LATE QUATERNARY FAULT ACTIVITY AND MECHANISM OF STRONG EARTHQUAKE AROUND THE BOUNDARY ZONE OF THE ORDOS ACTIVE BLOCK, IN CENTRAL CHINA
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摘要: 鄂尔多斯地块是位于中国大陆中心位置的典型活动地块,由于受西南部青藏高原和东部的太平洋板块远程作用的影响,地块各边界带构造活动特征和变形具有明显的特殊性和差异性。本文围绕多年来鄂尔多斯活动地块周缘边界带开展的活动断裂定量研究、地震孕育机制及活动特征等的结果,对地块周缘断裂活动特征及强震孕育机制进行系统总结。鄂尔多斯活动块不同边界带,断裂活动特征的不同是强震孕育环境差异的最直接的表现形式。构造变形样式复杂的鄂尔多斯活动地块西南边界,受青藏高原向北东挤压扩展的影响,断裂以走滑、逆走滑和逆冲为主要特征。而向北到西边界北段,以青藏高原最新扩展边界以右旋为特征三关口-牛首山断裂为界,北部的银川盆地表现为典型的断陷盆地,有右旋走滑特征,地震多以正走滑型为主。北边界河套盆地的以北侧的正断层为其控盆构造,历史和古地震多集中在边界断裂上。南边界的渭河盆地构造特征相对较为复杂,有两组正断裂组成,历史大地震多发生在盆地南缘,盆地中北部有中强地震发生。东边界山西地堑系有多个裂谷型盆地斜列组成,历史大地震表现为南强北弱,北部盆地受张渤构造带的影响,盆地走向和断层运动性质均发生了明显变化,多具备发生7级左右地震构造条件。综合认为,断裂活动特征典型的鄂尔多斯活动周缘各边界带,未来强震多发生在大地震离逝时间长的地震空区,或是构造带的转换和交汇区,也要注意一些新生断裂和没有发生过破裂的断层段发生6-7级强震的可能。Abstract: The Ordos block with typical boundary zone activity is located in the center of the Chinese Mainland. Under the joint influence of the remote action of the Tibet Plateau at the southwest and the Pacific Plate at the east, the tectonic activity and deformation of each boundary zone of the Ordos block have obvious specificity and differences. This paper systematically summarizes research achievements on active faults and the seismogenic mechanism of strong earthquakes around the Ordos block over the past few decades. The difference of fault activity in each boundary zones have determined the differences of strong earthquake breeding tectonic environment. At the southwest of the block, the faults are mainly characterized by strike slip, reverse strike slip, and thrust due to the influence of the northward compression and expansion of the Tibet Plateau, resulting in complex structural deformation styles within the boundary zone. In the northern section of the western boundary, the latest expansion boundary of the Tibet plateau is characterized by a dextral movement along the Sanguankou- Niushoushan fault. The Yinchuan Basin in the northern section of the west boundary zone is a typical fault basin, with the basin-controlling faults exhibiting dextral strike slip characteristics, and earthquakes are mostly of a normal strike slip type. On the northern boundary, the normal fault on the north side of the Hetao Basin is its controlling structure, and historical and ancient earthquakes characterized by normal faults are mostly concentrated on boundary faults. The southern boundary is mainly the Weihe basin, which has a relatively complex structure and is composed of two sets of normal faults. Large historical earthquakes mostly occurred on the southern edge, and moderate earthquakes also occurred in the central and northern parts of the Weihe basin. The Shanxi graben system on the eastern boundary is composed of multiple rift type basins, with strong seismic activity in the south and weak activity in the north. The northern basin is influenced by the Zhang-bo tectonic belt, and the basin trend and fault movement properties have undergone significant changes, with most of them having the conditions for ~M7. Overall, it is believed that the typical characteristics of fault activity are in the boundary zones around the Ordos active block. In the future, strong earthquakes will mostly occur in seismic gaps with long periods of large earthquakes, or in the transition and intersection areas of tectonic zones. It is also important to pay attention to the possibility of M 6-7 earthquakes occurring in some newly formed faults and fault segments without rupture.
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