Study On Site Response And Design Spectra Of Vertical Ground Motion Using KiK-net Seismic Data

Vertical ground motion is considered an important part in the seismic designs of critical structures but neglected or simplified when designing ordinary structures in many current seismic codes worldwide,the corresponding is many researcher’s primary attention to the properties of horizontal ground motion.But there are many evidences confirming that,not only for critical structures but also for ordinary structures,many structural failures are due to the effects of vertical ground motion,indicates the importance of the study of vertical ground motion.In this paper,linear site response of vertical ground motion,nonlinear site response of vertical ground motion,site Poisson’s ratio and vertical design spectra are investigated based on the Ki K-net database,aiming to reveal the mechanism of site effect and the statistic rule of vertical design spectra,which can be utilized as the reference for seismic fortifications and designs of structures.The main works and conclusions are listed as follows:(1)Linear site response of vertical ground motion.Surface-to-borehole spectral ratio technique is firstly utilized to estimate the Fourier amplitude ratios based on the seismic data.Then,natural frequencies and predominant frequencies are extracted.The dispersion of predominant frequency and the distribution of high-order natural frequency are studied based on the estimates.Main results:Predominant frequencies are mainly distributed within the first six order modes,the predominant frequency for vertical ground motion is more focused on the low-order mode.Relationships between the ratios of multiple natural frequencies to fundamental frequency and fundamental frequency can be described by the hyperbolic models,as a whole,the correlation of the higher-order mode is higher than that of the lower-order mode and the correlation for vertical ground motion is higher than that for horizontal ground motion under the same modal order.Moreover,it is found that the high-order modes for horizontal ground motion obtained by horizontal-to-vertical spectral ratio technique could be offset by the low-order modes for vertical ground motion,particularly the second order mode.(2)Nonlinear site response of vertical ground motion.Firstly,time-frequency analysis technique is used to expand the number of strong motion records,seismic interferometry is applied to the seismic data to estimate the seismic wave velocities.Then,the site modulus and strain can be obtained using the dynamic parameter method.Based on the theory of one-dimensional wave propagation and soil mechanics,a new model is proposed to describe the global site constrained modulus degradation curve considering the effects of groundwater and the vertical nonlinearity of site is investigated based on it.Main results:The global constrained modulus degradation curve has a low limit and it is negatively correlated with groundwater level and confining pressure.The horizontal peak ground acceleration threshold for vertical nonlinearity can be as low as about 30 cm/s~2,which is generally larger than that for horizontal nonlinearity.Vertical nonlinearity is restrained by the effects of groundwater and the global constrained modulus decreases slower than the shear modulus,indicates the nonlinearity of the site Poisson’s ratio.(3)Site Poisson’s ratio.Combining the said results and the theory of soil mechanics,a method for describing the variation of the site Poisson’s ratio with shear strain is proposed,and a higher resolution method is also developed for the sites where the groundwater level is below the soil layer combining horizontal-to-vertical spectral ratio technique.Considering the effects of rock layer,an adjusted effective vertical confining pressure is proposed to represent the average confining condition for the whole borehole including rock layer.Main results:Groundwater increases the small-strain site Poisson’s ratio and its influence has a threshold effect.Generally speaking,the small-strain Poisson’s ratio of soil site is larger than that of rock site,the site including the dense and waterproof rock layers has a smaller small-strain Poisson’s ratio compared with that including the porous and perviousness rock layers.Groundwater increases the shear strain threshold of the site Poisson’s ratio.Based on the studies of the effects of confining pressure,groundwater and rock layer type,an empirical model using site conditions for the site assessment of the small-strain Poisson’s ratio is proposed.(4)Vertical design spectra.The algorithm parameter studies and a method comparison for differential evolution and simulated annealing are conducted firstly to determine an appropriate method and its algorithm parameters,it is found that differential evolution is better than simulated annealing whether for accuracy,stability and efficiency.Then,the characteristic parameters of vertical design spectra are estimated using the method optimized in this study,their statistics and properties under different conditions are proposed and investigated,respectively.Main results:The effects of earthquake magnitude on characteristic parameters are more obvious than those of epicentral distance,on the whole,earthquake magnitude is positively correlated with the corner periods and negatively correlated with the amplitudes of the flat portion.There could be a loglog exponential relationship between the corner period of vertical design spectra and that of horizontal design spectra,and the proposed model may be used in other regions.Furthermore,the site factors for vertical design spectra are recommended based on the estimation results and previous research achievements.