Arma models for earthquake ground motions

This paper outlines the use of discrete, autoregressive/moving-average (ARMA) models for identification and estimation of parameters in models derived from analysis of uniformly digitized earthquake ground motion acceleration data. Such models are of equal generality as compared to continuous-time models and have a number of significant advantages for purposes of digital analysis and simulation. The structure of ARMA models is briefly described, their relation to continuous models noted, and results of their application to a number of recorded accelerograms summarized.     

Characteristics of frequency content of near-fault ground motions during the Chi-Chi earthquake

Introduction The dynamic response of structural systems subjected to earthquake ground shaking is sig-nificantly affected by the frequency content of input ground motions. When the frequency content of a predominant earthquake ground motion closely matches the natural period of a structural sys-tem, the dynamic response is significantly enhanced and thus may cause severe damage (Chopra, 1995). Therefore, it is of great importance to evaluate the frequency content of ground motions. In recent …     

Digital simulation of multivariate earthquake ground motions

In this paper a new generation procedure of multivariate earthquake ground motion is presented. The technique takes full advantage of the decomposition of the power spectral density matrix by means of its eigenvectors. The application of the method to multivariate ground accelerations shows some very interesting physical properties which allows one to obtain significant reduction of the computational effort in the generation of sample functions relative to multivariate earthquake ground motion processes. Copyright © 2000 John Wiley & Sons, Ltd.     

Synthetic earthquake ground motions on an array

Linear and especially non-linear analyses of spatially extended structures, such as pipelines and bridges, often requires specification of time histories of ground motion at an array of closely spaced points. As the number of dense accelerograph arrays worldwide is small, and the number of earthquake observations is limited, synthetic motions with desired characteristics become necessary. This paper presents a method for synthesizing such motions, which is an extension of the SYNACC method, developed first in the early 1970s for synthetic accelerations, velocities and displacements at a point, and later extended to synthetic near surface strains, rotations and curvatures of ground motion at a point. It consists of unfolding in time a site specific Fourier amplitude spectrum of ground acceleration, obtained by an empirical scaling model, by representing the ground motion as a superposition of traveling wavelets of Love and Rayleigh waves and body waves, which propagate with phase and group velocities consistent with the dispersion characteristic of the site geology, approximated by parallel layers. Uniform hazard Fourier spectra or any specified target Fourier spectrum can also be used. Derivations of the point strains, rotations and curvatures are also presented. The method is illustrated for scenario M6.5 and M7.5 earthquakes and three dispersion models.     

Near-field horizontal and vertical earthquake ground motions

Strong-motion attenuation relationships are presented for peak ground acceleration, spectral acceleration, energy density, maximum absolute input energy for horizontal and vertical directions and for the ratio of vertical to horizontal of these ground motion parameters. These equations were derived using a worldwide dataset of 186 strong-motion records recorded with 15 km of the surface projection of earthquakes between M_s=5.8 and 7.8. The effect of local site conditions and focal mechanism is included in some of these equations.     

Multifractal characteristic analysis of near-fault earthquake ground motions

This study aims to reveal the multi-scaling behavior and quantify the irregularity of near-fault earthquake ground motions from a new perspective of multifractal theory. Based on multifractal detrended fluctuation analysis, the multifractal characteristic parameters of acceleration time series for typical near-fault ground motions are calculated, and their correlations with two period parameters (i.e., mean period T_m and characteristic period T_c) and box-counting fractal dimensions are analyzed. Numerical results of strong nonlinear dependence of generalized Hurst exponents h(q) upon the fluctuation orders q indicate that near-fault ground motions present the multifractal properties and long-range correlation obviously. Furthermore, the scaling exponent h(2) of near-fault records has a strong correlation with their periods T_m and T_c, and strongly negative correlation with their box dimension. Moreover, h(2) can be regarded as a measure of frequency content and irregularity degree of strong earthquake ground motions. Finally, it is pointed out that the long-range correlation of small and large fluctuation is the major source of multifractality of near-fault ground motions.     

