Research In Crustal Deformation In Longmenshan Fault Zone And Earthquake Fault Parameters Inversion

With its burstiness and impossible accurate prediction, earthquake disasters may have great effect on human life,among them,the Wenchuan earthquake in 2008 caused huge losses of life and property to Chinese people.One of the greatest progress in Seismology in 20 th century is that we have discovered that earthquake take place in fault. With the development of space technology, GPS technology can accurately obtain the earth’s crust deformation information, combined with the geological and geophysical data,we can search for the natural law of earthquake gestation and occurrence and development, thus maximumly reduce the damage of the earthquake.Longmenshan fault has a complex geological structure, the geologic activity here is not stable.Taking the Longmenshan fault zone as an studying area, using the GRACE data to calculate the gravity change in the surrounding area around the fault zone,this paper focus on the research of distribution law of coseismic deformation, and build a velocity field model, and calculate the seismic strain changes distribution by using the method of triangle and quadrilateral method respectively.Combining GPS and leveling data,the inversion calculation on the fault model of Longmenshan fault is carried out. The paper’s main work and achievements are as follows:(1) The paper researches the dislocation theory about crustal deformation and satellite gravity.Based on the rectangular elastic half-space dislocation theory model numerical,the paper carries out a simulation calculation about the surface horizontal displacement, vertical change, and the distribution of gravity changes.When GRACE data is used to calculate the effect of Gaussian filter on eliminating the stripe error when surface gravity changes, the conclusion is that the result is good, when the smooth radius is 600 km.(2)The characteristics of co-seismic displacement distribution in Longmenshan fault are as follows: within the Yingxiu- Beichuan fault centered epicentral area, the co-seism decreases quickly the moment when it is away from the fault area; a strong shortening of the earth’s crust is formed in the horizontal direction in the Longmenshan fault zone; and the orderly deflection of the motion direction of GPS stations can be found in the vicinity on the west side of Longmenshan fault zone:to the south in the south,and to the north in the north.(3)The paper demonstrates a direct link between the occurrence of Wenchuan earthquake and GRACE gravity changes through calculation of Longmenshan fault zone and the average GRACE gravity field changes around it from 2004 to 2015,relative to change of gravity field in 2003,And through the calculation of stress field change is further evidence that the event.The GRACE gravity anomaly changes in the region near the Chengdu was significant before and after the wenchuan earthquake.(4) When using polyhedral function method to establish velocity field model of Longmenshan fault, the over-fit phenomena occured if too many nodes are chosen.The influence of smooth factor on the accuracy of model was studied, the conclusion is that the smooth factor has little influence on the precision of the model if the smooth factor value within the range [0.001 0.05].(5) Using GPS observation data, on the basis of triangle and quadrilateral method respectively,this paper calculated and analysed the horizontal principal strain,the vertical principal strain and the shear strain on the XY plane, XZ plane and YZ plane in Longmenshan district. Based on the analysis and comparison, the final results show that the horizontal strain near the fault is the accumulation of plus strain or minus strain,and compared with geological results,it correspond to the generally accepted fact: Qinghai-Tibet plateau plate move to the east and collided with the rigid south China block,and then the strain is highly accumulated on the edge of the two plates,thus leading to the earthquake.The vertical strain showed that the strain in the southeast of the main fault zone is negative, and it is positive in the northwest, and this indicates:the slight surface subsidence in southeast and a little surface uplifting in the northwest are identical with the geological results of the east side of Longmenshan fault zone,that is, the surface on the east side of.Longmenshan fault zone is given priority to decline.(6) When the inversion of the slip parameters of earthquake fault in Longmenshan fault zone is carried out by using Particle Swarm Optimization, the complex fault can be regarded as an unit combination of 22 sub-faults close to the real model.When using a single data inversion, there is a consistency between the results of GPS inversion and that of the leveling data inversion.They both indicate that the fault slip in the vertical fault direction is the most remarkable, and the second is in strike-slip direction,showing that the fault slip is given priority to the right-lateral movement and thrust movement, and the nature of the fault movement is basically identical to tectonic geological results.Taking λgpsl as unknowns, and the joint inversion of a variety of data is used together with the fault parameters at the same time, the inversion result is better than that obtained by using the conventional power allocation inversion.