Present-day Stress Field And Fault Interactions In The Sichuan-Yunnan Region Of China

The China continent can be divided into several tectonic blocks by nearly ME- and EW-trending faults. Most of strong earthquakes in China continent happen along the boundaries of these blocks. It means that blocks control earthquake occurrence. On the other hand, fault interactions, here just like influences of fault activities or earthquakes on regional stress field are very important to prediction of future seismicity So it is one on the key factors to understanding influences between different active faults when we study fault interactions from the point of view of active blocks. The Sichuan-Yunnan region is on the edge of the Eurasia and Indian plate collision zone, where there are complex tectonic deformation and high frequency of earthquakes. So this region is an ideal place for geosciences and taken as the target of the case study for this dissertation. Based on careful analysis of acquired data, including focal mechanism solutions, velocities of observation stations based on GPS measurements offered by Wang Qi et al (2001) and two earthquake catalogs including earthquakes above Ms5.0 since 1850 and above Ms2.0 since 1970, it is concluded that the stress field of the study region varies both in temporal and spatial scales, and the transferring order of active period of strong earthquakes along great active faults since last 150 years is obtained. Furthermore, this work discusses the problems how the present-day stress field forms, where are the block boundaries, and what is the relationship between block motions and boundary fault interactions and different block interactions. Numerical simulation concerning the problems of stress field and fault interactions is performed in this work. Bases on these results, a multi-block model with experimental simulation and numerical simulation is analyzed finally in order to help to understand the mechanical process and the deformation during block motions, which is aimed at the phenomenon of block interactions in the Sichuan-Yunnan region.1 The block boundaries of the Sichuan-Yunnan region and its motion stateBlock boundaries are commonly the same as active faults on the margins of the blocks, and the motion of these faults represents the movements of the blocks. The Sichuan-Yunnan rhombic block, moving towards SSE, is very typical in the Sichuan-Yunnan region and its boundaries are composed of the Xianshuihe fault, the Xiaojiang fault, the Honghe fault and the Jinshajiang faalt. This is a-controversial problem, and this study holds that not the Honghe fault, but the newly generated Longling-Lancang fault in SW of Yunnan province, together with the other two faults, the Xianshuihe fault and the Xiaojiang fault, probably form the new block boundaries, in which the block moves from north to south and rotates clockwise to a certain extent. There are some evidence that suggest these faults are the block boundaries. The P axes in the two sub-blocks on the east and west of the Honghe fault, as inverted from more than 1000 earthquake focal mechanism solutions between 1994 and 2000, do not show obvious difference that indicates the two sub-blocks are under the same dynamic circumstance and the Honghe fault does notnecessarily play a role of a block boundary. Seismicity along the great faults shows close relationship between Longling-Lancang fault and Xianshuihe fault, which are parallel and slip adversely, and this phenomenon indicates that the Longling-lancang fault exhibits block boundary's characteristics to some extent.The great Longmenshan-Ruili fault, striking N40癊, is not only a remarkable seismic zone, but also a gravity gradient belt and crustal thickness gradient belt. Geologic survey results show moving rates of two sub-blocks to the NW and SE of the Lijiang-Xiaojinhe fault, which is the middle segment of the Longmenshan-Ruili fault, indicating much difference and more that 33 percent movement rate decreases across it (Xiang Hongfa, 2002). GPS data also indicate that movement rates of observation stations on the both sides of the Longmenshan-Ruili fault h...