PERIOD-DEPENDENT NONLINEAR CHARACTERISTICS OF SURFACE/BOREHOLE SPECTRAL ACCELERATION RATIOS
Weak overburden layers at shallow surfaces would substantially affect the ground motion characteristic parameters of ground motion,such as spectral accelerations,and these influences regularly showed obvious nonlinearity with the increase of ground motion strength.Despite the differences between motions down the borehole and ground motions on bedrock,the surface/borehole spectral ratios could reflect the seismic site effect to some extent and overcome the shortcomings of the lack of reference bedrock stations in the standard spectral ratio method and the assumption of seismic point source in the generalized inversion method.In recent years,more and more scholars have studied the nonlinear dynamic characteristics of the soil body and its change trend based on the strong motion data by comparing the surface/borehole spectral acceleration ratios under different motion strengths.However,the research conclusions,especially the quantitative characterization of the nonlinear characteristics,still need to be improved due to the insufficient statistical sample size.It was found that the surface/borehole spectral acceleration ratio curves at the same station exhibited high similarity under different motion strengths.However,these spectral acceleration ratios significantly decreased as motion strength increased and shifted towards longer periods at certain period ranges.The surface/borehole spectral acceleration ratios at different motion strengths corresponding to the same period point cannot adequately reflect their nonlinear characteristics.It is more reasonable to characterize these features by the corresponding relationships between the points on the surface/borehole spectral acceleration ratio curve shapes at different motion intensities.Although some scholars have proposed methods for correcting the dominant frequency offset,there remains a lack of effective methods for fixing the period shift of the surface/borehole spectral acceleration ratio across the wholeentire period range,as well as for the quantitative characterization of nonlinear characteristic parameters throughout the wholeentire period range.Dynamic Time Warping(DTW)algorithm is a nonlinear alignment method that combines time alignment and measurement matching.The core idea of the Dynamic Time Warping(DTW)algorithm is to find the optimal mapping between two-time series by calculating the distance between their respective discrete points and identifying the best path connecting them.This process enables point-to-point matching between the two-time series,and the DTW distance(sum of distances along the best path)can be utilized to assess their similarity.In this study,we selected 166 700 strong-motion records from 180 stations in Japan's KiK-net network them.We grouped them by ground motion intensity to obtain the surface/borehole spectral acceleration ratios and their average values for each station under different intensity levels.A dynamic time warping algorithm was employed to effectively correct long-period shifts in the spectral acceleration ratio curves,which allowed for the precise extraction of both the period shift and amplitude attenuation values at various period points for each seismic station.Subsequently,a detailed statistical analysis was conducted to assess how these values varied about different levels of ground motion intensity.In addition,nonlinear characteristic parameter curves,dependent on both the period and seismic intensity changes,were derived for different site categories,accompanied by the establishment of corresponding empirical relationships.Furthermore,a novel method was proposed to predict the surface-to-borehole spectral ratio under conditions of strong seismic motion,utilizing the spectral acceleration ratio data obtained from weaker ground motion intensity scenarios.This approach is intended to offer more precise and detailed data support for adjusting nonlinear site effects in China's seismic design codes and seismic hazard zoning maps.Ultimately,the goal is to provide a more refined and comprehensive basis for enhancing the nonlinear site response adjustments in China's seismic design specifications and seismic motion parameter zoning maps.The quantitative analysis of period offset characteristics of the surface/borehole spectral acceleration ratios indicated that the period offsets were more significant under larger motion strength.Meanwhile the period offsets at different periods were not consistent,and the relative period offsets at the same period exhibited linear relationships with motion strength under the double-logarithmic coordinates.The relative period offsets could be reliably expressed by the period offset coefficient defined empirically.The period offset coefficients obtained at each station and on each site class(Ⅰ,Ⅱ,Ⅲ,and Ⅳ)of Chinese standards were all related to the period,which showed a trend of initially increasing and subsequently decreasing with the increase of the period.Furthermore,the period offset coefficients increased with the thickness and softness of the overburden layers,which indicated more significant period offsets on sites of thick and soft overburden layers.The quantitative analysis of the amplitude decay characteristics of the surface/borehole spectral acceleration ratios indicated that the spectral ratio amplitude was lower under more considerable motion strength.Meanwhile,the amplitude rates at different periods were not consistent,and the spectral ratio amplitude at the same period exhibited fine linear relationships with motion strength under the semi-logarithmic coordinates.The amplitude decay rate could be reliably expressed by the amplitude decay coefficient defined empirically.The amplitude decay coefficients obtained at each station and on each site class(Ⅰ,Ⅱ,Ⅲ,and Ⅳ)of Chinese standards were all related to the period.They approximately conformed to a Gaussian function under the semi-logarithmic coordinates,indicating that the amplitude decay rates were lower in the middle period range(the platform segment of spectral accelerations)compared to the shorter or longer period ranges.Moreover,the amplitude decay coefficients(＜0)decreased with the thickness and softness of the overburden layers,which indicated faster amplitude decay rates on sites of thick and soft overburden layers.Despite the differences between surface/borehole spectral acceleration ratios and soil surface/bedrock surface spectral acceleration ratios,the period offset coefficients and amplitude decay coefficients derived statistically could reflect the nonlinear characteristics of seismic site effects to some extent.According to the empirical relationships for relative period offsets and amplitude decay rates of surface/borehole spectral acceleration ratios on different site classes(Ⅰ,Ⅱ,Ⅲ and Ⅳ)of Chinese standards,the surface/borehole spectral acceleration ratios under stronger motions could be predicted reliably by surface/borehole spectral acceleration ratios under weaker motion.
ground motionengineering geological conditionlocal site effectsurface/borehole spectral acceleration ratiodynamic time warping algorithm
万方数据
维普
