Study On Site Effect In The Wenchuan Earthquake Using Strong-motion Recordings

The site effect has been a polular topic in the field of earthquake engineering for along time. Much effort has been made in studis on site effect by means of numericalsimulation and theoretical computation. However, some methods based onstrong-motion recordings are recently recognized and widely used, due to theirsimplicity and no need of many assumptions any more. In the2008Wenchuanearthquake, the China National Strong Motion Observation Network System(NSMONS) captured a large number of strong-motion recordings at free field both inthe main shock and aftershocks. This makes a good chance for studying the site effectin the Wenchuan earthquake. In this paper, based on a database of these recordings, ittakes a systematical study on the site classification, generalized inversion of siteresponse and identification of nonlinearity of site response. That manages to extend theapplication of strong-motion recordings in the field of scientific research andengineering practice, and promote the development of strong-motion observation inChina. The contents are summarized as follows:(1) The merits and drawbacks of three common-used HVSR (Horizontal-to-Vertical Spectral Ratio) methods of site classification are summarized. Then animproved HVSR method using entropy weight theory is proposed. The site classes of54permanent stations and66temporary stations in the Wenchuan earthquake areclassified by using these four methods. The comparisons between the results given byeach method shows that the new method has a more scientific rationality.(2) An updated classification criterion for six site classes is proposed and used inthe process of site classification of temporary strong-motion stations in the Wenchuanearthquake. The results comfirms that it can raise the classification accuracy andeffectively solve the problem that HVSR method can not be used for the sites whereHVSR curves appear multiple peaks or no any peak. Finally, a preliminary analysis ofthe correlation between station elevation and its natural period TGidentified by HVSRindicates that TGis generally smaller at higher elevations in Sichuan area.(3) A total of602strong-motion recordings obtained by28stations from96aftershocks of the Wenchuan earthquake are selected as a dataset. The source, path andsite effect of these recordings are separated by the generalized inversion technique(GIT). The inversion results have been verified as reliable by comparing the siteresponse at station62WUD using the GIT method and the the standard spectral ratio(SSR) method. For all28stations, the site predominant frequency Fpand the average site amplification in different frequency bands of1.0-5.0Hz,5.0-10.0Hz and1.0-10.0Hz are calculated based on the inversion results. Compared with the results from theHVSR method, it shows that the HVSR method can reasonably estimate the sitepredominant frequency but underestimates the site amplification. The linear fittingbetween the average site amplification for each frequency band and the Vs20(theequivalent uppermost-20m shear wave velocity) shows a good correlation.(4) A distance measurement called the asperity distance DAspis proposed toreasonably characterize the source-to-site distance for large earthquakes such as theWenchuan earthquake, and is verified to be much better than the other three kinds ofdistance measurements, including the rupture distance (DRup), fault distance (DFau) andhypocentral distance (DHyp). Then it’s used for generalized inversion of site responses inthe main shock of Wenchuan earthquake.(5) The S transform is suggested to be used for identifying the nonlinearity of siteseismic response. Combined with GIT method and HVSR method, it is practiced toidentify the site responses of33strong-motion stations behave linearity or nonlinearityin the main shock of Wenchuan earthquake. The result shows that the site responseappears obvious nonlinearity when PGA larger than300cm/s2or PGV larger than20cm/s. In this regard, the sites of station051AXT,051GYS,051GYZ,051JYC,051JYD,051JYH,051JZW,051MXN,051SFB and062WUD also behave significant nonlinearresponses. Compared with the results given by GIT method and HVSR method in thefrequency domian, it is verified that the S transform can effectively identify thenonlinerity behavior of site seismic response in the time domain, and have an ablitity todetermine the particular time when the nonlinearity occurs.(6) The correlations between the ratio of Fpunder weak motions and strong motion(RFp) and ground motion levels such as PGA and PGV, and site coefficients such as Vs20and Vs30are analyzed respectively. The results show RFphas a strongly positivecorrelation with PGA and PGV, but is uncorrelated with Vs20and Vs30. A new definitioncalled ADNL (absolute degree of nonlinearity) is proposed to characrize the degree ofnonlinearity of site seismic response. Then the ADNL of each site and other parameterssuch as fNL (frequency of nonlinerity), DNL (degree of nonlinearity) and PNL(percentage of nonlinearity) are calculated. The empirical regression analysis betweenthese parameters and PGA represents that each one has a strong correlation with PGA.