Study On Ground Motion Attenuation Relation And Uncertainty Of Aftershocks During Wenchuan Earthquake

On May 12,2008,an earthquake of magnitude 7.9 occurred in Wenchuan,Sichuan Province,China.The earthquake is the most destructive,the most widespread and the most difficult to do disaster relief after Tangshan earthquake.The records of earthquakes and aftershocks were basically completely preserved.These records are recorded by the digital strong-motion instruments which are of high quality and have data of stations and sites.From the statistics of earthquake disaster,it can be seen that the main reason for buildings structural damage is strong ground motion.As an effective estimation method of ground motion parameters,the attenuation relation of ground motion is the key of the effective prediction of seismic field distribution.The existing seis mic design of the norms generally ignore the role of aftershocks,and the uncertainty analysis of the factors affecting the ground motion is relatively small due to the small database of single field records.Based on the data of aftershocks in Wenchuan earthquake,the magnitude of seismic data conversion and site classification processing were done in this paper.Moreover the Wenchuan earthquake source information and site information database was established for analysis.Using the simplified model of Campbell-Bozorgnia in NGA program,the attenuation effect of aftershocks of Wenchuan earthquake is given by random effect algorithm based on Wenchuan medium-strong aftershock ground motion record.Because the of the aftershocks of Wenchuan earthquake is discontinuous and lacks large magnitude,the limit of the attenuation of the attenuation model is small which results in the ground motion attenuation model of discrete degree residuals in the near field larger than in the far field.Meanwhile the discretization of model residuals of large earthquakes is larger than medium and small earthquakes;the attenuation relations given in this paper are only suitable applicable to medium and small aftershocks and the estimated value of the large earthquake is smaller than the actual value.What's more,the ground motion attenuation function obtained in this paper is applied to seismic risk study.And the uncertainty correction was done.It is found that the modified exceedance probability has been greatly i mproved,especially in the low probability segment.Based on the error analysis of single station using NGA model,the uncertainty of attenuation relation is studied,the system errors caused by the source,path and site are analyzed.The results show that removing the systematic error separated from the error can effectively reduce the standard deviation of a single site,especially when a region is recorded to a certain extent.The error of attenuation relationship analysis can be greatly reduced by using single site record.