Analysis On Frequency Content Characteristic And Energy Spectrum Of Near-fault Ground Motions

At present, engineering properties of near-fault ground motions and their structural effects have attracted the close attention of researchers in earthquake engineering all over the world, and it will play a significant role in the development of the new contents and innovational technology of seismic engineering. Based on 150 near-fault ground motion recordings from Chi-Chi earthquake in Taiwan, China and Northridge earthquake in USA, the frequency content characteristic and energy spectrum of near-fault ground motions are analyzed in this paper.Firstly, this paper focuses on the frequency content characteristic analysis of near-fault ground motions with and without distinct pulses separately from Chi-Chi and Northridge earthquake in terms of the mean value, coefficients of variance and correlation analysis. Hilbert-Huang transform is implemented for the ground motions. And ten scalar period parameters of ground motions are calculated including seven smoothed periods and three predominant periods. Meanwhile, two new parameters, namely mean period of Hilbert marginal spectrum (Tmh) and improved characteristic period (Tg) are introduced. Moreover, the effect of source mechanism and velocity pulse on the frequency content parameters and non-stationary degree of near-fault ground motions is explored. Finally, comprehensive comparison and analysis indicate that Tmh, Tg and Tavg (average spectral period) can distinguish the low frequency components of near-fault ground motions, Tm (mean period of Fourier amplitude spectrum) and To (smoothed spectral predominant period) represent the moderate and high frequency components respectively. The variance coefficient of predominant instantaneous frequency of Hilbert spectrum (Hcov) can be regarded as an alternative parameter to measure the non-stationary degree of near-fault ground motions. Additionally, the velocity pulses and source mechanism remarkably affect the frequency parameters of near-fault ground motions.Secondly, this paper examines the influence of characteristics of near-fault ground motions, such as forward directivity effect, fling-step effect and hanging wall effect on the frequency content parameters and nonstationary property of ground motions. Numerical results indicate that frequency content parameters and the variance coefficient of predominant instantaneous frequency of Hilbert spectrum (Hcov) of ground motions at footwall sites are greater than those of ground motions at hanging wall sites, and the long period components ground motions at footwall sites are abundant. The main frequency content parameters of fault-normal components of ground motions approximate those of fault-parallel components. Moreover, the variance coefficients of predominant instantaneous frequency of Hilbert spectrum (Hcov) of normal components of ground motions are smaller than the respective value of parallel components, which means that the non-stationary degree of normal components isless.Finally, for the SDOF systems subjected to an ensemble of 150 near-fault ground motion recordings, the influence of the structural parameters of the SDOF systems and the velocity pulse of near-fault ground motions on the hysteretic energy spectrum and input energy spectrum of ground motions is investigated. The computational results illustrate that the damping, ductility level, period of SDOF system and velocity pulse of ground motions have significant effects on the energy spectra, and the effect of yield stiffness ratio on the hysteretic energy can be neglected. Further, the frequency content characteristics of ground motions and source mechanism are key factors for the form of energy spectra.