复杂场地条件下震源参数对断层附近长周期地震动的影响

基于显式有限元方法和运动学震源模型并利用昆明盆地三维地下构造模型,本文研究了震源参数对断层附近长周期地震动的影响.结果表明,断层的破裂方式、埋藏深度、破裂速度以及断层面上位错的不均匀分布对断层附近长周期地震动有重要影响.不同破裂方式下,破裂的方向性强的区域分布不同,由于破裂的方向性效应和复杂场地条件的共同作用,导致不同破裂方式的断层附近地震动分布差别很大.随着破裂速度的增加,方向性效应更加明显,断层附近的长周期地震动也随之增大;对于浅源地震,随着断层埋深的增加,地震动明显下降.对于埋藏深度很浅的断层,当Asperity靠近断层上沿时,会显著增大其在地表投影附近的长周期地震动.能否合理地估计这些基本震源参数,是预测未来发震断层周围地震动场的关键.     

Spatial variability effects of ground motions on cable-stayed bridges

In this paper, stochastic analysis of a cable-stayed bridge subjected to spatially varying ground motions is performed. While the ground motion is described by power spectral density (PSD) function, the spatial variability of ground motions is taken into account with the incoherence and the wave-passage effects. The incoherence effect is examined by taking into account two extensively used models. As the effect of the wave-passage effect is investigated by using various wave velocities, the effect of local soil conditions where the bridge supports are constructed is outlined by using homogeneous firm, medium and soft soil conditions. Solutions obtained for the spatially varying ground motions are compared with those of the specialised cases of the ground motion model. Stationary as well as the transient response analyses are performed for the considered bridge model. It is concluded that spatial variability and propagation effects of ground motions have important effects on the dynamic behaviour of the bridge and the variability of the ground motions should be included in the stochastic analysis of cable-stayed bridges.     

Near-fault ground motion bi-normalized pseudo-velocity spectra and its applications

Ground motions with forward-directivity effect in the near-fault region are obviously different from ordinary far-field ground motions. Design spectral models for this kind of motions have been proposed by correlating sim-ple pulses with parameters attenuation relationships in a previous study of the authors. To further test the applica-bility of the established design spectral model, we analyze ground motion pseudo-velocity response spectra (PVS), normalized pseudo-velocity spectra (NPVS) and bi-normalized pseudo-velocity spectra (BNPVS) of 53 typical near-fault forward-directivity ground motions. It is found that BNPVS not only has more salient features to reflect the difference between soil and rock sites, but also has less scattering to reveal the nature of forward-directivity motions. And then, BNPVS is used for prediction of design spectra accounting for the influence of site conditions, and the constructed design spectra are compared with those spectra established previously. It is concluded that site condition can heavily affect ground motions, buildings on rock can be even more dangerous than those on soil sites, in particular for ordinary buildings with short to middle vibration periods. Finally, pulse models are also suggested for structural analyses in the near-fault region.     

A stochastic model and synthesis for near‐fault impulsive ground motions

The orientations of ground motions are paramount when the pulse‐like motions and their unfavorable seismic responses are considered. This paper addresses the stochastic modeling and synthesizing of near‐fault impulsive ground motions with forward directivity effect taking the orientation of the strongest pulses into account. First, a statistical parametric analysis of velocity time histories in the orientation of the strongest pulse with a specified magnitude and various fault distances is performed. A new stochastic model is established consisting of a velocity pulse model with random parameters and a stochastic approach to synthesize high‐frequency velocity time history. The high‐frequency velocity history is achieved by integrating a stochastic high‐frequency accelerogram, which is generated via the modified K‐T spectrum of residual acceleration histories and then modulated by the specific envelope function. Next, the associated parameters of pulse model, envelope function, and power spectral density are estimated by the least‐square fitting. Some chosen parameters in the stochastic model of near‐fault motions based on correlation analysis are regarded as random variables, which are validated to follow the normal or lognormal distribution. Moreover, the number theoretical method is suggested to select efficiently representative points, for generating artificial near‐fault impulsive ground motions with the feature of the strongest pulse, which can be used to the seismic response and reliability analysis of critical structures conveniently. Finally, the simulated ground motions demonstrate that the synthetic ground motions generated by the proposed stochastic model can represent the impulsive characteristic of near‐fault ground motions. Copyright © 2014 John Wiley & Sons, Ltd.     

