Study On The Distribution Characteristics Of Crustal Stress Field In Chinese Mainland

The research of crustal stress field is an important part of the study of the engineering geology, a basic foundation of testing large geotechnical engineering construction on mechanical environment, an important way to study the prediction and prevention method of geological hazards such as earthquake and landslide, a key of Pushing plate theory from the kinematic to the dinamics. In nearly centenary time, not only measurement technology, analysis method and theoretical research of in-situ stress have made great progress and many valuable results, but also accumulated a large amount of observation data. With the measured in-situ stress data, we researched the present state of the in-situ stress field and the strong earthquakes’disturbance impact to it by numerical simulation method in the Chinese Mainland.Firstly, we collected and analysed the measured in-situ stress data of3586on the Chinese Mainland1474points by the method of hydraulic fracturing and overcoring. We have solved the problem of uneven distribution of the measured in-situ stress data along the depth by the equal depth group summarized method, and given the stress magnitude and orientation characteristic in the upper crust within the measured depth range on Chinese Mainland and each study area. And through the method of subtracting the effect of gravity from the measured stress, we analysed the role of the tectonic stress field. And we have given the orientation characteristic and magnitude range of the horizontal tectonic stress on Chinese Mainland and each active block.Secondly, we have built a finite element model including the present main active faults and actual layered structure of the lithosphere (ground, Moho, and lithosphere). And we also introduced the spherical structure in the model to make the numerical model and the geological model more consistent. Based on this model, we used gravity and plate tectonic as the main influencing factors, the research results of the measured in-situ stress in the upper crust as the target controls, to make the inversion analysis of the present in-situ stress field in the Chinese Mainland, and gave the magnitude estimation of it.Thirdly, we made the numerical simulation research about the stress field adjustment law caused by strong earthquake in Chinese Mainland, with the method of reduce the seismogenic fault elements group modulus to simulate the loss of carrying capacity when the faults were unstable. Thus we could explore the annual time scale of a wide range of changes in the stress field and the corresponding strong seismic activity migration mechanism.We have made the following key findings:First, the statistical analysis of the measured shallow crust stress data.(1) The maximum horizontal principle stress σH, the minimum horizontal principle stress σh and the vertical stress σV in shallow crust of China all increase linearly with depth:σH=0.0229D+4.738, σh=0.0171D+1.829, σv=0.0272D; The range of σT and σt in shallow crust of China are:4.738<σT<0.0139D+4.738and1.829<σt<0.0081D+1.829, respectively. Regression equation of horizontal tectonic differential stress in China mainland is:σT-σt=0.0058D+2.912with a gradient of5.8MPa/km, and surface value at Okm is3MPa.(2) The maximum horizontal tectonic stress σT, minimum horizontal tectonic stress σt both increase linearly with depth. The intermediate regression value of σT1(at2000m depth) is various in research regions, the maximum value is45.6MPa while the minimum value is26.8MPa. According to intermediate regression σT1value, the ranking (from largest to smallest) of research regions is:Qinghai-Tibet, north section of north-south seismic belt, south China, middle section of north-south seismic belt, north China, south section of north-south seismic belt, northwestern China, northeast China. Horizontal tectonic differential stress σT—σt increase linearly with depth and the maximum and minimum of the intermediate regression value of σT—σt is25.3MPa and13.0MPa, respectively. Ranking of research regions based upon the intermediate regression σT—σt value is:Qinghai-Tibet, northwestern China, north section of north-south seismic belt, south China, south section of north-south seismic belt, north China, middle section of north-south seismic belt, northeast China. In general, the stress magnitude is much higher in western than in eastern China. This indicates that the strong collision of Indian plate with Eurasian plate dominates the present tectonic stress field in Chinese mainland.(3) Compared to most other study regions, the crustal stress magnitudes in the Qinghai-Tibet block which in under the northward compression is low in shallow depths and high in deep.Second, numerical simulation analysis of the in situ stress field.(1) We built a three-dimensional spherical shell model of the Chinese continental lithosphere. And we made the path to estimate the present in-situ stree field in Chinese Mainland, with the method of numerical simulation invertion analysis, and with the research results of the measured in-situ stress in the upper crust as the target controls.(2) The simulation results showed that the general direction characteristics of maximum horizontal stress basically spreads radially from the center of Tibetan Plateau. The directions of maximum horizontal stress gradually rotate clockwise from NS to NNE, NE, NEE, SE and are consistent with the result of focal mechanism solution.(3) The stress states in different study regions are evidently different. The stress magnitude is obviously lower in the center of Qinghai-tibet active block and higher in the surrounding area of the block.(4) The maximum and minimum tectonic stress σH and σh were generally compressive at the depth of2000metres in Chinese Mainland shallow crust, and the magnitude range are14.5MPa<σH<58.0MPa、3.8MPa<σh<26.7MPa.Third, the research of stress field adjustment caused by strong earthquake in Chinese Mainland.(1) Killing one elements group will cause the stress field’s adjustment in large area. Over55%of elements’equivalent stress increased more than0.1MPa. The change of direction of principal stress was less than20degree in99.7%area.(2) Setting up initial stress field greatly influenced the perturbed stress field caused by killing elements group.(3) By killing the elements group of November11th2001Kunlun mountain Ms8.0strong earthquake area, the perturbed stress field was corresponded with the actual observed value of in-situ stress and the strain step of drilling strain observation.