Numerical Simulation Analysis On Site Effects In The Meizoseismal Area Of The Minxian-Zhangxian Ms6.6 Earthquake

The Loess Plateau area is the most frequent area of strong earthquakes in China.The post earthquake scientific expedition of the Zhang Xian Ms6.6 earthquake in Minxian found that the thickness and topographic conditions of the site cover layer in the epicenter area had a significant amplification effect on the ground motion.Taking the 6.6 magnitude earthquake area of the Minxian,Zhangxian as the research area,this paper uses the servo velocity seismograph to carry out high density pulsation observation on the site of the epicentre area.The spectral analysis and H/V spectrum analysis of the observed signals are used to obtain the excellent frequency and its distribution characteristics,and then the thickness of the loess cover layer in the study area is retrieved.Based on the aerial flight telemetry of unmanned aerial vehicle(UAV)in this area,relevant data are extracted to analyze the terrain and geomorphology in the study area,and the 3D model of the area is established by irregular grid method.By extracting the data of the thickness and topographic features of the cover layer,the DEM data of the surface and bedrock layer are obtained by the compound algorithm,and the generated land is further processed by the Rhino.The 3D model of real terrain in general finite element software is established.Finally,the two dimensional finite element numerical simulation of the thickest section of the Loess covering layer in the real terrain is selected,and the dynamic response characteristics of different thickness of the cover layers and the elevation sites are compared and analyzed.(1)the data collected from the seismological area of the seismological epicentre of the Minxian and Zhangxian show that the peak of the excellent frequency in the study area is concentrated in 2-5Hz.By inversion of the thickness of the covering layer of the severely affected villages,it is found that the serious affected settlements are all in the thick loess area over 20 meters above the loess,indicating the thickness of the loess cover layer.There is a certain relationship between the degree and the magnification effect of the site.(2)The irregular grid method is adopted to create 3D model of real terrain,which optimizes the structure of the algorithm and reduces the memory capacity while ensuring the modeling accuracy.(3)The extreme earthquake area of Minxian,Zhangxian is very complicated and the ravines are vertically and horizontally.The maximum altitude difference is 620 m.The coverage of the high altitude area is thick,and the coverage of the lower elevation is thin.The thickness of the overburden is 54 m and the thinnest part is 0m.(4)The acceleration amplification factor of the site has a certain relationship with the thickness of the overburden and the elevation of the site.The stress value and the absolute change of S12 also have a certain relationship with the terrain and elevation.Based on post-earthquake investigations,it was found that the most serious Yongxing Village was located in the thickest area covering 2600-2700 meters,and the calculation results were basically consistent with the actual disaster situation.