一种改进的近断层脉冲型地震动模拟方法

本文基于小波包技术的随机地震动模拟方法,提出一种改进的参数化随机近断层脉冲型地震动模拟方法。然后,通过识别和提取近断层脉冲型地震动数据库中脉冲型地震动的特征参数,建立了基于震源、传播路径和场地特征等参数的脉冲模型参数预测方程。最后,通过模拟实际记录和误差分析检验了改进的模拟方法的有效性。结果表明:应用改进的模拟方法得到的地震动时程无论在波形、频率特性还是峰值上均与实际记录具有较好的一致性。改进的模拟方法在保留地震动时频非平稳性的基础上,能够有效地提高近断层脉冲型地震动的模拟效果,并且能够很好地体现脉冲型地震动的主要特征。     

Simulation of synthetic ground motions for specified earthquake and site characteristics

A method for generating a suite of synthetic ground motion time‐histories for specified earthquake and site characteristics defining a design scenario is presented. The method employs a parameterized stochastic model that is based on a modulated, filtered white‐noise process. The model parameters characterize the evolving intensity, predominant frequency, and bandwidth of the acceleration time‐history, and can be identified by matching the statistics of the model to the statistics of a target‐recorded accelerogram. Sample ‘observations’ of the parameters are obtained by fitting the model to a subset of the NGA database for far‐field strong ground motion records on firm ground. Using this sample, predictive equations are developed for the model parameters in terms of the faulting mechanism, earthquake magnitude, source‐to‐site distance, and the site shear‐wave velocity. For any specified set of these earthquake and site characteristics, sets of the model parameters are generated, which are in turn used in the stochastic model to generate the ensemble of synthetic ground motions. The resulting synthetic acceleration as well as corresponding velocity and displacement time‐histories capture the main features of real earthquake ground motions, including the intensity, duration, spectral content, and peak values. Furthermore, the statistics of their resulting elastic response spectra closely agree with both the median and the variability of response spectra of recorded ground motions, as reflected in the existing prediction equations based on the NGA database. The proposed method can be used in seismic design and analysis in conjunction with or instead of recorded ground motions. Copyright © 2010 John Wiley & Sons, Ltd.     

Simulation of orthogonal horizontal components of near‐fault ground motion for specified earthquake source and site characteristics

A procedure to generate horizontal pairs of synthetic near‐fault ground motion components for specified earthquake source and site characteristics is presented. Some near‐fault ground motions contain a forward directivity pulse; others do not, even when the conditions for such a pulse are favorable. The proposed procedure generates pulse‐like and non‐pulse‐like motions in appropriate proportions. We use our recent stochastic models of pulse‐like and non‐pulse‐like near‐fault ground motions that are formulated in terms of physically meaningful parameters. The parameters of these models are fitted to databases of recorded pulse‐like and non‐pulse‐like motions. Using these empirical “observations,” predictive relations are developed for the model parameters in terms of the earthquake source and site characteristics (type of faulting, earthquake magnitude, depth to top of rupture plane, source‐to‐site distance, site characteristics, and directivity parameters). The correlation coefficients between the model parameters are also estimated. For a given earthquake scenario, the probability of occurrence of a directivity pulse is first computed; pulse‐like and non‐pulse‐like motions are then simulated according to the predicted proportions using the empirical predictive models. The resulting time series are realistic and reproduce important features of recorded near‐fault ground motions, including the natural variability. Moreover, the statistics of their elastic response spectra agree with those of the NGA‐West2 dataset, with the additional feature of distinguishing between pulse‐like and non‐pulse‐like cases and between forward and backward directivity scenarios. The synthetic motions can be used in addition to or in place of recorded motions in performance‐based earthquake engineering, particularly when recorded motions are scarce.     

The effect of spatially varying earthquake ground motions on the stochastic response of bridges isolated with friction pendulum systems

In this paper, a comprehensive investigation of the effect of spatially varying earthquake ground motions on the stochastic response of bridges isolated with friction pendulum systems is performed. The spatially varying earthquake ground motions are considered with incoherence, wave-passage and site-response effects. The importance of the site-response effect, which arises from the difference in the local soil conditions at different support points of the isolated bridge, is investigated particularly. Mean of maximum and variance response values obtained from the spatially varying earthquake ground motions are compared with those of the specialised cases of the ground motion model. It is shown that site-response component of the spatially varying earthquake ground motion model has important effects on the stochastic response of the isolated bridges. Therefore, to be more realistic in calculating the isolated bridge responses, the spatially varying earthquake ground motions should be incorporated in the analysis.     

