Estimation Of Near Fault Ground Motion Field Of Jiuzhaigou Mw6.5 Earthquake

The main reason of building damage caused by earthquake is strong ground motion,so the reliability of seismic input determines the reliability level of structural seismic analysis to a large extent.The near fault ground motion is affected by the source with the significant spatial variation,the strong amplitude and the short duration,which may produce large velocity pulse and has a strong influence on engineering structure.Near fault strong earthquake records contain complex information of near fault ground motion,but usually the information from stations is limited,which can not describe all ground motions in the study area.So it is necessary to use simulation method to meet the needs of the study of ground motion.At present,the ground motion can be estimated by combining the physical focal mechanism,the deterministic model of wave propagation path and local site conditions.The simulation results are helpful to reduce and avoid the future earthquake disasters,and provide technical support for seismic analysis and seismic risk analysis of major projects.This paper takes Jiuzhaigou earthquake as the research object.The source parameters are inversely calculated by using the strong earthquake observation data.The ground motion in Jiuzhaigou near fault area is estimated by using the deterministic ground motion simulation method based on the seismic wave propagation theory.The main contents of this paper are as follows: firstly,according to the crust structure of Jiuzhaigou area,the deep and shallow crust velocity structure model of Jiuzhaigou area is established,including P-wave velocity,S-wave velocity,thickness and density of each layer.With the discrete wave number method of horizontal layered medium,the ground motion process of a point in half space caused by shear dislocation source is calculated.The amplification effect of near surface soil layer is considered by one-dimensional nonlinear soil layer response analysis method.Based on the near-field strong ground motion records of 34 stations of Jiuzhaigou earthquake,BSI broadband spectrum inversion method is used to inverse the corner frequency of sub sources,index term of source time function and source slip distribution.The results show that the optimal parameters of the inversion can ensure that the response spectrum deviation of the simulation results at 34 stations does not exceed 0.6 natural logarithm units at0.1-10 hz.Taking Jiuzhaigou earthquake intensity distribution map as a reference,the target ground motion simulation area is selected.The strong ground motion in the target area is simulated by using corner frequency,focal slip distribution and horizontal stratified discrete wave number method.The time history of ground motion velocity at each point is obtained,and the distribution diagram of peak velocity and acceleration is formed.The long axis of the simulated acceleration contour line is basically consistent with the direction of the causative fault.The peak value appears in NS direction,the maximum acceleration is 383 gal,and the velocity is 37.8 cm/s.Finally,the geometric mean of acceleration in two horizontal directions is compared with the actual Jiuzhaigou seismic records and attenuation relation.The engineering applicability and improvement direction of inversion of ground motion parameters are discussed,which provides reference for the future ground motion analysis and disaster prevention strategies in this area.