Study On Site Responses Of Strong Motion Stations In Sichuan Area

Site effects is one of the important studies in earthquake engineering,which has important theoretical significance and practical value for the studyof the spatial distribution of ground motion, site of seismic safety evaluation,site selection and seismic fortification of buildings. Based on research statusof the site classification and site effect at home and abroad, utilizing the thestrong motion records in Wenchuan great earthquake, the site effects of stongmotion stations in Sichuan are studied by traditional spectral ratio method andhorizontal to vertical Fourier spectral ratio (HVFSR) method. Its main workand achievements are as follows:1. Firstly, utilizing the shear wave velocity profile of112strong motionstations in Sichuan, Vs30is estimated by Boore’s method. Then, the siteclassification of112stations is carried out by Chinese Code Seismic Designof buildings (GB5011-2010) and American Uniform Building Code (UBC,1997). The results show that: Accodding to hinese Code Seismic Design ofbuildings (GB5011-2010), among the site classes of the strong motion stations,majority is II class. In112strong motion stations in Sichuan, I1class with2,II class with103, II or III class have7sites.2. Utilizing the acceleration time history of56stations in the aftershocks,a site-classification scheme based on the predominant period of the site isdetermined from horizontal to vertical Response spectral ratio (HVRSR)method.The results are as follows: Among, SC-I:(Rock/stiff soil), SC-II:(Hardsoil)/SC-III(Medium soil), SC-IV:(Soft soil) and SC-V(Rock/stiff soil), thenumber of sites of class are24,16,4,3,9, respectively. In56stations, thepercentage of class SC-I,SC-II,SCII,SC-IV,SC-V is42.86%28.57%7.14%5.36%16.07%. So, there are40of class SC-I and SC-II, dominated by classSC-I sites.3.Utilizing the acceleration time history of55stations in Wenchuan greatearthquake mainshock and aftershocks, the maximum amplification factor andthe corresponding frequency of the stations are estimated by horizontal tovertical Fourier spectral ratio (HVFSR) method to differentiate the sites ofthe stations whether nonlinear site response occurs. The results are as follows:SC-I, SC-II, SC-III, SC-IV, SC-V sites of five categories where nonlinearresonse occurs, and the corresponding number of stations are:12,6,2,0,4.So, the percentage of each class SC-I,SC-II,SCII,SC-IV,SC-V of nonlinear siteresponse is63.16%35.29%50%0%44.44%.4. Utilizing the acceleration time history of23stations in Wenchuangreat earthquake mainshock and aftershocks, the the amplification effects ofsoil sites in Sichuan are estimated by traditional spectral ratio method. And8 of the study stations are differentiated by traditional spectral ratio methodwhether nonlinear site response occurs.The results showed that: If you canchoose the appropriate reference site, using the traditional spectral ratiomethod can better estimate the site amplification.