Long-period ground motions from the 2003 Tokachi-oki earthquake

We study characteristics of long-period ground motions from the 2003 Tokachi-oki earthquake (Mj 8.0), a large interplate earthquake, based on spatial distribution maps and attenuation relationships for four kinds of peak ground velocity (PGV) value. The first kind (PGV(WB)) is obtained from a maximal value of vector sum of the three-component, wide-band velocity seismograms, and the other three kinds (PGV(BP10), PGV(BP20), and PGV(BP30)) are obtained from a maximal value of vector sum of the three-component, narrow band-pass filtered velocity seismograms (the central periods are 10, 20, and 30 s). The spatial distribution maps for all kinds of PGV value show azimuth dependence; the PGV values in Hokkaido, northern side of the epicenter are larger than those in Tohoku, southwestern side of the epicenter, when compared at a comparable distance. We find that the features result from the radiation pattern of long-period surface waves, that is, the source effect. The attenuation relationships show the following trends: The PGV(WB) values are larger than the sum of the PGV(BP10), PGV(BP20), and PGV(BP30) at distances (D) less than 200 km, while the PGV(WB) values are comparable to the sum of the PGV(BP20) and PGV(BP30) at D > 200 km. This indicates that the PGV(WB) values at D < 200 km are affected by ground motions with periods less than 10 s, while long-period surface waves mainly contribute to the PGV(WB) values at D > 200 km. The basin site effects generate a patchy pattern in the spatial distribution maps and a large scattering in the attenuation relationships for the PGV(WB) and PGV(BP10) values. Finally, we conclude that the PGV(WB) values from the 2003 Tokachi-oki earthquake are controlled by the radiation pattern of long-period S and surface waves and various basin site effects.     

The inelastic vibration absorber subjected to earthquake ground motions

Two storey bilinear hysteretic structures have been studied with a view to exploring the possibility of using the dynamic vibration absorber concept in earthquake-resistant design. The response of the lower storey has been optimized for the Taft 1952, S69°E accelerogram with reference to parameters such as frequency ratio, yield strength ratio and mass ratio. The influence of viscous damping has also been examined.     

On the significance of phase content in earthquake ground motions

A number of problems relating to characteristics of harmonic phase angles contained in earthquake ground motions are discussed, which have been mostly overlooked in the past in contrast with the importance placed on harmonic amplitudes. Particularly, the significance of the concept of phase differences in certain properties of earthquake ground motions is emphasized. A few applications of this new concept to earthquake engineering problems are illustrated as well.     

Variation of earthquake ground motions within a very small distance

At present, dense strong motion observation networks have been established in Japan. One of the important findings based on these networks is that strong ground motions are quite site-dependent. Characteristics of observed ground motions at nearby stations can exhibit a significant variation, even when the stations are within several hundreds of meters. These observations raise one important question; if characteristics of strong ground motions exhibit large variations even for smaller regions, we should be concerned about the application of observed or predicted ground motions for the assessment of structures. In particular, if ground-motion parameters such as PGA, PGV, Spectral Intensity, etc., exhibit large variations for smaller regions, their use for the seismic design and practice will be subject to restriction. In other words, the evaluation of variation of these parameters is an important issue. From such a point of view, the authors investigated the variation of observed ground motions within a very small distance in this study. First of all, fifteen couples of adjacent strong motion stations in Japan, where the distance is within 100 m, were listed up based on our field reconnaissance. Then, microtremor measurements were carried out at each of the station pairs. Next, variation of recorded earthquake ground motions for the station pairs was examined based on various ground motion parameters and response spectra. Moreover, we investigated the key factor which is affecting the variation of observed ground motions.     

地下水位变化对地震地面运动的影响

本文以Biot提出的流体饱和多孔介质波动理论为基础, 建立了成层地基模型, 把地下水位以下的饱和土层用水饱和多孔介质模拟, 地下水位以上土层用气饱和多孔介质模拟. 通过研究入射平面简谐波在成层地基中的传播, 分析了地下水位变化对地震地面运动的影响. 结果表明: P波入射下, 当土体骨架相对刚度较小时, 地下水位变化对地表位移尤其是竖向地表位移幅值影响较大, 随着地下水位的下降, 竖向位移逐渐增加, 相对应的峰值频率逐渐减小; 当土体骨架相对刚度较大时, 地下水位变化对地面运动影响不大.      

