An earthquake struck Wyoming two years ago that made little sense, scientifically speaking—but experts seem closer to solving the mystery, the BBC reports. Called the Wind River Earthquake, it hit with 4.7 magnitude in an area that rarely sees such seismic power. Hardly surprising, since the Wind River area has little tectonic-plate movement that would normally trigger such an earthquake. But a new study in Earth and Planetary Science Letters says the quake may not have originated with tectonic plates grinding against each other at all. The Wind River Earthquake may have started deeper, in what the BBC calls "the Earth’s hotter and more viscous mantle." Such a quake might be caused by crust falling into the mantle, which lies between the Earth's higher crust and deeper core.
These deeper quakes remain "a highly controversial topic," the scientists write, but this one "occurred well within the mantle, and likely over 20 km [12.4 miles] deeper than the base of the crust." If true, the finding makes the Wind River Earthquake one of the three deepest ever recorded in the area. Still, some things don't fit: While such deep quakes can occur in volcanic regions, when fluid or magma flows in the Earth's mantle, they usually affect a smaller area; the Wind River quake ruptured nearly 11 million square feet, notes Geology in Motion. And Wind River is noticeably far from the nearest volcanic region. So what's up? Perhaps the mantle was so brittle that it failed and triggered the quake, say researchers, who admit that the causes remain debatable. (Read about naked tourists blamed for a mountain quake.)