Study On Near-fault Design Ground Motion Parameters Of Loess Site

As a special soil,loess is widely distributed in China.The Loess Plateau,as the main area of loess distribution,has a complex geological background and frequent tectonic activity,which causes serious seismic hazards,especially in the near-fault area,and often brings great damage to people’s lives and property safety.Therefore,it is particularly important to reasonably determine the design ground motion parameters in order to improve the seismic resistance of buildings(structures)in loess areas.In this paper,based on the strong motion records in the database of seismic landslide and strong motion records of the Loess Plateau,the relationships of ground motion parameters such as design response spectrum characteristic period,platform value,attenuation index and PGA with magnitude,epicentral distance,fault distance and site type were statistically analyzed,and by comparing with the code values,some meaningful conclusions have been drawn.The main work and achievements of this paper are as follows :(1)Based on the database of seismic landslide and strong motion records of the Loess Plateau,the acceleration time curves with complete waveforms were screened out,and after baseline correction and filtering using Seismo Signal,the response spectra were calculated and calibrated using the Nelder-Mead simplex algorithm to obtain the response spectrum calibration parameters and PGA values for 1,731 strong motion records of 87 earthquakes.(2)Based on the calibration results of the design response spectrum,the relationships of the characteristic period,platform value and attenuation index of the design response spectrum with the magnitude,epicentral distance and site category were statistically analysed.The results show that the characteristic period,platform value and magnitude are positively correlated and the attenuation index is negatively correlated with magnitude for the same category of sites;the relationship between characteristic period,platform value and attenuation index and epicentral distance is more complex.(3)Combining the results of existing studies and empirical formulas for fault distance,the fault distances of 1,251 strong motion records for 24 earthquakes of magnitude M > 4 were determined.A total of 231 strong motion records within a fault distance of 60 km from the loess Class II site were selected to explored the characteristics of the design response spectrum characteristic period,plateau values,attenuation index and the variation of PGA with fault distance.The results show that the characteristic period and platform value of the design response spectrum in the range of 60 km from the fault in the loess Class II site increase with the increase of the magnitude,and the attenuation index decreases with the increase of the magnitude.The relationship between the characteristic period,platform value and attenuation index of the design response spectrum and the fault distance is not obvious.The ratio of vertical and horizontal PGA of strong motion records is larger than the code value,and it gradually increases with the decrease of fault distance and the increase of magnitude.(4)Compared with the values in the current code for seismic design of buildings(GB 50011-2010)(2016 edition),the platform value and attenuation index of the design response spectrum of loess site obtained in this paper are larger than the code value.The characteristic period value of loess Class II site is smaller than the code value;the enhancement coefficient in the range of 10 km near the fault is smaller than the standard value.