Stress Mohr's circle and envelope line analysis of high angle normal fault area in the south of Qinshui basin

In the area of fracture development, influenced by the main fracture, a series of associated small faults and fractures are usually developed in stratigraphic rocks, and the Mohr's circle and envelope lines of rocks usually show different change regularities. The high-angle and near-vertical normal faults in the upper Paleozoic strata in the south of Qinshui basin are extremely developed. The research results show that,compared with the structurally intact rocks that are far away from the fault, the Mohr's circle of the normal fault disk moves to the left, its radius decreases at the same time, and its cross section angle(θ_1) is larger,which is consistent with the results that high angle normal faults are developed. For the normal faults under the same stress mechanism, θ_1 value does not change with the depth. In weak fault zone, there are some secondary small faults, or fractures formed after the high angle normal fault zones are formed, which are fractured zones from the core observation, its layer distribution angle is usually less than 30 °, and it is consistent with the lower weak fault belt angle(θ_2) in the Mohr's circle. With the increase of the normal fault dip angle, the shear resistance strength of rocks will increase gradually.