Why can earthquakes occur far from tectonic plate boundaries?

Scientists think that about 90% of earthquakes occur in areas near tectonic plate boundaries , but sometimes they can occur in unexpected remote locations.

There have been a number of earthquakes recorded around the world that occurred far from tectonic plate boundaries, such as an earthquake near New Madrid, Missouri in the winter of 1811, thousands of miles from the nearest fault. The 7.2 to 8.2 magnitude earthquake then violently shook this area, causing a series of strong aftershocks collectively known as the 1811-1812 New Madrid earthquake.

According to researchers, earthquakes typically form along boundary lines. These areas experience more earthquakes because the Earth's interior - specifically the mantle - moves Earth's tectonic plates, causing them to separate and collide with each other . The cracks in between these plates are called faults and they are fragile. So when pressure begins to build up at these weak spots, the plates can rupture, causing the planet to shake. This is what we feel like earthquakes, explains Attreyee Ghosh, a geophysicist at the Center for Earth Sciences at the Indian Institute of Science in Bangalore.

But every now and then, a tremor can occur in the middle of a tectonic plate. Scientists call this an intraplate earthquake . Exactly why it happened remains a mystery, said Christine Powell, a geophysicist at the University of Memphis. She and other scientists have studied places with high concentrations of intraplate earthquakes, called intraplate seismic zones , such as areas in the central and eastern United States. After studying these areas, experts have come up with several hypotheses to explain why earthquakes can occur in unexpected places.

The 2011 Louisa County, Virginia, earthquake occurred far from the plate tectonic boundary. (Photo: Damage from the 2011 earthquake in Louisa County, Virginia).

A 2001 study hypothesized that intraplate earthquakes may be caused by old glaciers. About 20,000 years ago, most of North America was covered under a giant ice sheet and the ground was greatly pressed down. As the ice sheet melts, the ground slowly rises, so earthquakes can be the result of this adjustment. However, evidence for this hypothesis is scant.

Another idea is that intraplate earthquakes are occurring around old faults within tectonic plates. "Over billions of years, the Earth's crust has been splitting and coming together, old faults leaving scars. When forces move inside the plates and put too much pressure on these old faults, they can work again ," Ghosh said.

The complex structure of the Earth's crust and interior may also be a factor . Sometimes, remnants of an ancient rock get stuck in the middle of a plate, causing instability, according to a 2007 study in Geophysical Research Letters. Powell, who co-authored a study on the upwelling phenomenon in 2016, said pipes carrying hot liquid could add pressure, leading to movement 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 cause earthquakes. According to a 2013 review in the journal Science, wastewater fluids from these operations are pumped into deep wells, which can seep into fractures, lubricate old faults and cause seismic activity. For example, fracking was tied to several earthquakes in Ohio in 2015.

Scientists are trying to better understand these complexities through data from projects like EarthScope, which uses sensors to understand the dynamics beneath the Earth's crust. When the project first began, Powell recalls, some scientists didn't think the sensors would find anything that could lead to earthquakes except on the West Coast, where the plate boundary is. But the project "really expanded our understanding of what's going on inside the Earth ," Powell said. "It was a remarkable experiment."

Understanding intraplate earthquakes is essential because they cause quite a bit of damage to people living in these seismic zones. Three earthquakes in New Madrid, Missouri in 1811-1812 caused considerable destruction, even changing the course of the Mississippi River and causing it to temporarily flow backward. A 5.8 magnitude earthquake in Virginia shook Washington, DC in 2011, causing damage to monuments and cathedrals.