Abstract: Due to the interaction between the Tibetan plateau, the Alxa block and the Ordos block, the western margin of Ordos(33.5°~39°N, 104°~108°E)has complex tectonic features and deformation patterns with strong tectonic activities and active faults. Active faults with different strikes and characteristics have been developed, including the Haiyuan Fault, the Xiangshan-Tianjingshan Fault, the Liupanshan Fault, the Yunwushan Fault, the Yantongshan Fault, the eastern Luoshan Fault, the Sanguankou-Niushoushan Fault, the Yellow River Fault, the west Qinling Fault, and the Xiaoguanshan Fault.
    In this study, 7 845 earthquakes(M≥1.0)from January 1st, 1990 to June 30th, 2018 were relocated using the double-difference location algorithm, and finally, we got valid locations for 4 417 earthquakes. Meanwhile, we determined focal mechanism solutions for 54 earthquakes(M≥3.5)from February 28th, 2009 to September 2nd, 2017 by the Cut and Paste(CAP)method and collected 15 focal mechanism solutions from previous studies. The spatial distribution law of the earthquake, the main active fault geometry and the regional tectonic stress field characteristics are studied comprehensively.
    We found that the earthquakes are more spatially concentrated after the relocation, and the epicenters of larger earthquakes(M≥3.5) are located at the edge of main active faults. The average hypocenter depth is about 8km and the seismogenic layer ranges from 0 to 20km. The spatial distributions and geometry structures of the faults and the regional deformation feature are clearly mapped with the relocated earthquakes and vertical profiles. The complex focal mechanism solutions indicate that the arc-shaped tectonic belt consisting of Haiyuan Fault, Xiangshan-Tianjingshan Fault and Yantongshan Fault is dominated by compression and torsion; the Yellow River Fault is mainly by stretching; the west Qinling Fault is characterized by shear and compression. The structural properties of the fault structure are dominated by strike-slip and thrust, with a larger strike-slip component. The near-north-south Yellow River Fault is characterized by high angle NW dipping and normal fault motion.
    Based on small earthquake relocation and focal mechanism solution results, and in combination with published active structures and geophysical data in the study area, it is confirmed that the western margin of Ordos is affected by the three blocks of the Tibetan plateau, the Alax and the Ordos, presenting different tectonic deformation modes, and there are also obvious differences in motion among the secondary blocks between the active faults. The area south of the Xiangshan-Tianjingshan Fault has moved southeastward since the early Quaternary; the Yinchuan Basin and the block in the eastern margin of the Yellow River Fault move toward the SE direction.

Key words: seismic relocation, focal mechanism solution, active fault, tectonic stress field, western margin of Ordos

摘要： 文中针对1990—2018年间发生在鄂尔多斯西缘的地震事件, 采用双差定位法获得其中4 417个事件(M≥1.0)的精确定位结果; 利用CAP方法求解了54个地震(M≥3.5)的震源机制解, 并收集了15个前人获取的震源机制结果, 综合研究了区域内地震事件的空间分布规律、 主要活动断裂的深部几何结构和区域构造应力场特征。 小震精定位结果揭示M≥3.5地震震中位于主要活动断裂的边缘, 刻画出较为清晰的断裂几何学特征。 震源机制解反映海原断裂、 香山-天景山断裂和烟筒山断裂以压扭性质为主; 黄河断裂以伸展为主; 西秦岭断裂表现为压扭性质。 综合小震精定位和震源机制解结果, 并结合研究区内已发表的活动构造和地球物理资料, 证实了鄂尔多斯西缘受青藏高原、 阿拉善和鄂尔多斯3个地块的共同作用, 表现出不同的构造变形模式, 同时活动断裂之间的次级块体也存在明显的运动差异。 香山-天景山断裂以南的区域自第四纪早期整体向SEE运动, 而银川盆地以及黄河断裂东缘的块体向SE运动。

关键词: 小震精定位, 震源机制解, 活动断裂, 构造应力场, 鄂尔多斯西缘