Interrelationship Of Active Faults And Geo-stress

Earthquakes and other disasters caused serious losses to the country and people. It makes great significance to understand the seismogenic mechanism, influence factors and risk evaluation. In-situ stress is the reason of tectonic activities in the crust. To research the crust stress state is an important means to cognize crust tectonic deformation and dynamics. Active faults have an close relationship with geologic hazard like earthquakes for example. It is not only reflect on the consistence of space distribution of active faults and earthquakes, but also the uniformity of faults active feature and the coseismic ruptures along the faults accompanied by earthquakes. Thus, there must be some connection between active faults and stresses and it is more important to research the stress states near active faults.In this thesis, a large number of references are collected to summarize the research status of active faults and stresses which laid foundation to the following study. In-situ stresses were measured by the hydraulic fracturing method to determine the present-day stress state in Longmenshan Fault zone. The test results show that the relative stress magnitudes are σu> σh> σv, and the horizontal principal stress intensity, in0.54-5.24, is higher than general statistics. This kind of stress state is beneficial to reverse fault activity. Besides, the stress distribution characteristic in Longmenshan Fault zone is compared to the results of previous studies to discuss the basic feature of stress state and its particularity.Research of relationship between active faults and geo-stress were carried out using the measurement data and numerical simulation respectively. Preliminary conclusions were obtained by discussing the distribution characteristic of shear stress and principal stress, and the stress evolution with time near the fault, that is, stress increases with distance from the fault in the calm period before the earthquake, and tends approaching to the regional stress value. But this distribution regular seems to be gradually reversed with time and change of the fault property such as frictional strength, stress is increasing near the fault and is decreasing far away from, stTess seems to decrease with distance from the fault. The distribution of stress is not the same even in one fault. The magnitudes of principle stresses in the northern and southern segments are apparently higher than the ones in the middle section, and the effect strength of stress in the southern segment is stronger than that in the northern part which is still in the high stress environment. Stress values at the hanging wall is much higher than that of the foot wall, implying unbalanced stress environment in the Longmenshan Fault zone. Fault activity is discussed by the relative size of shear stress and faultsliding criteria, and critical equation taking the maximum horizontal principal stress as reference is summarized. The result indicating that the reverse fault activity is apt to be triggered.Taking advantage of numerical simulation, different effect due to single fault and multiple faults is analyzed. It shows that single fault has limited impact to stress and the inlluence sphere is expanding when there exit other faults. In addition, the direction of the maximum principal stress is change to some extent. Considering the change of the friction coefficient, distribution of stress is also effected. Relatively low stress area is reduced and advantaged accumulation region is more obvious near the fault with increase of the friction coefficient. For the angle between the direction of maximum principal stress and fault strike, is almost right angle in the case of low value of friction coefficient, and seems to reduce with the increase of friction coefficient. Evolution of stress in3000a is calculated, and the results shows that the stress concentration zone is transferred to Longmenshan Fault zone and indicating differential stress concentration in the hanging wall and the footwall. Changing of stress values are slow down in the years of2000--3000, consistent with the recurrence interval of Wenchuan earthquake,2500a--6000a, obtained by previous studies.