Why do earthquakes occur far from plate boundaries?

It is commonly assumed that earthquakes only occur near tectonic plate boundaries, and about 90% of earthquakes occur in these areas. These boundaries include, for example, the San Andreas Fault, which runs almost along the western 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, occurred in the winter of 1811, thousands of miles from the nearest fault, yet the magnitude 7.2 to 8.2 quake shook the area violently, triggering a series of powerful aftershocks collectively called the New Madrid 1811-1812 . Earthquakes.

How was this possible? How do earthquakes occur far from plate boundaries?

First, by comparison, it is important to understand the way conventional earthquakes form along boundary lines. These areas experience more earthquakes because the Earth's interior – the mantle – moves the planet's tectonic plates, causing them to split and collide. The cracks between these plates, called faults, are fragile. So, when stress starts to build up at these weak points, the plates can break, sending terror across the planet. This is what we feel when earthquakes occur, explained Atriy Ghosh, a geophysicist at the Center for Earth Sciences at the Indian Institute of Science in Bangalore.

Related: How Many Tectonic Plates Does Earth Have?

But from time to time, a tremor can occur in the middle of tectonic plates. Scientists call this an intraplate earthquake. Christine Powell, a geophysicist at the University of Memphis, said the exact reason why this happened is still a big mystery. She and other scientists have studied places with high concentrations of earthquakes within plates, called intraplate seismic zones. These areas are found, for example, in parts of the central and eastern United States. After researching these areas, experts have some theories about why earthquakes occur in unexpected places.

One possible explanation is that intraplate earthquakes may be caused by ancient glaciers, a 2001 study suggested. About 20,000 years ago, much of North America was covered by a giant ice sheet, and the land was heavily overburdened. As the ice sheet melted, the ground slowly rose, so earthquakes may be a result of this modification. However, evidence for this theory is sparse. “The direction of the earthquake axis and the glacial isostatic adjustment do not match,” Ghosh said.

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Another idea is that intraplate earthquakes occur around ancient faults in tectonic plates. For billions of years, the Earth's crust has split and come back together, leaving old wounds with scars. When forces propagate into the plates and put a lot of stress on these ancient faults, they may be reactivated, Ghosh said.

The complex structure of the Earth's crust and interior can also be a factor. Sometimes, the remains of an ancient rock slab get stuck in the middle of the plate, causing instability, hypothesized a 2007 study in Geophysical Research Letters. Powell, who co-authored a study on this fluctuation in 2016, said hot fluid tubes could increase pressure, leading to movement on the planet's surface.

Hydraulic factoring, or hydraulic fracturing, the process of injecting water, sand and chemicals into subterranean rock to extract oil or gas, can also cause earthquakes. Wastewater fluids from these processes are injected into deep wells, which can seep into fractures, lubricate old faults and cause seismic activity, according to a 2013 review in the journal Science. For example, fracking was linked to a number of earthquakes in Ohio in 2015.

Scientists are trying to get a better understanding of these complexities through data from projects like EarthScope, which uses sensors to capture the dynamics beneath the Earth's crust. Powell recalls that when the project first started, some scientists didn't think the sensors would find anything that could generate earthquakes except within the West Coast, where the plate boundaries are located. Powell, who lives in Tennessee, said the project “really opened our eyes to what's happening on our land here.” “It was a great experience.”

It is important to understand intraplate earthquakes because they pose a great danger to people living in these seismic zones. The three earthquakes that struck New Madrid, Missouri, in 1811–1812 caused so much devastation that they temporarily changed the course of the Mississippi River and caused it to flow backwards. A 5.8-magnitude earthquake in Virginia shook Washington, D.C., in 2011, damaging monuments and cathedrals.

“No one is thinking about earthquakes in the central and eastern United States,” Powell said. “We have to be prepared. You have to realize that earthquakes can happen in these places.”

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