Current in-situ stress state and seismic risk in Sichuan-Gansu-Shaanxi border area

The Sichuan-Gansu-Shaanxi border area has experienced many great earthquakes with complicated tectonic deformation and tectonic activities, which is the reason why this area is important for study of seismic risk. In order to better understand in-situ stress environment of Tianshui area and to evaluate the seismic risk, it is necessary to conduct deep borehole in-situ stress measurement. In this study, data were gathered from 9 in-situ stress sensors installed in deep borehole (600 m) in the southwest of Gangu County, Gansu Province and 14 in-situ stress sensors installed in deep borehole (400.12 m) at Sandui Town, Sichuan Province. The in-situ stress data reveal that the geostress level of the region is relatively high and both horizontal principal stresses in two tests are larger than the vertical stress (S_H >S_h >S_v). The direction of the current maximum horizontal principal stress is N41°E in Gangu borehole and is N85°W in Sandui borehole. The directions of the current maximum horizontal principal stress reflect that the present activity property of the north margin of west Qinling fault is thrust and sinistral slip, and that the present activity property of the Qingchuan fault is thrust and dextral slip. The activity of the north margin of west Qinling fault and the Qingchuan dault was analyzed by Coulomb friction sliding criterion, and the results reveal that the current in-situ stress levels have reached the critical condition of sliding instability. Based on the in-situ stress state and earthquake migration data, it is suggested that more attention should be paid to the Sichuan-Gansu-Shaanxi border area. The results obtained by the authors have great significance for analysis of active faults, long-term monitoring of in-situ stresses, assessment of the regional geological environment and geological disasters prevention.