STABILITY AND STRESS MEASUREMENT NEAR THE QINGCHUAN FAULT IN THE NORTHERN LONGMEN MOUNTAINS

After the M8.0 Wenchuan earthquake, it becomes an important concern that whether aftershocks direct to Hanzhong along the Qingchuan fault and a strong aftershock also occur.The study on in situ stress measurements was carried out at Ningqiang region in the northeastern section of the fault in order to master the changes of the coulomb stress in the Qingchuan fault after the earthquake and to identify the development trend of aftershocks, using the hydraulic fracturing in situ stress measurement techniques on June 2 to 28, 2008.The size, orientation and distribution of the tectonic stress in the fault were obtained respectively by the hydraulic fracturing and stress relieving in three boreholes.According to the measured stress data, the direction of the maximum level principal stress is NE-NEE near the fault, and does not accord with the SN-NNE direction of the regional tectonic stress.It is shown that the stress state near the fault changes obviously compared to the regional stress field, affected by the impact of fault activity.The relationship between horizontal and vertical stress is σ_H>σ_h>σ_V at the measurement depth.The study reveals that the stress state is conducive to the activities of reverse fault and is basically the same as the geological data.By using sliding friction Coulomb Guidelines and friction strength from 0.6 to 1.0, it can be seen that present stress state near the fault has reached or exceeded the sliding friction threshold of the thrust fault, indicating that present tectonic activity is mainly thrust movement and it is impossible for the strike-slip movement to occur in the measured area.The thrust fault activity is less likely in the eastern section of the fault but much more likely in western part of the Qingchuan fault.