Study On The Gravity Field Variation Of The Longmenshan Fault Zone Based On The Disclination-Dislocation Model

The Longmenshan fault zone has the characteristics of special geological structure background, high frequency of earthquake occurrence, and has attracted much attention of scholars in China and elsewhere in the world. The study on the characteristics of crustal movement and fault activity in the Longmenshan fault zone can not only provide scientific evidence for the uplift of the Qinghai-Tibetan Plateau, but also help to understand the characteristics of the change of the potential field during the formation of the earthquake.Dislocation theory only describes the slip of fault, and the fault rotation factor is taken into account by disclination theory, which is the complement of dislocation theory, has been applied to the study of three-dimensional displacement field. The disclination theory is extended to the study of gravity field in this paper. Based on the dislocation theory, the three-dimensional model of the dislocation-disclination has been established. The model is applied to the study of the displacement field and the gravity field in the Longmenshan fault zone. The main researches and results are as follow:1. The formula to express relationship between spatial gravity change and fault three-dimensional slide and rotation is derived by using the formula of point source dislocation theory. The spatial gravity variation caused by the three-dimensional rotation of simple fault is simulated and analyzed. The results show that: When the fault rotates with a single component ω1, the spatial gravity distribution region has obvious asymmetry. However,the gravity variation shows the symmetrical distribution characteristic of the axial direction along the fault dip direction, when a single fault occurs with a single component ω2 or ω3;Another simulation proves that the rotation effect of the ω1 will break the spatial symmetry of the distribution of the whole gravity change results when a single fault rotates in two ways with ω1 and ω2 at the same time. In addition, when the fault rotation occurs with ω2 and ω3 at the same time, the rotation effect of the ω2 will play a leading role in the spatial gravity variation results. The greater the fault depth, the smaller the value of the spatial gravity variation. The greater the rotation of the fault, the greater the amount of spatial gravity variation, and the more obvious the asymmetry of the image.2. The surface displacement field induced by the Longmenshan fault movement issimulated by using the two models including dislocation-disclination model and dislocation model. The displacement field residual, residual mean, and variance of the two models are calculated and compared. The results show that: compared with the dislocation model, the results of the displacement field calculated by the dislocation-disclination model have a smaller degree of dispersion and deviation. The simulated values of the dislocation-disclination model are in better agreement with the measured results of GPS in both magnitude and direction.3. The dislocation-disclination model is used to calculate the spatial gravity variation caused by the crustal movement in the Longmenshan fault zone, and the results have been compared with the calculation results of the dislocation model. The results show that:compared with the dislocation model, the calculation results of dislocation-disclination model which takes the factors of fault rotation into account make a more complete description of the gravity field variation, and provide a result which is more close to the true reflection of the crustal movement effect. The dislocation-disclination model accomplishes the purpose of complement and refinement of the dislocation model.