| Severity of seismic hazard is affected by the site effect. Structural response is the form of seismic hazaed. Nonlinearity of site seismic response can provide scientific evidence for seismic design of buildings. These have been a popular topic in the field of scientific research and earthquake engineering. The site effect and nonlinearity of site seismic are important and practical for spatial distribution of earthquake motion, evaluation of seismic safety for engineering sites, site selecting of buildings and earthquake protection. In the 2008 Wenchuan earthquake, the China National Strong Motion Observation Network System captured a large number of strong-motion recordings at free field both in the main shock and aftershocks. This makes a good chance for studying the site effect and nonlinearity of site seismic response. In this paper, based on the mainshock, aftershocks, microtremors, strong-motion observation data in a reinforced concrete building, it takes a systematical study of the site effect and identification of nonlinearity of site response. That manages to provide scientific foundation for earthquake protection, extend the application of strong-motion recordings in the field of scientific research and engineering practice, and promote the development of strong-motion observation in China. The contents are summarized as follows:(1) Research progress of the site effect and site seismic nonlinear response and structural response are reviewed. Traditional spectrum ratio, generalized inversion technique, numerical simulation, H/V spectrum ratio, response spectrum method are introduced, then H/V spectrum ratio method and response spectrum method are proposed. The merits and drawbacks of these methods are summarized.(2)Strong-motion records and site conditions of Wenchuan earthqueak are collected and arranged. Strong-motion records of 25 stations(3 rock stations,15 Ⅱ soil site stations,7 Ⅲ soil site stations) are counted, then characteristic of these records are summarized for the next identification of nonlinearity of site response.(3)PGA and other characteristics of the main shock are showed in the paper. Predominant frequency, site amplification times, Tg, βmax, these are obtained by H/V spectrum ratio method and response spectrum method, to analysis the site effect. The results show that:H/V spectrum ratio of FAS is better than H/V spectrum ratio of VRS. Least square fitting method based on coordinate transformation is superior to obtain the parameters of site.(4)A total of 1008 strong-motion recordings obtained by 25 stations from 338 aftershocks of the Wenchuan earthquake are selected as a dataset. The data is divided into 3 groups as:PGA smaller than 100 gal, PGA between 100 gal and 200 gal, PGA lager than 200 gal. The comparisons between the results given by H/V spectrum ratio method and response spectrum method show that the site seismic nonlinear response are existence, but the applicability of each method is different. The results show that: nonlinear response of Ⅰ rock site is smaller than Ⅱ soil site, nonlinear response of Ⅲ soil site will be obtained in the sixth chapter.(5)Microtremors that obtained from aftershocks of Wenchuan earthquake are proposed after analytic study. Mean value of microtremors existse big different between the daytime data and the nighttime. The equal principle is suggested to be used for intercepting microtremors from aftershocks data. Those data are pre-event recordings. Identifying the nonlinearity of site seismic response by microtremors is meaningful to site effect study.(6) The comprehensive comparisons between the results from mainshock, aftershocks, microtremors show that the site effect and the site seismic nonlinear response is different from Ⅰ rock stations, Ⅱ soil stations and Ⅲsoil stations. In order to obtain representative results, data of Maoxiandiban station, Lixiantaoping station and Wudu station are used. The result shows that the site response appears obvious nonlinearity when PGA in different scale. The results show that:nonlinear response of Ⅰ rock site is smaller than Ⅱ soil site, nonlinear response of Ⅲ soil site is the biggest in the three different sites.Overall, H/V spectrum ratio of FAS applies to aftershocks and microtremors; H/V spectrum ratio of VRS applies to mainshock and big PGA aftershocks.(7)Discussing on validity of structural response by H/V ratio method. In the paper, we analysis structural response of a reinforced concrete building based on excitation of microtremors and passing subway trains. We conclude that structural analysis using the ambient seismic noise(microtremors) as excitation input may provide an economical and flexible way to supplement the traditional strong ground motion analysis using only accelerograms in building vulnerability assessment to seismic hazard. |
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