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《Chinese Journal of Geophysics》 2016-01
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Characteristics of the Japan and IBM subduction zones:Evidence from gravity and distribution of earthquake sources

XING Jian;HAO Tian-Yao;HU Li-Tian;SUH Man-cheol;KIM Kwang-hee;Key Laboratory of Petroleum Resources Research,Institute of Geology and Geophysics, Chinese Academy of Sciences;University of Chinese Academy of Sciences;Kongju National University;Department of Geological Sciences,Pusan National University;  
The Japan and the IBM subduction zone are located in the place where the Pacific plate,the Philippine Sea plate and the Eurasian plate meet.Both the subduction zones act as typical areas where the"subduction factory"takes effect,bringing on themselves great significance for deep research.Using the distribution of earthquake sources and satellite gravity data,this paper studies the characteristics of these two subduction zones.Firstly,a free-air gravity anomaly map was compiled to delineate the regional structure of the subduction zone.From the free-air gravity anomalies,the isostatic anomalies on the premise of the Airy-Heiskanen model were calculated to reflect isostatic characteristics of the crust.Secondly,information of seismic-source distribution of earthquakes with magnitude greater than 3was acquired from USGS.Four graphs of epicenters relevant to different ranges of depths were drawn,indicating the characteristics of the distribution of earthquakes of different depths.A correlation between the isostatic anomalies and big earthquakes was detected and discussed.Thirdly,by means of the information of earthquakes collected,the Wadati-Benioff zones of Japan and IBM were analyzed by dint of cross-sections across/along the subduction zone.A contour map of depths of earthquakes in the subduction zone was also prepared to show the shape of the subducted slab.Fourthly,with the purpose of demystifying the crustal structure of the Japan and IBM subduction zones,a 2.5Dgravity inversion was conducted on two profiles located in the Japan and the Izu-Bonin subduction zones,respectively using the free-air anomalies available.(1)The free-air anomaly map shows a zone with a great gradient in the subduction zone,with positive anomalies on islands and negative anomalies along the trench.The isostatic anomaly map manifests the large deviation from isostasy in the subduction zone,except for the KyushuPalau Ridge which has been away from the locality where subduction takes effect.(2)The distribution of earthquakes delineates a characteristic of"zone"accumulation especially in the arc,and few earthquakes are detected inside the basin.Earthquakes tend to occur in the place with a great gradient on the isostatic anomaly map.(3)The Wadati-Benioff zone of Japan has a large width and a small dipping angle,with a subducting direction of NWW.Sparseness of earthquake distribution at the depth of more than 200 km is recognized.As for the Izu-Bonin Wadati-Benioff zone,the subducting direction is from NWW to SWW,with a narrower breadth and a deeper depth and big dipping angle.The distribution of earthquakes of the intermediate depths is sparse as well.This sparse area becomes larger in the south of the Izu-Bonin Wadati-Benioff zone to the central IBM where deep earthquakes are not detectable.The Wadati-Benioff zone of Mariana shows a wider breadth and a deeper depth from north to south,with the subduction zone trending in SWW to NWW gradually,and NNW at the southernmost of IBM where a relatively smaller dip angle,a lower depth and a shorter breadth are discerned.(4)In the Japan subduction zone,the Pacific plate has a thin sedimentary layer without accretion wedge detected in the trench.Layers 2and 3are subducted with a homogeneous distribution of density.The upper mantle of the Pacific plate is serpentinized.The crust of the overlying arc is divided into four layers:a sedimentary layer,basement layer,upper crust and lower crust.No obvious serpentinization is discerned on the mantle wedge.In the Izu-Bonin subduction zone,the Pacific plate has a sedimentary layer of heterogeneous density distribution.Layers 2and 3are subducted with a large gradient of density variation in relation to the depth.The upper mantle of the Pacific plate is not serpentinized largely.The crust of the overlying plate is divided to four layers:a sedimentary layer,basement,middle crust and lower crust.The lower crust and mantle wedge are serpentinized so strongly that the Moho discontinuity is not detectable.More earthquakes occur in the Japan subduction zone and are distributed uniformly.Most earthquakes in the Izu-Bonin subduction zone are inside the slab.(1)The Kyushu-Palau ridge is inclined to the state of isostasy.(2)The Wadati-Benioff zones of Izu-Bonin and Mariana have disparate characteristics.The results indicate that the subducted slab of Izu-Bonin finally stays on the 660 km transition zone while that of Mariana penetrates nearly vertically into the lower mantle.Such differences are due to several factors such as the gravity effect of the subducted slab,plate movement,viscosity in the slab and the age of lithosphere subducted.(3)Slab windows exist in the central and southern IBM as a result of collision of the trench and the Ogasawara plateau and of accommodation of the reduced volume the slab must occupy,respectively.The lack of earthquakes in the Yap-Belau subduction zone,however,is due to its proximity to the Euler pole,which results in its slow movement that produces few earthquakes.(4)The subducted crust and mantle in the Japan subduction zone are less coupled than those in the Izu-Bonin subduction zone and the subducted crust in Izu-Bonin has a more obvious phase transition.(5)The coupling between the oceanic plate and the overlying plate is better in the north than in the south along the Japan-IBM subduction zone,due to the movement of plates,which is the same reason accounting for the fact that the dipping angle of the subducted slab becomes smaller from the south to the north along the Japan-IBM subduction zone.
【Fund】: 国家油气重大专项(2011ZX05008);; 中国地质调查国土资源部海岸带综合地质调查工程“中国海及邻域地质地球物理及地球化学系列图项目”(GZH200900504-207);; 中国科学院专项(XDB06030200);; 科技部专项(2013YQ120357);; 国家重大科研装备研制项目(ZDYZ2012-1-08-01 ZDYZ2012-1-08-04 ZDYZ2012-1-08-05);; 国家自然科学基金项目(41210005 41374139 41304079 41476033 41404050 41404114 91428204)共同资助
【CateGory Index】: P315.726
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