Empirical Estimate Model For Ground Motion Of Wenchuan Earthquake Zone

On12May2008, an earthquake with Mw7.9occurred in Wenchuancounty, Sichuan province, China, which gave rise to many landslides,structural damage and collapses and resulted in widespread damage and agreat number of casualties. The main cause of those damage is strong groundmotion. The research of strong ground motions is a important part ofEarthquake Engineering. In particular, the research of strong ground motion inWenchuan earthquake is significant to other seismic areas of China.The ground motion is complex of source effect, crustal attenuation andsite condition effect.The long period component of ground motion isdeterministic and is given by representation theorem, which can simulate thelong period wave form very well. However, the precision of green fuction islimited by the heterogeneity and complexity of crust. Meanwhile, the highfrequency component of ground motion is stochastic, because of thecomplexity of source rupture process, the heterogeneity of crust and thevariety of site condition. As a result, empirical model is a more reliablemeasure for the prediction of ground motion.There are two empirical models for the prediction of ground motion,attenuation relationship model and stochastic model. Attenuation relationshipmodel is suitable for the region with sufficient strong motion records, such asEast America, Japan and Taiwan. Because chinese strong motion observationwork just initiate and the strong motion records is sparse, the attenuationrelationship for China are not derived from strong motion records, but fromother data, such as intensity data. Since the ground motion is characteristic ofregion, the reliability of this mothod need to be verified by the strong motionrecords in the future. For the region with limited strong motion data,stochastic model is a better choice. At present, many researcher usestochastic model, which are inverted from the strong motion records in WestAmerica or East Canada, to simulate the ground motion of Chinese earthquake.However, there are many aspects which result in ground motion differencesbetween the earthquake in China and other regions. Firstly, differentgeological structure and type of faulting classification will cause differentsource parameters. Secondly, the difference of crustal thickness andgeological structure will lead to different attenuation function. Thirdly, siteamplification factor is mainly determined by the surface soil, which isstrongly characrictic of region. As a result, the mothod, which use foreignparameter to simulate the ground moiton of earthquake in china, need Chinesestrong motion data to validate and update in the future.This thesis use attenuation relationship model and stochastic modelseparately to fit Wenchuan aftershocks (Ms3.8~6.3) and develop empirical ground motion model for Wnchuan aftershocks. At last, the ground moiton ofWenchuan mainshock are simulated by finite fault stochastic model.NGA Campbell-Bozorgnia model are used to fit Wenchuan aftershockdata, which take Vs30to describe the site classification. However, the drillingdepth of most strong motion stations in Sichuan and Gansu provinces is lessthan20meters. In order to extrapolate the profile of20meters to30meters,the empirical extrapolation model for the estimate of Vs30are derived from theconstruction report of147strong motion stations in Sichuan and Gansuprovinces. Based on the strong motion records of64wenchuan afterhshock,we get a new attenuation relationship, which consideres the site effect.Moreover, the estimate of Vs30can offer facility for the application of foreignmodel to Chinese data.Since the Wenchuan aftershocks are characteristic of concentratedSource location and multiple records at single station, the bias of attenuationrelationship regression can be decomposed into source bias, path bias,sitecondition bias and occasional bias. The results show that, if single path isconsidered, the sigma can be reduced to50%of the corresponding sigma forattenuation relationship regression.Based on the13wenchuan aftershocks, we invert the Q model separatelyfor the hanging wall and footwall of Longmenshan fault, stress dropes of the13afterhocks and site amplification factor for the shallow soil sites anddeep soil sites at Southwest China. Then the point source stochastic model isconstructed. Through the comparison of point source stochastic model andattenuation relationshiop model, it is noted that the point source stochasticmodel can simulate the near-fault ground motion better.Finally, finite fault stochastic model is used to simulate Wenchuanmainshock data. The fit of data show that the empirical model can simulatethe near-fault motion very well. The constructed model can offer artificialground motion for structure engineering and can be used to predict the groundmoion of large magnitude earthquake in Souchwest China.