A Study On The Rock Site Spectrum And Its H/V Ratio Of Earthquake In Subduction Zone

As the input ground motion for evaluation of site effect in a ground motion prediction equation(GMPE),predicting reasonable and reliable rock site response spectrum is an important part in the GMPE.However,the models for deriving response spectrum and the horizontal to vertical(H/V)spectral ratios at an engineering bedrock site in the subduction zone are not available yet.This study addresses this issue and to develop suitable models based on the work from previous studies.Another important feature for the H/V response spectral ratios is the effect of impedance ratios.It is possible that the effect of impedance ratio is larger for the horizontal components than for the vertical component,leading to that the H/V ratios depend on the site impedance ratios.This study will also investigate this assumption and model the effect of impedance ratios.This study used the results of existing GMPEs for the horizontal components and HN spectral ratios,i.e.,the within-events residuals of site class(SC)I sites that have a site period less than 0.2s.The GMPEs and the H/V models were derived by others in the previous studies.The data used in this study are from KiK-net and K-net strong-motion networks,has a large number of records from four categories of earthquakes,namely,shallow crustal,upper mantle,subduction interface and subduction slab events.These events have reliable source parameters and the recording sites have reliable shear-wave velocity profiles down to the engineering bedrock.The models were derived from the within-events residuals from the SC I sites based on a random effects model that has terms for magnitude,fault-top depth,source distance(anelastic and geometric attenuation rates),site period terms and a constant term representing the earthquake categories.The model can be used to calculate the deamplification ratios for the engineering bedrock sites for the horizontal response spectrum and H/V spectral ratios in a range of various model parameters.In addition,the effect of the impedance ratio on the H/V response spectral ratio is modelled and the model parameters are presented in this study.The residual distribution with respect to each term in a graphic form was used repeatedly to test the regression equation form and the rationality of variable selection.In this study,the within-event residuals were divided into the within-site and between-site components.The within-site residuals are mainly associated with the path effect,and the between-sites residuals are related to the site effects.When the distribution of residuals are unbiased with respect to all terms and the coefficients for these terms are statistically significant,the final models were achieved by minor smoothing to the regression coefficients.The within-event standard deviation is also decomposed into two parts:the within-site and the between-site standard deviations.The magnitude of between-site deviation indicates whether the site effect is well modeled and within-site deviation can be used to gauge whether the path effects are well simulated.Compared with the standard deviations of the GMPEs for the horizontal and vertical components,the standard deviations from the model for the H/V spectral ratios are small.