THE PRESENT STATE OF STRESS IN CHINA AND RELATED PROBLEMS

The present state of stress in China and concerned problems are discussed and analysed in this paper based mainly on in situ stress measurements.1. Regional characteristics of the present state of earth's stress of ChinaBun through by the Taihangshan mountain North China is separated into two distinct regions: first, east of the mountain, the compressive principal stress is neasly in the E Wdirection; second, west of the mountain, the direction is about N-S. South China is marked by stress orientation predominately in the N Wdirection, and the orientation of stress in the Northwest China is in the N-N-Edirection. In Southwest China., however, the stress pattern is more complex: the orientations of compressive principal stresses differ from each other in various regions. On the whole, the relative magnitudes of principal stress in China tend to be smaller in the east than the west2. Variation of state of earth's stress with depthIn most areas the horizontal principal stress is larger than the vertical, though exceptions to this pattern caused by different tectonism are occasionally seen. The ratio Kof average horizontal stress to vertical is somewhat scattered ranging from 0.5 to 4 down to a depth of 100 m., but there exists a linear relation between depth and Kof 0.5-2 below the depth of 100m. Available data taken recently in China all indicate that generally the direction of horizontal principal stress varies little with depth except for the Tangshan region where not only the orientation but the magnitude of the horizontal principal stress is distinctly different above and below the depth of 150 m.3. The state of stress in the vicinity of faultsAs the state of stress is different in different regions, so is the behaviour of active faulting. In Tangshan region the vertical principal stress is moderate (i.e. max >xv >Hmin), a condition propitious to the development of strike slip faulting; whereas, if the vertical stress is the minimum principal stress (i.e. Hmax>Hmin >v),it is prone to induce thrust faulting as in the Jinchuan region of Gansu province. The in situ stress measurements carried out at the Tancheng-Lujiang fault show that the nearer to the fault the less is the shear stress.4. The stress state in regions of strong earthquakesFollowing the stress measurements in the regions of strong earthquakes such as Tangshan, Longling, Haicheng and Xingtai etc. all results show that the stress magnitude measured in the epicentral locality immediately after a major earthquake is distinctly less than that measured in surrounding regions and the direction of the maximum principal stress is found to deviate from the orientation of the regional principal stress.5. The relationship between the state of earth's stress and crustal stability is briefly discussed in the paper as well.