We know that after a strong earthquake there are always aftershocks: tremors of lesser intensity in the same region where the earthquake occurred. This happens because the earth’s crust continues to settle around the fault where the earthquake occurs. They may occur several hours or days after the main event and gradually decrease in intensity. But a new study says they could take years to happen. Until centuries later.
For example, the central and eastern United States may still be experiencing the effects of earthquakes that occurred in the 14th century, according to a study published in The New York Times Magazine. Journal of Geophysical Research: Solid Earth. Researchers from Wuhan University and the University of Missouri calculated that 16% to 30% of the current earthquakes in this region may be part of the same events. reported almost 200 years ago.
The team focused on three historical episodes: an earthquake near southeastern Quebec, Canada, in 1663; a trio of earthquakes near the Missouri-Kentucky border between 1811 and 1812; and the 1886 Charleston, South Carolina earthquake. Estimates of the magnitude of these events ranged from 6.5 to 8.0. These are the largest earthquakes in recent stable history of North America.
This area is called stable because it is calmer because it is far from tectonic plate boundaries. Therefore, earthquakes do not occur here as often as on the west coast of North America. That is why doubts arise about the origin of its modern seismicity.
How did you determine that these earthquakes were still causing aftershocks?
Regions near the epicenters of these historic earthquakes are still seismically active. This can have three causes: precursors to future earthquakes, background seismicity—the normal level of seismic activity for a region—or aftershocks from past earthquakes.
According to the United States Geological Survey (USGS), it is impossible to distinguish previous earthquakes from background seismicity until a larger earthquake occurs. But scientists can distinguish copies.
Aftershocks cluster around the epicenter of the original earthquake. For this reason, a group of scientists studied earthquakes that occurred within within a radius of 250 kilometers from the epicenters of three selected historical episodes. They focused on earthquakes greater than or equal to magnitude 2.5, since anything weaker is difficult to record reliably.
The team used a statistical approach called nearest neighbor method. The idea, broadly speaking, was to compare the distance, timing and magnitude of two earthquakes to see whether they were aftershocks or unrelated background seismic activity.
If the distance between two earthquakes is less than expected for background events, then one earthquake is most likely an aftershock of the other. Aftershocks occur near the epicenter of the original earthquake, while background seismic activity can occur anywhere in the region.
Final results
The study found that the 1663 aftershock sequence near southeastern Quebec, Canada, has ended. Thus, modern seismicity in this area is not related to the old earthquake.
Now, near the Missouri-Kentucky border, researchers have found that about 30% of all earthquakes that occurred between 1980 and 2016 were probable aftershocks. major earthquakes that occurred in the area between 1811 and 1812. And in Charleston, South Carolina, the same thing happens with about 16% of movements were recorded between 1980 and 2016.
This means that most tremors in these areas are more related to background seismicity, which is a sign of continually building stress. The sequence of aftershocks weakens over time, but the build-up of stress could cause stronger earthquakes in the future.
“To make a hazard assessment for the future, we really need to understand what happened 150 or 200 years ago,” Susan Hough, a USGS geophysicist who was not involved in the study, said in a statement. “That’s why it’s important to use modern techniques to solve this problem.”
Source: Hiper Textual
