Analysis And Research On Site Effect Of Ludian Ms6.5 Earthquake Based On Strong Ground Motion Observation

Ludian M_S6.5 earthquake caused serious seismic damage in 2014.The Longtoushan Town,located near the epicenter,has the most serious and ununiformed damage.Related researches showed that the special seismic source and local ground motion effect are main reasons for serious seismic damage in Longtoushan town.Based on strong ground motion records of main shock,aftershock and site pulsation to analyze site effect.In the process of analysis,the following methods are used,they are described as following.(1)The stochastic finite fault method,which can reflect characteristics of earthquake source.(2)HVSR spectrum ratio method,which can effectively identify dominant period of site.(3)Soil equivalent linearization method for seismic response process of soil layer.For contrast,drawing into Qianchang site to analyze site effect.The main conclusions are as follows:1)Based on strong ground motion records of base rock to inverse main source parameters.The result shows that the seismic stress drop is about 190.87MPa,the moment magnitude is 6.18 and the corner frequency of seismic is 0.6323Hz.Comparing synthetic ground motion and observation records,we can conclude that the source parameters are reliable.In addition,the synthetic ground motion can meet analysis requirements of site effect.2)The spectral ratio results of Longtoushan site show that when main shock occurred,the predominant period was about 1.0s,which corresponds to the 4.45 of the spectrum ratio curve.Moreover,the micro seismic spectral ratio was less than 2.0 and the predominant period was about 0.32s,which is close to test results of borehole shear wave velocity.The DNL parameter of Longtoushan site is 5.02,which is greater than4.0.In summary,we can conclude that when main shock occurred,the significant nonlinear effects occurred in Longtoushan site.Site soil equivalent linear reaction results showed that the amplification of ground motion is significantly,the PGA is up to 1090.9gal,which is higher than the bedrock input value.3)The spectrum ratio results of main shock and aftershock show that the site of Qianchang isn't type I,the predominant period of main shock and aftershock are 0.30s and 0.25s respectively.So,we can conclude that when main shock occurred,the site of Qianchang had a slight nonlinear effect.From the site nonlinear effect degree,seismic response spectrum,seismic attenuation of Longtoushan site and Qianchang site,we can conclude that nonlinear effect results the wide platform of response spectrum.On the contrary,due to the thin soil layer,better soil quality and single soil layer,there is no significant site effect at Qianchang site,even the bedrock ground motion is very strong.4)The seismic damage of Longtoushan town is most serious and the distribution of the seismic damage is extremely ununiformed.The specific performance is as follows:(1)The seismic damage of Laoji town is more serious than Xinji town.When down to slope,the seismic damage is gradually lessen.(2)The PGA of bedrock in Laoji town is between 320 gal and 440 gal,the PGA of borehole 4 and 5 is higher than other boreholes.(3)The results of synthetic ground motion show that the ground motion of Laoji town is higher than Xinji town,which is main reason for different seismic damage.The soil layer of Laoji town is thinner,which is about 20~30m and input motion of bedrock is greatly amplified after the soil layer reaction.The distribution of ground motion is much ununiformed in different periods.The soil layer of Xinji town is very thick,which is more than 100m,and there is only on type of soil.Although the PGA is higher after soil layer reaction in Xinji town,but the long period seismic motion of the site is more significant because of the thicker cover layer.Therefore,there is no serious damage of frame structure.In addition,the building of Xinji town is frame structure,which is seismic designed and in good quality,But in Laoji town,many houses are made in brick with poor seismic performance.Therefore,seismic design is main reason for seismic difference of Xinji town and Laoji town.