200 years ago," notes Hough. "So applying modern methods to address the problem is important."

The results of this research have been published in JGR Solid Earth and can be accessed here.