Something unusual is happening in Eastern Tennessee where the ground seems to be shaking gently from time to time, only to quickly pass and leave people wondering what really happened.
As seismologists say, this is a soft ruble of an earthquake where it shouldn’t be, far from boundary of any tectonic plate. These shakes, sometimes quiet but often devastating, are difficult to understand. Now geologists might have a clue for what has been causing these shakes, as the answer lies deep beneath us.
Earthquakes are caused by the discharge of tension built up between Earth’s tectonic plates, or that’s what we learn in school.
However, every now and then there are tremors far from plate boundaries, known as ‘intraplate’ earthquakes. There are hundreds of these intraplate earthquakes yearly and it’s rather difficult to explain them.
Geologists recently identified a shared characteristic of a number of such earthquake locations.
The Canadian district Charlevoix, US county of New Madrid, and a huge part of East Tennessee regularly experience earthquakes above a 2.5 magnitude regardless of their location and distance from the plate boundaries.
These territories share a similar geology far underground as well.
“We present a new hypothesis that bigger seismic zones are blocked to places where the large-scale basement structures have been damaged by concentrated crustal deformation,” said a pair of researchers from the University of Kentucky and the University of Memphis, in a recently conducted study.
This CCD, or concentrated crustal deformation, includes motions that at some point of our planet’s history lowered the force of the ancient rocky layers that make up the deepest parts of a continental crust.
According to the researchers, the basement structures underneath the places where these continual intraplate quakes occur, are associated with ancient plate reorganizations that were left hundreds of millions of years ago, only to have been reactivated over time.
The perfect example for this would be the Charlevoix Seismic Zone (CSZ), an area that stretches 85 kilometres (53 miles) along the Saint Lawrence River in Quebec, and has experienced five earthquakes with a magnitude higher than 6 since 1663.
Not only the seismic zone is located on set of faults deep in its basement, it’s also a site of a meteor impact that struck approximately 360 million years ago.
Most of the earthquakes in this region are concentrated exactly where this meteor collision occurred, so now researchers produced numerous models in an attempt to make sense of the area’s seismicity.
However, it’s almost certain that these deformations caused by the impact played a key role in the earthquakes.
The New Madrid Seismic Zone has a different approach to this mystery and according to them the deformations were the product of repeated massaging of the crust following the breakup of the supercontinent Rodinia which happened over half a billion years ago.
When speaking about Eastern Tennessee, tension surrounding an unforeseen complication within one of its deep faults resulted in something named “releasing bend”. This means an extending of the crust along the fault that resulted in another kind of deformity.
“Although the mechanisms producing the CCD differ, the regionally restricted CCD serves to focus seismicity in these three zones” , was written in the research paper.
These deformations are probably not the only piece of the puzzle, but they sure are necessary.
More studies are about to be conducted in order to determine the exact cause of each crustal deformation individually and given the challenges in predicting most earthquakes, knowing more about these distinctive deformities is vital if we’re to arm ourselves against future activity.