[S-CG46] 岩石―流体相互作用の新展開：表層から沈み込み帯深部まで

[SCG46-20] スラブ残骸からの流体と2013年北米上部マントル地震

*趙 大鵬¹、Liang Xuran²、豊国 源知¹、Hua Yuanyuan²、Xu Yigang² (1.東北大学、2.中国科学院広州地球化学研究所)

キーワード：地震、上部マントル、スラブ、プルーム

Since the advent of plate tectonics, it has been generally considered that intermediate-depth (60-300 km) and deep-focus (300-680 km) earthquakes take place in the subducting slabs and form the so-called Wadati-Benioff deep seismic zone. However, on 21 September 2013 two earthquakes (M4.8 and M3.0) occurred at ~71-75 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. In this study, we use both local and teleseismic data to determine detailed 3-D 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 the dense continental lithosphere. Another high-V body exists at ~300-500 km depths, which may reflect a remnant of the subducted Farallon slab. A significant low-velocity (low-V) zone appears at ~200-300 km depths between the two high-V bodies, and the low-V zone exhibits seismic radial 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 reactions of the subducted slab remnant, which was promoted by the nearby hot Yellowstone plume. We deem that the 2013 Wyoming upper-mantle earthquakes were caused by the ascending fluids and structural heterogeneities related to the slab remnant and the Yellowstone plume.