|Abstract:The Ebian-Jinyang Fault Zone is the boundary zone between Liangshan fold fault uplift and Yibin-Leibo tectonic fault. The seismicity parameters of b value shows that the northern section of the fault is at a low stress level. To find out the crustal stress state in this area, the hydraulic fracturing crustal stress measurement of two bore holes was carried out respectively in 2013 and 2014. The measurement results show that the maximum horizontal principal stress direction of the two boreholes is between NW 50° and NW 68°,tending to be NW-NWW, which is consistent with the results of focal mechanism solution and regional stress field; the stress level of DPS-1 hole, which is located on the axis of the syncline, should be higher than that of the surrounding area; the SH value is between 5.89MPa and 14.50MPa. At present, the stress level is low, which is consistent with the seismicity parameters of b value; the SH of the Ebian area whose depth is more than 300m is similar to the value of the stress near the southern part of the Ebian-Jinyang Fault Zone and the Anning River Fault Zone, and as the depth increases, the SH is less than that near the southern part of the Ebian-Jinyang Fault Zone and the Anning River Fault Zone; the principal stresses of DPS-1 hole in three dimensions are expressed as Sv＞SH＞Sh, which is beneficial to the activity of normal fault and is inconsistent with the thrust property in the Ebian-Jinyang Fault Zone. The main reason is the weakening of thrust tectonic activity in the northern part of Ebian-Jinyang Fault Zone, and consequently, the intensity of its tectonics is already weaker than the regional stress field. The three-dimension stress of YJP-1 hole at a depth of less than 150m shows a thrust stress state, while that at a depth of more than 225m shows a strike slip stress state. As the depth increases, the stress state will change to a normal fault stress state. It is also a reflection of the weakening of thrust tectonic activity in the northern part of Ebian-Jinyang Fault Zone. Based on the measured data, combining the Coulomb failure criterion and the stress Mohr"s circle, the relationship between μm ( the ratio of maximum shear stress to average principal stress) and stability of the fault is discussed, and the possible effects of earthquake, topography and geological structure, and the stress adjustment prior to the MS5.0 earthquake of Jinkouhe on the stress state of the measured hole area are analyzed qualitatively. The results show that topography and geological structure have little influence on the stress state of the measured hole area.
The μm of DPS-1 hole in the condition of effective stress is high,which is beneficial to the sliding of normal fault, but it will restrain the activity of reverse fault. This shows that Ebian-Jinyang Fault Zone is more stable, and is consistent with the actual situation. Stress adjustment prior to the earthquake has little influence on the stress state of DPS-1 hole. All these results are of great significance to understand the background stress field in the northern part of Ebian-Jinyang Fault Zone.