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. 
 
 Follow Matt Walker and BBC Earth on twitter. ||||| Yellowstone is an active volcano. Surface features such as geysers and hot springs are direct results of the region's underlying volcanism. Credit: National Park Service 
 
 DENVER — A supervolcano blasting Yellowstone National Park to smithereens may capture the imagination, but the region's real risk comes from earthquakes, researchers reported here Sunday (Oct. 27) at the Geological Society of America's annual meeting. 
 
 "The pervasive hazard in Yellowstone is earthquakes," said Robert Smith, a seismologist at the University of Utah. "They are the killer events." 
 
 Smith and his collaborators analyzed 4,520 earthquakes in and around Yellowstone that struck between 1985 and 2013. Their goal: Create the best picture ever of the magma chamber hidden beneath the park's colorful hot springs and spectacular geysers. A side benefit was a better view of the seismic risk from nearby faults. [Infographic: Geology of Yellowstone] 
 
 Constant trembling 
 
 One of these faults triggered the most destructive earthquake ever recorded in the Rocky Mountains — the deadly magnitude-7.3 Hebgen Lake quake in 1959. The epicenter was about 15 miles (24 kilometers) north of West Yellowstone. 
 
 Smith said the probability of another magnitude-7 or larger earthquake on one of the major faults near Yellowstone is 0.125 percent. The number reflects the chance an earthquake will occur in any given year, based on past records. 
 
 The annual probability of a Yellowstone supereruption is a much smaller 0.00014 percent, Smith said. 
 
 Yellowstone National Park is cradled inside a gentle depression created by a giant volcanic eruption 640,000 years ago. The ground collapsed, leaving a bowl-shaped caldera. It was the third in a series of massive eruptions, the first of which exploded 2.1 million years ago. 
 
 A mantle plume (also called a hotspot) feeds Yellowstone's supereruptions. Hotspots are massive rising blobs of hot rock from Earth's mantle, the layer beneath the crust. As the planet's tectonic plates trundle over hotspots, the plumes punch through the crust, forming volcanic chains like Hawaii or the Idaho's Snake River Plain and Yellowstone. 
 
 In the millennia since the last massive volcanic blowout, magma has again built up beneath Yellowstone. The park trembles constantly with tiny earthquakes as gas and hot fluids course through underground fractures, escaping from the molten rock below. 
 
 Beneath Yellowstone 
 
 Led by graduate student Jamie Farrell, the University of Utah group used these tremors like a CT scan, building a precise image of the underground magma reservoir. 
 
 However, Yellowstone's magma chamber isn't just a giant pool of molten rock. What's called a partial melt — small interconnected zones of magma filling fractures and small spaces — fills 6 to 7 percent of the crust beneath Yellowstone, Smith said Sunday. "The Yellowstone crustal reservoir is 250 percent larger than previously imaged," Smith said. 
 
 The actual volume of molten magma is somewhere between 200 to 600 cubic km (50 to 145 cubic miles), he said. 
 
 The reservoir is shaped like a dog's knobby chew toy, with one end about 9 miles (15 km) below the center of Yellowstone National Park, and the other rising to the northeast, about 3 miles (5 km) below the surface. 
 
 The shallow end extends 12 miles (20 km) northeast of the caldera rim created 640,000 years ago, Smith said. That distance matches the total tectonic drift of the North American plate over the Yellowstone mantle plume since that time, he said. 
 
 Email Becky Oskin or follow her @beckyoskin. Follow us @OAPlanet, Facebook & Google+. Original article on LiveScience's OurAmazingPlanet.
– 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.)