It's commonly assumed that earthquakes occur only near the boundaries of tectonic plates, and roughly 90% of earthquakes do happen in these areas. These boundaries include, for example, the San Andreas Fault, which runs roughly along the west coast of California, where the North American and Pacific plates meet.

But not all earthquakes occur along plate boundaries. For example, an earthquake near New Madrid, Missouri in the winter of 1811 was thousands of miles from the nearest fault, yet the magnitude 7.2 to 8.2 quake violently shook the region, triggering a series of powerful aftershocks collectively called the 1811-1812 New Madrid earthquakes.

So how was this possible? How do earthquakes happen far away from plate boundaries?

First, as a point of comparison, it's important to understand the way conventional earthquakes form along boundary lines. These areas experience more earthquakes because Earth's interior namely, the mantle move the planet's tectonic plates, causing them to split apart and collide. The cracks in between these plates, called faults, are fragile. So, when stress starts accumulating at these weak points, plates can break, sending a shudder through the planet. This is what we feel as earthquakes, explained Attreyee Ghosh, a geophysicist at the Centre for Earth Sciences at the Indian Institute of Science in Bangalore.

Related: How many tectonic plates does Earth have?

But every once in a while, a shudder can happen in the middle of a tectonic plate. Scientists call this an intraplate earthquake. Exactly why it happens remains much of a mystery, said Christine Powell, a geophysicist at the University of Memphis. She and other scientists have studied places with a high concentration of intraplate earthquakes, called intraplate seismic zones. These zones exist, for example, in parts of the central and eastern United States. After researching these areas, experts have some theories as to why temblors may occur in unexpected places.

One possible explanation is that intraplate earthquakes may be caused by old glaciers, a 2001 study proposed. Around 20,000 years ago, much of North America was covered under a giant ice sheet, and the ground was weighed down considerably. As the ice sheet melted, the ground slowly rose, so the earthquakes may be the result of this adjustment. Evidence for this theory, however, is sparse. "The orientation of the earthquake axis and the glacial isostatic adjustment doesn't match," Ghosh said.

Another idea is that intraplate earthquakes are occurring around old faults on the insides of tectonic plates. For billions of years, Earth's crust has split apart and come back together, and old wounds leave scars. When forces propagate through to the plates' interior and put too much stress on these old faults, they may get reactivated, Ghosh said.

The complicated composition of Earth's crust and interior could also be a factor. Sometimes, remains of an ancient slab of rock gets stuck in the middle of a plate, causing instability, as posited by a 2007 study in Geophysical Research Letters. Pipes of hot fluids could add pressure, resulting in movement on the planet's surface, Powell said, who co-authored a study on this upwelling in 2016.

Hydraulic factoring, or fracking the act of injecting water, sand and chemicals into underground rocks to extract oil or gas can trigger earthquakes, too. Wastewater fluid from these operations are injected into deep wells, which can seep into cracks, lubricate old faults and cause seismic activity, according to a 2013 review in Science. For instance, fracking was tied to a number of earthquakes in Ohio in 2015.

Scientists are trying to get a better understanding of these complexities with data from projects such as EarthScope, which use sensors to capture the dynamics underneath Earth's crust. Powell recalls that, when the project first started, some scientists didn't think the sensors would find anything that could lead to the generation of earthquakes except for within the West Coast, where the plate boundary was. But the project "really opened our eyes to what is going on inside our Earth here," said Powell, who is based in Tennessee. "It was a remarkable experiment."

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It's important to understand intraplate earthquakes because they pose a considerable risk for people who live in these seismic zones. The three earthquakes in New Madrid, Missouri in 1811-1812 caused considerable destruction, even altering the course of the Mississippi River and causing it to temporarily run backward. A magnitude 5.8 quake in Virginia shook Washington, D.C. in 2011, damaging monuments and cathedrals.

"Nobody thinks about earthquakes in the central and eastern U.S.," Powell said. "We must be prepared. You have to be aware that earthquakes can happen in these places."