The Study Of The Present Stress At The Northeast Of Longmen Shan Faults

The great catastrophes and losses were caused to our country and peoples by the Wen chuan Ms8.0 earthquake happened on the 12the in the May,2008. In this earthquake, about more than 69227 people were died,374643 people were hurt and 17939 people were disappeared. The economical losses to the country amounted to 8451 billions. The Wen chuan Ms8.0 earthquake was another more serious disastrous earthquake after the Tangshan earthquake happened in 1976.The occurrence of the earthquake has great relationship with the stress condition around the active faults. When the stress in the area increases to a certain value, the strain energy has to be released and the earthquake may happen at this time. In the early 1960s, professor Li siguang initiated the technology of measuring and monitoring the in-situ stress in order to provide the technical supports for the prediction of the earthquakes. With the development of several years, some researchers have made great achievements and verified the relationship between the occurrence of the earthquakes and the in-situ stress. Therefore, it important for the researchers to study the stress field around the active faults in order to find out the mechanism of the earthquakes.However, there were little information about the in-situ stress around the Longman shan faults. Besides, the results show that the earthquakes usually happen at the active faults with the discontinuous deformation, which are inclined to generate the stress concentration. The much results about the landscape, geological structures, GPS observations and the historical earthquakes along the longmen shan faults have showed that the activity of longmen shan faults are discontinuous and the faults are divided into two southwest and northeast sections. The activity of the southwest section has been strong since the late Pleistocene with many moderate and small magnitude earthquakes, where the activity of the northeast section has been weak.The areas with the high stress and results of the seismic hazard derived form the abnormal value of b show that the stress state in the Jiangyou and Beichuan have been high for a long time. Owed to the analysis above, it is considered that the areas of Jiangyou and Beichan with discontinuous activities may be the key areas which are inclined to generate the earthquakes. Therefore, it is important for the researchers to research the seismic hazard that stress measurements were initiated in the areas of Jiangyou and Beichuan after the Wen chuan Ms8.0 earthquake in order to obtain the stress data close to the faults.This paper based on the project named "monitoring the stress along the main faults in the eastern of Qinghai-Tibet Plateau" presents the stress state and the seismic hazard which are derived from the seismic data, geological data, the activities of the faults and the numerical simulation of stress field in the studying area.The contents of this paper as follows:1,Systematically collect and analyze the data of the structural geology, the stress and the rock mechanics which were done by the predecessors.2,Systematically collect and analyze the data of the activities of the faults.3,Hydraulic fracturing in-situ stress measurement at three wellbores.4,Analyze the present stress state along the northeast of the longmen shan faults.5,Analysis of the seismic hazard in the studying areas.6,Analyze the tectonic stress field along the longmen shan faults.7,Two dimension numerical simulation of the tectonic stress field along the northeast of longmen shan faults.8,Three dimension numerical simulation of the tectonic stress field at the vicinity of ZK02 along the northeast of longmen shan faults.Based on the above works, some conclusions are drawn as follows:1,The results that the relationship among the three principal stresses is SH(the maximum horizontal principal stress)> Sh(the minimum horizontal principal stress)> Sv(the vertical stress) show that horizontal stresses are dominant in the areas.2,The discrepancy has been existed between the stress field near the northeast of longmen shan faults and the distance to the faults.(1) The affects of the horizontal stress are gradually increased vs the increasing distance to the faults.(2) The value of the maximum horizontal principal stress and the gradient vs the depths are gradually increased vs the increasing distance to the faults.(3) The value of the maximum shear stress and the horizontal shear stress are gradually increased vs the increasing distance to the faults.3,Compared to the previous stress data, the present stresses at the areas of Jiangyou and Beichuan in the northeast of the longmen shan faults are much higer.4,The orientation of the maximum horizontal principal stress is NE～NEE, which accords with the mechanism of the right-lateral strike slipping of the fault and is different form the regional tectonics tress field.5,Owed to the Wen chuan Ms8.0 and its aftershock, the stresses in the Jiangyou and Beichuan areas in the northeast of the longmen shan faults have been gradually decreased. There will be unlikely to generate the large magnitude earthquakes, where, the small magnitude earthquakes may occurred in some areas.