A new approach of selecting real input ground motions for seismic design: The most unfavourable real seismic design ground motions

This paper presents a new way of selecting real input ground motions for seismic design and analysis of structures based on a comprehensive method for estimating the damage potential of ground motions, which takes into consideration of various ground motion parameters and structural seismic damage criteria in terms of strength, deformation, hysteretic energy and dual damage of Park & Ang damage index. The proposed comprehensive method fully involves the effects of the intensity, frequency content and duration of ground motions and the dynamic characteristics of structures. Then, the concept of the most unfavourable real seismic design ground motion is introduced. Based on the concept, the most unfavourable real seismic design ground motions for rock, stiff soil, medium soil and soft soil site conditions are selected in terms of three typical period ranges of structures. The selected real strong motion records are suitable for seismic analysis of important structures whose failure or collapse will be avoided at a higher level of confidence during the strong earthquake, as they can cause the greatest damage to structures and thereby result in the highest damage potential from an extended real ground motion database for a given site. In addition, this paper also presents the real input design ground motions with medium damage potential, which can be used for the seismic analysis of structures located at the area with low and moderate seismicity. The most unfavourable real seismic design ground motions are verified by analysing the seismic response of structures. It is concluded that the most unfavourable real seismic design ground motion approach can select the real ground motions that can result in the highest damage potential for a given structure and site condition, and the real ground motions can be mainly used for structures whose failure or collapse will be avoided at a higher level of confidence during the strong earthquake. Copyright © 2007 John Wiley & Sons, Ltd.     

利用BP神经网络技术建立自贡地区地震动地形效应模型及其在汶川地震中的效验

地形效应是地震学、地震工程学和工程地震学中非常重要的研究课题.地震震后调查发现: 地形起伏变化引起了更严重的地震灾害, 地震仪器也记录到复杂的三维地形对地震动具有放大效应.关于地形激发的地震动效应问题, 目前仍处于定性研究阶段, 得到了一些定性的结论: 如地形效应表现为与地形坡度相关、与山脊和山谷相关、与山形坡面形态相关、与山体高宽比相关、与地形变化的尺度相关等等.这些定性结论没能给出影响地形效应的主要因素, 如何预测地形效应仍是难题.本文通过谱元数值模拟方法, 计算了自贡地区的地震动地形效应的放大系数, 同时引入BP神经网络技术, 揭示了地形放大系数可以用不同尺度地形上的变化来表述, 即可以用地形高程、地形变化梯度及其高阶变化梯度来表示.据此为参量, 文中建立了地形效应BP神经网络的预测模型, 并将该预测模型的结果与自贡地形台阵在汶川地震中的主震记录获得的地形放大系数结果进行了对比, 给出了预测结果与实际结果的差异范围, 讨论了造成差异的成因, 该论文提出的研究方法为更全面的研究地形效应提供了重要的研究思路.

     

用随机模拟方法研究设定地震地面运动

用随机模拟方法研究设定地震地面运动可分两步进行：（1）采用中小地震的数字观测资料确定研究区路径、场地参数;（2）把确定的参数应用到研究区的强地面运动随机模拟中. 为此,本文首先采用了一种可行的方法,分离软基岩场地效应和非弹性衰减的影响,并把二者分别确定出来. 研究区北部中软基岩场地的平均效应在2～4Hz频率范围为15倍左右;研究区的S波品质因子为QS＝278f[KG*2]0.346. 把这些参数用于研究区的场地和路径模型中,并选择单拐角频率震源谱模型,随机模拟了研究区未来中强地震可能在北天山中段可能造成的地面运动,模拟加速度时程和反应谱可以服务于本地区的地震灾害预测和建筑物可靠性验算.     

