| Tectonic stress field research is a very important branch of geodynamics, the tectonic stress distribution in the Certain range of the Earth comprehensively reflect a variety of power acting on geological bodies which are related to tectonic movements, and closely relate to the research of tectonic movement and the power source problem. revealing the tectonic stress field distribution law will not only help the situation of various tectonic interpretation, but also provide important clues to explore force source of the tectonic movement, so ancient and modern tectonic stress field and its evolution has been study as geotectonic and geodynamic important elements, especially for the modern tectonic stress field research, are in favor of a better understanding to the problems that the new tectonic movement, regional stability and seismic risk.Xiaojiang fault is located in the Sichuan-Yunnan region, it is a famous active fault in north-south zone, Xiaojiang fault and its adjacent areas'structure is extremely complex, and stress adjust drastically, there had taken place several large earthquakes in history, in order to a clear understanding of stress changing in space and time in this region, in this study, using focal mechanism data, studed tectonic stress field characteristics of the middle part of Xiaojiang fault and adjacent areas with the method of sliding fitting. With 740 earthquake wave data of ML≥2.5 recorded by Kunming digital seismic net work in middle section of Xiaojiang fault and its adjacent area which occurred during in January,1998 to January 2009 , all focal mechanism were calculated out ,and their characteristic were analyzed ; the stress field was inverted with the method of FMSI(Gephart 1990), and its change characteristics in time and space were analysed.This study includes the following aspects:(1) In this paper,With 740 earthquake wave data of ML≥2.5 in Xiaojiang fault and its adjacent area which occurred during in January,1998 to January, 2009 , using vertical SV-wave and P-wave amplitude ratio, all focal mechanism were calculated out ,and their characteristic were analyzed ;(2) According to regional tectonic feature of middle section of Xiaojiang fault and its adjacent area, the stress field was parted into two zone as the boundary of xiaojiang falut, they are A and B(including xiaojiang fault) zone; in order to research the xiaojiang fault's stress in detail, the xiaojiang fault zone was picked out for independent analysis; the research field also parted by 1°×1°and with 0.5°slip trapeze zone. based on the mentioned above, the stress field about A,B and fault zone were inverted with the method of FMSI(Gephart 1990). The result shows that: the fault's mainly strain-slip way in middle section of Xiaojiang fault and its adjacent area is lateral, the A ,B and fault zone respectively has 81.28,69.23 and 72.97 percent lateral fault; the stress inversion also indicated that, the azimuth of maximum principal stressσ1 respectively in A, B, and Fault zone are approximately NNW,the stress action is mainly horizontal, and strike-slip faulting is dominated in the study area. The 1°×1°and with 0.5°slip trapeze parted zone stress inversion shows that: in the region Central Yunnan block, the azimuth of maximum principal stress change from NNW to NWW, but the region in the east of the LUzhi River fault and west of the PUdu river fault, the main compressive stress direction in the region shows that the North and South slip is much obvious, the main stress direction of Xiaojiang fault zone is mainly NW, in the Dianzhong Block, the stress field shows the direction of principal compressive stress is NW-NNW.(3) based on the spatial stress field analysis of the middle part of Xiaojiang fault and its adjacent areas, the regional stress field in the sub-region bounded by the larger seismic which occurred in this area, were respectively inverted during the time of before and after the shock, the results showed that: the stress field of Xiaojiang fault and its adjacent areas in the time period on zoning change significantly, in the A zone, the principal compressive stress field orientation not change obviously before and after several moderately strong earthquakes;but in the northern part of B zone and the Xiaojiang fault zone , the principal compressive stress field orientation change obviously before and after most shocks, this phenomenon indicate that the northern part of the area B and Xiaojiang fault zone's stress adjust relatively intensively, but the central Yunnan block is relatively stable.
|
PDF Full Text Request