| The site effects have always been a hot topic as it directly affects the severity of the earthquake disaster in the field of earthquake engineering.As one of the site effect,nonlinear site effect also has the attention of scholars both at home and abroad.Many results show that the strain of soil and damping ratio increase but the shear modulus decreases under the effect of strong ground motion,so predominant frequency of site move to low frequency and the site amplification has a significant boost in the long period of time.Therefore,the study of nonlinear site effect is the most important thing for the location of engineering structure and the improvement of seismic theory,especially in calibrating the design response spectrum.At the same time,it is crucial to study the characteristics of nonlinear site effect and threshold of it.China National Strong Motion Observation Network System(CSMNC)recorded a large amount of strong-motion records with high quality during 2007 to 2013 which has provided a good opportunity for us to study.Generally speaking,the paper based on the above theory and data,study the nonlinear site effect of sichuan region,the main work and conclusions are as follows:1.Selected strong motion recordings in the 50 stations according to certain principles and handled all the records with scientific data processing system.Separated the records on the basis of the value of the PGA into different levels.Calculated the average curve of each levels based on the Fourier amplitude H/V spectrum ratio method than identified the predominant frequency to analyze the nonlinear site effect on the next step.2.Reviewed the relationship and correlation between four parameters(Rfp?DNL?ADNL?PNL)and the peak ground acceleration(PGA)on the nonlinear site effect of 50 stations.Determined the nonlinear site effect by stations occurred DNL and estimated the threshold value of parameters when nonlinear site effect appeared.The results showed that for most stations are showing the characteristic of nonlinear under the action of strong motion site and 28 stations appeared nonlinear site effect by identifying the four parameters.This paper gives threshold of nonlinear site effect of PGA is 100 cm/s2,the threshold of ADNL is 0.2,and the threshold of Rfp is 0.75.3.Analyzed parameters of nonlinear from different time periods and identified the predominant frequency of weak shocks using the different period of seismic records of data and microtremor data in 2007 and 2009 combined with the geological conditions.The results showed that the values of parameters recovered gradually over time under the action of same weak vibration.The response of the site in the medium hard soil or the weak earthquake corresponding to the linear seismic response or the transient nonlinear behavior.The predominant frequency can be recovered in a short period.The seismic response of the site in the soft soil site or the action of strong motion is corresponding to the nonlinear effect,and the recovery period of predominant frequency is usually for 1 year.Whether or not the site soil can be recovered to pre earthquake state decided by the geological structure of the site and the level of the ground motion. |
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