Study On Site Dynamic Characteristics Of Weihe Basin Based Onwenchuan Strong Earthquake Data

In recent years, with the development of strong motion observationtechnology, local site on ground motion amplification has been paid more andmore attention. Basin as a special field, the seismic response has its owncharacteristic. In May12,2008, a magnitude8earthquake occurred inWenchuan County, Sichuan Province. Buildings destroyed in different degreewere found in Weihe basin with500~800km distance from wenchuanearthquake epicenter. According to the post-disaster, Xi’an City and ChencangDistrict of Baoji City, located in Ⅵ, Ⅶ intensity anomaly area.The articlebased on the background of Weihe Basin, using the records of Wenchuanearthquake, analyse the site dynamic characteristics of Weihe Basin. The maincontents are as follows:1. We analyse the Fourier spectrum and response spectrum of Weihebasin stations and calculate the predominant frequency and magnificationbeneath each station of soil site using the reference station method and H/Vspectral ratio method, respectively. Based on the analysis of responsespectrum, we found nearly half of stations’s the decline of response spectrumexceeds design response spectrum. In some stations the platform also beyondthe design response spectra. Structures which are designed according to theseismic code are in danger in the earthquake. The long period componentsseismic wave in the basin is mainly due to the presence of surface waves. Thefundamental predominant frequency ranges0.1~0.4Hz. The ability of the siteamplification is stronger with the stations of Xi’an, Weinan, Chencang andPuchen. Among them, Xi’an and Chencang is agreed with intensity anomaliesin the post-disaster survey.2. We gain the dispersion curve of Rayleigh wave based on the multiplefiltering theorys. It calculates the dispersion curves of source and stationconsidering the influence of finite fault. We find the propagation direction ofsurface wave by painting radial and vertical if meeting the inverse relationinto ellipse. The group velocity between station and source is nearly constantvalue, about2.8km/s. Using the double station correlation theory, we getGreen function of the surface wave and calculate the group velocity dispersionrelation. Comparison the results of the dispersion curves, we can accuratelyextract to the Rayleigh wave group velocity dispersion curves for10~30s. Inthis band, the group velocity varies from1.6~3.6km/s, about20secondsappears the minimum of the group velocity.3. We inverse the site using the Rayleigh wave group velocity dispersioncurves. By using the damped least square method, Fit the theoreticaldispersion curve and the measured dispersion curve to get the structure of theearth. The inversion results show, at a depth of40km wave velocity boundarylayer are obvious, namely the Moho. The values of Shear wave velocity of Xi’an-Linyi, Huxian-Dali, Caotan-Wanrong, Xianyang-Lindian and Yanliang-Heyang, Pucheng-Hejin stations at the Moho of are4.4km/s,4.5km/s,4.1km/s,4.4km/s,4.2km/s,4.3km/s;The shear wave velocity values at20,30,40kmhave a similar results with using the P wave inversion by others. It proves thatthe results are reasonable.