Findings from a new study published in the AGU’s Journal of Geophysical Research: Solid Earth suggest that the central and eastern United States may still be experiencing aftershocks from some of the strongest earthquakes in recorded U.S. history that struck in the 1800s. Researchers focused on three historic earthquake events estimated to range from magnitude 6.5-8.0 and found that around 30% of all earthquakes from 1980 to 2016 near the Missouri-Kentucky border were likely aftershocks from the major earthquakes that struck the area between 1811 and 1812. The research suggests that some modern earthquakes in these regions could be long-lived aftershocks of past quakes, foreshocks that precede larger earthquakes, or background seismicity, which is the normal amount of seismic activity for a given region.
The stable continental interior of North America is located far from plate boundaries and has less tectonic activity than regions close to plate boundaries, such as North America’s west coast. This raises questions about the origins of modern seismicity in the central and eastern United States. Identifying the cause of modern earthquakes is important for understanding the future disaster risk in these regions. The study found that background seismicity is the dominant cause of earthquakes in all three of the study regions, which could be a sign of continued strain accrual. However, some faults can creep along without building up strain, meaning that strain accrual can lead to larger earthquakes in the future while modern seismicity may have little to no damage impact at the moment.
These findings highlight the importance of bringing modern methods to bear on the problem of understanding the historical origins of the seismicity in these regions. It is crucial for assessing a region’s modern seismic risk to monitor creep and background seismicity in addition to any aftershocks. Scientists can use the findings from the new study to develop a hazard assessment for these regions in the future.