Koji Nakamura of the Japan Meteorological Agency addresses a news conference next to a map showing an earthquake epicenter off the coast of Fukushima prefecture on Nov. 22. (TORU HANAI/REUTERS)

The 6.9-magnitude earthquake last week off the coast of Fukushima, Japan, probably happened along the same fault that ruptured in 2011, unleashing a massive 9.0-magnitude temblor and resulting tsunami that caused widespread destruction. What makes this part of the world so susceptible to big earthquakes?

Japan lies along what is called the Pacific Ring of Fire, an imaginary horseshoe-shaped zone that follows the rim of the Pacific Ocean, where many of the world’s earthquakes and volcanic eruptions occur. According to the U.S. Geological Survey, 81 percent of the world’s largest earthquakes happen in this belt.

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Within the Pacific Ring of Fire, several tectonic plates mash and collide. In what are known as subduction zones, one plate bends and slides underneath the other, causing the oceanic crust to sink into the Earth’s mantle.

“From Alaska down to Japan and the Philippines, all the way down around the western Pacific — and then the boundary of the west coast of South America and central America — are all big subduction zones,” said Robert Smith, an emeritus professor of geophysics at the University of Utah.

Japan itself sits atop a complex mosaic of tectonic plates that grind together and trigger earthquakes and volcanic eruptions, Smith said.

The recent earthquake off Fukushima was centered about 80 miles southwest of the epicenter of the Tohoku 2011 quake. This means that the latest temblor could be an aftershock of the earlier quake, according to seismologists.

“There’s been a whole sequence [of aftershocks] since the 2011 earthquake,” Smith said. “These enormously big earthquakes have aftershocks that can continue for tens to hundreds of years. It’s very common.”

The 2011 earthquake released hundreds of years of pent-up stress within the subduction zone and triggered an enormous tsunami that inundated the Fukushima Daiichi power plant, eventually causing a nuclear meltdown. While last week’s quake was not as powerful, the entire region is still at risk of big earthquakes.

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The Tohoku quake “was one of the biggest earthquakes we’ve recorded historically, but the fact is, the seismic hazard of the whole subduction zone is extremely high, so large earthquakes are more common there than other places,” Smith said.

In April, a 7.0-magnitude earthquake struck the Kumamoto region in southern Japan, two days after a 6.2-magnitude temblor shook the same area.

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