Simulation of spatially correlated earthquake ground motions for engineering purposes

A new model to simulate spatially correlated earthquake ground motions is developed. In the model, the main factors that characterize three distinct effects of spatial variability, namely, the incoherency effect, the wave-passage effect and the site-response effect, are taken into account, and corresponding terms/parameters are incorporated into the well known model of uniform ground motions. Some of these terms/parameters can be determined by the root operation, and others can be calculated directly. The proposed model is fi rst verif ied theoretically, and examples of ground motion simulations are provided as a further illustration. It is proven that the ensemble expected value and the ensemble auto-/cross-spectral density functions of the simulated ground motions are identical to the target spectral density functions. The proposed model can also be used to simulate other correlated stochastic processes, such as wave and wind loads.     

地震面波产生的地震动转动分量研究

本文利用弹性波动理论对地面转动分量，即瑞利（Rayleigh)波和乐夫（Love)波产生的转动分量进行了研究，给出了相应的计算公式和计算方法，特别注意到面波的散射效应对转动分量的影响，并将这一特性引入到转动分量的求取中，使问题的解决更切合于实际，最后选取实际地震记录，利用得到的公式计算出地震面波产生的转动分量。     

Site dependence of far-source ground motions during the Wenchuan earthquake

This paper aimed to examine the site dependence and evaluate the methods for site analysis of far-source ground motions. This was achieved through the examination of frequency content estimated by different methods based on strong ground motions recorded at twelve far-source stations in Shandong province during the Wenchuan earthquake. The stations were located in sites with soil profiles ranging from code classes Ⅰ to Ⅲ. Approaches used included the Fourier amplitude spectrum (FAS), the earthquake response spectrum (ERS), the spectral ratio between the horizontal and the vertical components (H/V), the spectral ratio between the spectra at the site and at a reference site (SRRS), and coda wave analysis (CWA). Results showed that major periods of these ground motions obtained by FAS, ERS and H/V ratio methods were all evidently larger than site dominant periods; the periods were also different from each other and mainly reflected the frequency content of long period components. Prominent periods obtained by the SRRS approach neither illuminated the long period aspect nor efficiently determined site features of the motions. The CWA resulted in a period close to site period for stations with good quality recordings. The results obtained in this study will be useful for the evaluation of far-source effect in constructing seismic design spectra and in selecting methods for ground motion site analysis.     

利用高频GPS技术进行强震地面运动监测的研究进展

回顾了高频(1 Hz) GPS在地学应用中的研究进展和主要的数据处理方法,全面比较了高频GPS与地震仪的工作原理与性能,认为高频GPS可为后者进行有效的补充;分析了高频GPS应用于监测短周期强震地面运动的技术优势,以及在处理方法和实际的地学应用中存在的不足;通过对高频GPS的应用领域与发展前景的研究,对我国开展高频GPS数据的处理方法与地学应用研究提出了自己的观点和看法.     

基于EMD和VARMA模型的地震动仿真

通过EMD方法将地震动分解成若干固有模态函数,提出了用固有模态函数的时变VARMA建模实现地震动仿真的思路。算例分析表明,该方法充分利用了固有模态函数的特性,解决了直接基于ARMA或VARMA模型建模的仿真方法所面临的模型判阶的难题,并可同时考虑地震动的强度和频率非平稳特性,使仿真地震动与实际地震动在能量时频分布特性上具有较好的一致性且样本统计性较好,弹性及弹塑性反应谱拟合精度较高。     

Cumulative demand of the earthquake ground motions in the near source

In this paper the effects of near‐fault earthquakes are investigated through the study of parameters related to both input energy and plastic cycles demand. Based on the results of two different regression analyses performed with 128 near‐fault and 122 far‐field registrations, respectively, the Cosenza and Manfredi adimensional Id index was found to be the best seismological parameter correlated to the number n and amplitude m of plastic cycles in the vicinity of the source as well as far from it. Analytical formulations for n,m and for the equivalent number of plastic cycles neq are proposed for near‐fault and far‐field regions. The study of Id and neq shows the tendency of such parameters to increase as the distance increases. Copyright © 2003 John Wiley & Sons, Ltd.