In-Situ Stress Measurements and Slip Stability of Major Faults in Beijing Region, China

In order to estimate the upper crustal stress state and slip stability of major faults after the Wenchuan Ms 8.0 earthquake, five boreholes, i.e., Pinggu, Shisanling, Xifengsi, Miyun, and Lisiguang Memorial Hall borehole ranging from 600 to 1000 m were drilled in Beijing region. Hydraulic fracturing method was used to conduct in-situ stress measurements in these five boreholes, and stress monitoring equipment was installed at appropriate depth and monitoring stations were constructed. Those data derived from the in-situ stress measurements reveal that the gradient coefficients of the maximum horizontal stresses and minimum horizontal stresses versus depth are 0.0328 and 0.0221, respectively; and the magnitudes of lateral pressure coefficient such as Kav, KHv and the ratios (KHh) of the maximum horizontal stress to minimum horizontal stress are consistent with previous studies. However, the ratios (μm) of the maximum horizontal shear stress to the average horizontal stress are relatively low. It is noted that the dominant direction of the maximum horizontal stress in Beijing region is ∼EW which is the same with the tectonic stress field of North China. However, sub-regional stress field induced by faults which shows deviation from the orientation of North China, has been also revealed. In addition, the stress regimes inferred from in-situ stress data imply that the maximum horizontal stress (σH) is the maximum principal stress (σ1), while the intermediate (σ2) and the minimum principal stress (σ3) show some variations. It is suggested that these variations may be caused mainly by the regional tectonic evolution and the activity of faults near the boreholes. Based on these characteristic parameters, we can point out that the maximum horizontal stress in the upper crust of Beijing region can be defined as dominant stress, while the horizontal shearing stress is relatively weak. Finally, slip stability of major faults in Beijing region is estimated according to Column friction criterion and Byerlee's law, together with the frictional coefficients being assumed to be 0.2, 0.4, 0.6 and 1.0. The results reveal that when the frictional coefficient ranges from 0.6 to 1.0, the recent crustal stress state of Beijing region does not yet reach the expected limit of fault slip. The stress values of the Xifengsi borehole will reach the expected limit when the frictional coefficient is weakened to 0.4, revealing that the Babaoshan fault stands chance of slipping; the Xiadian-Mafang fault and the Huangzhuang-Gaoliying fault will approach to the expected limit of slip and will stand chance of slipping under this hypothesis, but only with very low possibility. However, it should be pointed out that if the frictional coefficient is weakened to as low as 0.2, the major faults in Beijing region will be unstable and slip may occur under the present-day crustal stress state. Conclusions in this paper would be of great significance for studies on tectonic stress field and seismogeology in Beijing region, even North China.