Cause of Enigmatic Upper-Mantle Earthquakes in Central Wyoming

Earthquakes deeper than 60 km generally occur in subducting slabs. However, on 21 September 2013 two earthquakes (M 4.8 and 3.0) occurred at ∼71 to 75 km depths in the upper mantle beneath central Wyoming in the stable North American continent, where there is no actively subducting slab at present. The cause of the two events is still unclear. Here, we present detailed 3D P-wave isotropic and anisotropic tomography down to 750 km depth under Wyoming and adjacent areas. Our result shows that the two Wyoming events took place within a high-velocity (high-V) body at 0–160 km depths, which may be part of dense continental lithosphere. Another high-V body exists at ∼300 to 500 km depths, which may reflect a remnant of the subducted Farallon slab. A significant low-velocity (low-V) zone appears at ∼200 to 300 km depths between the two high-V bodies, and the low-V zone exhibits seismic anisotropy that VP is greater in the vertical direction than that in the horizontal direction. The low-V zone may include ascending fluids from dehydration of the subducted slab remnant, which was promoted by the nearby hot Yellowstone plume. It is highly possible that the ascending fluids induced the 2013 Wyoming upper-mantle earthquakes.