Why do earthquakes happen far away from plate boundaries?

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.