Scientists believe that about 90% of earthquakes occur near tectonic plate boundaries but occasionally, they can strike in unexpected locations far away from these boundaries.
Globally, several earthquakes have been recorded far from tectonic plate boundaries, such as one near New Madrid, Missouri in the winter of 1811, which was thousands of miles from the nearest fault line. This earthquake, with a magnitude between 7.2 and 8.2, violently shook the area, causing a series of strong aftershocks collectively known as the New Madrid earthquakes of 1811-1812.
According to researchers, normal earthquakes typically form along plate boundaries. These areas experience more earthquakes because the Earth’s interior—specifically the mantle—moves the tectonic plates, causing them to separate and collide with each other. The fractures between these plates are known as faults and are very prone to breaking. Therefore, when pressure begins to build up at these weak points, the plates can fracture, causing the planet to shake. This is what we perceive as an earthquake, explains Attreyee Ghosh, a geophysicist at the Earth Science Center of the Indian Institute of Science in Bangalore.
However, occasionally, a tremor can occur within a tectonic plate. Scientists refer to this as an intraplate earthquake. Christine Powell, a geophysicist at the University of Memphis, states that the exact reason why this happens remains a mystery. She and other scientists have studied areas with a high concentration of intraplate earthquakes, known as intraplate seismic zones, such as regions in the Central and Eastern United States. After researching these areas, experts have proposed several hypotheses explaining why earthquakes can occur in unexpected places.
Earthquake in Louisa County, Virginia in 2011, occurring far from tectonic plate boundaries. (Photo: Damage from the 2011 earthquake in Louisa County, Virginia).
A study from 2001 proposed that intraplate earthquakes could be triggered by ancient glaciers. About 20,000 years ago, much of North America was covered by a massive ice sheet, which pressed down on the ground significantly. As the ice melted, the land slowly rose, so earthquakes could be a result of this adjustment. However, evidence supporting this hypothesis is still limited.
Another perspective suggests that intraplate earthquakes occur around old faults within the tectonic plates. “Over billions of years, the Earth’s crust has split apart and merged again, leaving scars from old faults. When forces act within the plates and create excessive pressure on these old faults, they can reactivate,” Ghosh states.
The complex structure of the Earth’s crust and interior may also be a factor. Sometimes, remnants of an ancient slab of rock can become trapped between plates, causing instability, according to a 2007 study in Geophysical Research Letters. Powell, a co-author of a 2016 study on this uplift phenomenon, mentions that hot fluid conduits might add pressure, leading to movements on the planet’s surface.
Hydraulic fracturing (fracking)—the act of injecting water, sand, and chemicals into underground rock to extract oil or gas—can also induce earthquakes. According to a 2013 review in the journal Science, wastewater from these activities is pumped into deep wells, which may seep into fractures, lubricating old faults and triggering seismic activity. For example, fracking has been linked to several earthquakes in Ohio in 2015.
Scientists are striving to better understand these complexities through data from projects like EarthScope, which uses sensors to grasp the dynamics beneath the Earth’s crust. Powell recalls that when the project first began, some scientists did not believe the sensors would detect anything capable of causing earthquakes outside of the West Coast, where there are plate boundaries. However, the project “really expanded our understanding of what is happening inside the Earth,” Powell states. “It was a remarkable experiment.”
Understanding intraplate earthquakes is crucial as they can cause significant damage to those living in these seismic areas. The three earthquakes in New Madrid, Missouri in 1811-1812 caused considerable devastation, even altering the flow of the Mississippi River, temporarily causing it to flow backward. A magnitude 5.8 earthquake in Virginia shook Washington, D.C. in 2011, damaging major monuments and churches.
