A mysterious earthquake that struck the centre of the US was one of the deepest earthquakes of its type ever recorded, say scientists. 
 
 The quake, known as the Wind River Earthquake, struck the state of Wyoming in 2013 with a magnitude of 4.7 Mw (Moment magnitude scale). 
 
 This earthquake occurred well within the mantle 
 
 The quake was strange for various reasons. 
 
 The Wind River area of Wyoming is usually quiet, seismically speaking. In the past 60 years since records have been collected, few earthquakes in the region have been recorded with magnitudes of 4 Mw, while just one surpassed 5 Mw. 
 
 So by regional standards the Wind River earthquake was unusually powerful. 
 
 However, the region is also tectonically quiet - that is, there is little movement in the Earth’s tectonic plates below the site. The nearest tectonic movement occurs 200 km away to the northwest, at the so-called Yellowstone Hotspot, an area that produces volcanic activity across the states of Oregon, Nevada, Idaho, and Wyoming. 
 
 A new study published in the journal Earth and Planetary Science Letters has now revealed what may have happened. 
 
 Earthquakes usually occur as tectonic plates in the Earth’s crust grind into each other along fault lines, releasing pent up energy, or as slabs of crust descend deeper into the Earth’s hotter and more viscous mantle. 
 
 They can occur tens of kilometres deep in the crust, or much deeper, at depths of a few hundred kilometres. 
 
 Earthquakes that occur near the surface produce more shaking, and those occurring at a depth of less than 70 km (43 mi) are classified as shallow-focus earthquakes. 
 
 The extreme depth of the Wind River Earthquake poses some interesting questions 
 
 Now scientists have evidence that the Wind River earthquake actually occurred deeper within the Earth, in its mantle, rather than within the Earth’s crust. 
 
 Their measurements suggest the earthquake occurred at a depth of 75 km. 
 
 In the local area, known as the Wyoming Craton, the Earth’s crust is no deeper than 40-50 km, forming part of the cold, stable lithosphere that underlies much of North America. 
 
 “This earthquake occurred well within the mantle, and likely over 20 km deeper than the base of the crust,” report the researchers in the journal. 
 
 That would make it among the three deepest earthquakes yet identified to have occurred in a region covered by a stable continental crust. 
 
 It could mean the mantle is brittle 
 
 The scientists, Dr T Craig of the PSL Research University in Paris, France, and R Hepburn of the UK’s Atomic Weapons Establishment, based in Reading, say they are aware of only two comparably deep earthquakes, which occurred in the Earth’s mantle underneath a stable continental lithosphere. 
 
 In 1979, an earthquake struck at depth of 90 km under Randolphe, Utah, some 40 km into the Earth’s mantle. 
 
 In 2000, another quake struck under the Arafura Sea, at a depth of 61 km, some 25 km into the mantle. 
 
 The extreme depth of the Wind River Earthquake “poses some interesting questions”, say the researchers. 
 
 Deep seismic activity can occur in volcanic regions, but it’s usually at a much smaller magnitude. Also, the Wind River location is not near any active surface volcanoes and is unlikely to be connected to the volcanic region 200 km away at the Yellowstone Hotspot. 
 
 The size and type of shock produced by the Wind River earthquake also suggests it was produced by tectonic forces, rather than by the movement of magma or fluid in the Earth’s mantle. 
 
 That could mean the mantle is brittle under the Wind River Craton, and it failed, producing the shock. 
 
 But that doesn’t match our current understanding of the forces needed to deform and break the mantle in such areas, say the researchers, leaving the true cause of the quake open to debate. 
 
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– 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.)