Scientists at the University of Kentucky and the University of Memphis may have learned why earthquakes often occur in places they aren’t usually expected. The slow, steady grind of tectonic plates and the tension released by tectonic activity are typically cited as the primary cause of earthquakes. However, hundreds of “intraplate” earthquakes occur each year in places that are far from regions where plates meet. Such hot spots for intraplate earthquakes include Charlevoix, Quebec, New Madrid, Missouri, and the eastern third of Tennessee. The researchers believe that these intraplate earthquakes may be caused in part by concentrated crustal deformation at the lowest levels of the continental crust.
In a study published in the science journal Tectonics, researchers presented their case that these areas of unusual seismic activity may be affected by damage to the underlying crust. “We present a new hypothesis that major seismic zones are restricted to places where the large-scale basement structures have been damaged by concentrated crustal deformation (CCD),” write co-researchers Christine Powell and William Thomas. CCD refers to any damage in geological history to the solid rock, deepest layers of a continental crust. Damage incurred millions of years ago may reemerge in the form of increased seismic activity.
Though CCD likely contributes to all, each region has its own unique geological story to its unusual seismic activity. For example, the New Madrid Seismic Zone in the Midwestern United States is the result of folding of the local crust during the collapse of Rodinia, a supercontinent that fell apart hundreds of millions of years ago. In Eastern Tennessee, the increased seismic activity is caused by a sudden twist within one of the area’s deep faults. “Although the mechanisms producing the CCD vary, the regionally restricted CCD serves to focus seismicity in these three zones,” write Powell and Thomas. The researchers conclude that while CCD likely impacts these intraplate-earthquake-prone areas, it is not the only contributing factor. There is more to the story.