Motions Of The Ordos Block

The China continent can be divided into several tectonic blocks by nearly NE- and EW-trending faults. Meanwhile strong earthquakes in the China continent usually cluster in time and space. The relationship between these two major features of the China continent is studied in this thesis, and several types of block motions are identified based on grouping of earthquakes. It is one of the key problems for understanding seismicity to study block motions. The Ordos Block is a typical block which has simple interior structures, and active seismicity around it So the Ordos block is taken as the target of the case study for mis work. The results show that the Odors Block is moving as a whole body. When the faults along the northern and southern boundaries of the Ordos block are active, its eastern boundary is inactive. However, if the faults along the eastern boundary are active, the northern and southern are inactive. The motion process of the Ordos Block since last 1000 years is also analysed in the thesis. And the motions of the Ordos block and the stress field around it are modeling with the numerial method.1 Block motions and Grouping of earthquakes in the China continentThis work attempts to find out corresponding relation between tectonic blocks and earthquakes in groups that were identified by previous studies. It has been found that most of earthquake groups are associated with block boundaries. Four types of block movement are identified based on relations between epicenters and blocks.(1) Movement along the single boundary of a block (or between two neighboring blocks) (A): Earthquakes in a group occur between two blocks, and epicenters in a group distribute along the single boundary of a block (neighboring blocks) or a combined block. A combined block refers to two or more blocks that are combined as one unit in some extent of time and space which controls where and how earthquakes occur. (2) Movement of a single block (a combined block) (B): Earthquakes in a group occur around a block or a combined block. This type can be associated with motions (slips, rotation, tilt, or compound movement) of a block or a combined block. (3) Movement of multi-blocks (C): Earthquakes in a group occur around two or more blocks. It demonstrates that blocks are active in the same time. (4) Movement in block interiors (D): Earthquakes in a group occur in the interior of a block. Isolated earthquakes other than earthquakes in groups mentioned above are called earthquakes of non-grouping. If we consider isolated earthquakes as earthquakes in groups which include only one earthquake, all types of block movement identified with isolated earthquakes are movement along a single boundary of a block (or combined blocks). The frequency of each type is different. If earthquakes of non-grouping are counted, the movement along a single boundary of a block (or combined blocks) is popular in four types, but movement of a single block dominates in modes if grouping earthquakes are only counted. Activity of blocks is also different from each other. Based on the block's frequencies related to earthquake groups, it is counted that the Taihangshan Mountain and the North China block are most active in eastern China, and theChuandian and the Kunlun-Songpan block are most active in western China.2 The Ordos Block moveing as a whole body and alternative activity of boundaries around itThe study about the Ordos block motions includes following aspects. (1) Geological data indicate that the graben basins around the Ordos block where the Loess Plateau is located, represent major extentional tectonic features since late Cenozoic time in north China, except some thrusts along the southwestern margin of the block. The extension dominates in this region. The kinematics of graben systems indicates that the regions along the northern and southern margins of the Ordos are left-lateral extension belts, and the western and eastern margins of the Ordos are right-lateral extension belts. The kinematics of the faults shows that normal faults are...