California isn’t the only area at risk for the shake, rattle and roll action of earthquakes. The southeastern United States was considered one of the most stable areas in the country until rare earthquakes began to occur. The mystery behind the tremors may have been discovered through recently published research.
When a 5.8 magnitude earthquake struck near Mineral, Virginia in 2011, its impact was felt from the Carolina’s to Washington D.C. It was strong enough to damage the famous National Cathedral.
This was weird because earthquakes usually strike at the edges of tectonic plate boundaries, not in the middle. The southeastern United States is more than 1,056 miles from the nearest edge of the North American Plate. The plate covers all of North America, Greenland and parts of the Atlantic and Arctic oceans.
New research has found that pieces of the Earth’s mantle are braking off and sinking back into the planet. Periods of accretion (when new material is added to the plate) and rifting (when plates split apart) have been weakening the plate. These pieces have been likely falling off for at least 65 million years. The process has been weakening the remaining plate, making it more prone to slipping and causing earthquakes during the process.
Berk Biryol, a seismologist at the University of North Carolina in Chapel Hill and lead author of the study says,
This region has not been active for a long time. We are intrigued by what was going on and how we can link these activities to structures in deeper parts of the earth. This was an interesting finding because everybody thought that this was a stable region, and we would expect regular plate thickness.
Through a CAT scan, getting a 3D image of a mantle sample, the researchers discovered that there was a periodic breaking off of the mantle. Areas of the North American Plate have become more dense and were pulled downward into the mantle through gravity and, at certain times, the densest parts broke off from the plate and sank into the asthenosphere below. In turn, the asthenosphere, lighter and more buoyant, surged in to fill the gaps created by the missing mantle chunks, cooling down to become new rock. This process could be the cause of the earthquakes.
Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate. The (seismic) zones that are active will continue to be active for some time.
These types of earthquakes are not as predictable as those occurring at plate boundaries. The present study may help scientists understand the dangers these earthquakes may cause.