Measurement results of in-situ stress in Guyuan area of Ningxia on the southwest margin of Ordos block and its causation analysis

The southwestern margin of the Ordos block has experienced many great earthquakes with complicated tectonic deformation and tectonic activities.In order to understand in-situ stress environment of southwestern margin of the Ordos block and to evaluate the seismic risk, the authors carried out the in-situ stress measurement by hydraulic fracturing in two boreholes in the Guyuan area of Ningxia.Combined with other measured data and focal mechanism solutions, the authors discussed the tectonic stress field characteristics of the southwestern margin of the Ordos block.Some conclusions have been reached:(1) The results show that the relationship of principal stress of the two boreholes in the study area is S_H > S_v > S_h, belonging to the strike-slip stress state.The azimuth of maximum horizontal principal stress near the borehole is N59°W; however, it is different from the P-axis orientation of focal mechanism solutions on the northeastern margin of the Tibetan Plateau.It is considered that the formation of the NWW strike-slip shear stress environment on the southwestern margin of the Ordos block is mainly affected by the Haiyuan fault and Liupanshan fault zone.The current stress environment may be the result of the combined effect of local structure and regional tectonic stress fields.(2) The crustal activity in the study area is discussed using the Mohr-Coulomb criterion and Byerlee's law under the premise that the friction coefficient is 0.6~1.0.The in-situ stress state of the southwestern margin of the Ordos block has not reached the sliding critical condition of the shallow fault and is in a relatively stable crustal stress state.The results have great significance for analysis of active faults as well as assessment of the regional geological environment and geological disasters prevention.