CHARACTERISTICS AND IMPLICATIONS OF IN-SITU STRESSES IN SOUTHEASTERN TIBETAN PLATEAU

This paper examines the variation characteristics of principal stress, lateral pressure coefficient and stress accumulation level with depth in the five tectonic stress zones(i.e., the Bayankala Mountain, Longmenshan-Songpan, Motuo-changdu, Sichuan-Yunnan and southwestern Yunnan stress zones) within the southeastern Tibetan Plateau. It uses the statistical regression analysis based on an extensive collection of 900 groups of in-situ stress data. The stress direction characteristics were also analyzed based on a combined analysis of focal mechanism with the orientation of the maximum horizontal in-situ stress. The results show that: (1)In the Bayankala Mountain, Longmenshan-Songpan and Motuo-changdu stress zones, the stress gradients are relatively large, and the deep stress is dominated by the horizontal component; while, in the Sichuan-Yunnan and southwestern Yunnan stress zones, the stress gradients are relatively small, and the deep stress is dominated by the vertical component; showing a stress pattern of "strong in north and weak in south". (2)The ratio of average differential stress to average effective stress(μ_m), which is a parameter measuring the stress accumulation level, is relatively high in the Bayankala Mountain, Longmenshan-Songpan and Sichuan-Yunnan stress zones, suggesting a state closer to the limit equilibrium of friction, and thus a higher probability of earthquake. (3)The dominant direction of the maximum horizontal stress of the southeastern Tibet Plateau rotates clockwise around the eastern Himalayan syntaxis, which is consistent with the GPS velocity field and the dominant direction of the maximum horizontal stress derived from the focal mechanism in the middle and upper crust. Based on these results, we preliminarily concluded that the stress pattern of "strong in north and weak in south" may be owing to the dominances of block lateral extrusions in north and lower crustal flows in south. Finally, using rock burst risk as an index, the stability of major underground projects in each stress zone was evaluated based on the stress state. The results show that: the rockburst risks within 0~2 km buried depth in the southeastern Tibetan Plateau are mainly low, moderate and high and that extremely high rock burst risk only exists in local areas. Furthermore, the buried depths of the same rock burst risk level at different stress zones have obvious differences.