An improved gravity method to horizontal tectonic stresses and its applications in Tibet

It is significant for identifying mass movement patterns to invert horizontal tectonic stresses at different depths underneath Tibet.In recent years,a large number of achievements focusing on two-dimensional tectonic stresses have been obtained from gravity data.However,three-dimensional tectonic stresses in Tibet are still unknown or debatable.Therefore,in the present study an improved method to multilayer horizontal tectonic stresses using gravity observations is developed.The inverted multilayer horizontal tectonic stresses are in agreement with those from previous studies.In addition,rich tectonic structure and development can be revealed from the inverted multilayer horizontal tectonic stresses:(1)the distribution of horizontal tectonic stresses at various depths shows strong correlation with that of the tectonic elements,where major faults and earthquake epicenters are corresponding with stress highs and the stable basins are consistent with stress lows.(2)the mass movement patterns of whole Tibet present clockwise,and the material movement directions in the west and east are approximately southnorth and east-west,respectively.(3)in eastern Tibet,the eastward materials caused by the south-north extrusion between Indian and Eurasian plates are divided into two parts by the stable Sichuan Block,one flowing nearly southeast and the other moving almost northeast.The inverted multilayer horizontal tectonic stresses may provide direct evidences for mass movement patterns in Tibet.

Geostress measurements near fault areas using borehole stress-relief method

To minimize negative effects of geostress distribution on mining safety near the fault areas, the UPM40 triaxial geostress testing system was introduced to conduct in-situ geostress measurements at three sites and nine points by the borehole stress-relief method. The results of strain?confining pressure curves show that rock masses at the three measuring sites exhibit comprehensive linear elasticity in spite of various fissures or cracks within rocks. Horizontal and vertical stress components distribute discrepantly near the fault areas, and the maximum lateral pressure coefficient is as high as 6.15. The maximum principle stress ranges from 8.01 to 14.93 MPa, and stress directions are in the range of N78.07°W?N17.55°W. Geostresses near fault areas are dominated by the horizontal tectonic stresses, while the lower values, compared to those under similar geological conditions are due to stress release by the fault. Additionally, the fault and shear stress nearby are partially responsible for asymmetric elongation and southwesterly migration of orebodies.