The Comparison And Analysis Of Wenchuan Pre-earthquake Deformation And Coseismic Deformation

The most effective measure to reduce the earthquake disaster is earthquake prediction based on earthquake precursor information.The study of mechanism of earthquake triggering and the crustal deformation monitored by modern space measurements before earthquake is of great significance to form the system of earthquake prediction methods.In this paper,based on the fault physical parameters of Longmenshan fault zone,fault slip dislocation model which build a connection between the surface displacement and fault slip is established.By using the genetic algorithm method with coseismic surface GPS measurements,coseismic fault slip parameters is inverted.According to the fault slip parameters,the moment magnitude of the earthquake is estimated.In order to find earthquake precursor information and analysize the pre-earthquake crustal movement trend near the fault,biharmonic green function interpolation method is used to interpolate the third phase surface velocity field before earthquake.The results of the tomographic imaging data explain the mechanism of earthquake triggering.Specific work is as follows.First,this paper introduces the fault dislocation model,the basic principle and research progress of inversion algorithm.Analytical relationship of fault slip and the surface displacements are given.Genetic algorithm is selected as the fault parameter inversion algorithm.The fault parameter inversion program is written in MATLAB.A numerical example is used in order to verify the reliability of the program.Then,159 pre-earthquake GPS measurements and 38 coseismic GPS measurements are selected to make a research near the Lomgmenshan fault.In order to invert the coseismic fault slip parameters,the original data are transformed form ITRF spatial coordinates to local coordinate system.Results show that the coseismic strike-slip is-3.029 m,the coseismic dip-slip is 4.725 m.The coseismic slip tendency is consistent with the coseismic reversion which is thrust strike-slip with right-lateral slip.The moment magnitude is estimated in order to check the inversion results.In addition,biharmonic green function interpolation method was used to interpolate the third phase velocity field before the earthquake based on scattered GPS measruements.The results show that the velocity of the crust movement paralleld to the fault strike is faster than that perpendicular to the fault strike.The earth's crust in the direction parallel to the fault is in compression.On the contrary,the earth's crust in the direction perpendicular to the fault is compacted.The characteristic of crustal movement near the fault before earthquake is identical with coseismic crustal movement.There is a low displacement area on the south of Chengtu before earthquake and the range of the area is expanding.Xiongpo anticline has a strong crustal fault block in direction of dip direction.The local geological conditions explain the reason that fault began to burst form epicenter toward the north.Finally,the low velocity layer exists in deep crust is analyzed.The crust under the fault provide an advantage condition for the upward movement of low velocity layer.