Ground Motion Modeling for Seismic Hazard Analysis in the Near-source Regime: An Asperity Model

—A new, yet simple, method using the asperity model to estimate ground motion in the near-source regime for probabilistic seismic hazard analyses is proposed in this study. This near-source model differs from conventional empirical attenuation equations. It correlates peak ground motions with the local contributing source in terms of the static stress drop released non-uniformly on the causative fault plane rather than with the whole seismic source in terms of magnitude. Here the model is simplified such that ground motions at a rock or firm soil site near extended vertical strike-slip faults are dominated by direct shear waves. The proposed model is tested by comparing its predictions with strong ground motion observations from the 1979 Imperial Valley and the 1984 Morgan Hill earthquakes. The results have revealed that ground motions in the near-source region can be adequately predicted using the asperity model with appropriate calibration factors. The directivity effect of ground motion in the near-source region is negligible for high-frequency accelerations. The cut-off frequency (?_max?) at a site is an important parameter in the near-source region. Higher values of ?_max yield higher estimates of peak ground accelerations. For high-frequency structures, ?_max should be carefully estimated. In the near- source region both non-uniform and uniform source models can produce non-stationary high-frequency ground motions. Peak motions may not be caused by the nearest sections of the fault (even if the uniform source model is considered).     

脉冲型地震动潜在破坏势参数的相关性分析

为了在众多参数中挑选其中最有代表性的参数,来解释和反映脉冲型地震动对结构的潜在破坏能力,以338条脉冲型地震动记录作为研究对象,分析地震动参数与中低层结构响应的相关性。选取了14个常用地震动参数,对各地震动参数之间的相关性进行分析,从中选出7个代表性地震动参数;并将脉冲型地震动输入中低层结构模型中计算结构响应,分析代表性地震动参数与结构响应的相关性,与基于非脉冲型地震动的相关性计算结果进行对比。选用了3层和7层2个RC框架结构作为中低层结构代表,其基本周期为0.62s和0.89s。结果表明:对于脉冲型地震动,对于3层结构时与结构响应相关性最好的为EPV,对于7层结构时与结构响应相关性最好的为PGV,因此可以用PGV和EPV作为表征脉冲型地震动对中低层结构潜在破坏能力的参数;而对于非脉冲型地震动,与结构响应相关性最好的参数为PGV,可以用PGV作为表征脉冲型地震动对中低层结构的潜在破坏能力的参数。因此,通过地震动参数来解释和表征脉冲型地震动对结构的破坏能力是可行的。     

近断层地震动的基本特征

本文对近断层地震动的基本特征作了详细分析,这些基本特征主要包括近断层强地震动的集中性、地表破裂、地面永久变形、破裂的方向性效应、近断层速度大脉冲和上盘效应。这些特征虽然在一次地震中不一定同时出现,但它们是已经被强震观测资料和数值模拟证实了的,在模拟和预测近断层地震动时,必须充分考虑这些特点,合理的近断层地震动模拟或预测模型和方法应当在结果中再现这些特点。     

风化半圆形河谷对柱面SH波的散射解析解

风化河谷地震效应对建在此类场地上工程结构(如大坝、桥梁)的动力稳定性具有潜在的威胁,为揭示河谷风化层对地面运动的影响,利用波函数展开法推导风化半圆形河谷对线源柱面SH波散射问题的解析解。计算不同震源位置条件下风化半圆形河谷的地表位移反应和地面运动放大因子,分析地面运动的幅值和形态,发现随着震源距离的增大,在河谷附近地震动的放大和衰减的交替更频繁。     

地形效应影响下地震动参数与斜坡稳定性的相关性研究

地形对地震动的影响比较复杂, 考虑地形放大效应的地震滑坡稳定性分析需要选择合适的地震动参数. 本文使用自贡地形影响台阵记录到的2008年汶川M_S8.0地震主震加速度记录, 分析了地震动峰值加速度、 阿里亚斯烈度以及90%能量持时随地形高度的变化, 探讨了地形效应作用下峰值加速度和阿里亚斯烈度与地震动作用下斜坡稳定性的相关性. 结果表明: ① 地形场地对峰值加速度和阿里亚斯烈度均有显著的放大效应. 地形放大效应较为复杂, 其整体上随台站高度的增加而增大, 水平向的放大效应大于竖直向. 水平向峰值加速度的放大系数为1.1—1.8, 阿里亚斯烈度的放大系数为1.2—3.3; 竖直向相应放大系数分别为1.1—1.3和1.2—1.7. ② 地形对地震动持时也有一定的放大效应, 但不同高度、 不同分量的放大效应没有显著差异, 其放大系数均约为1.3. ③ 阿里亚斯烈度和峰值加速度均能很好地表征地形对地震动的影响, 与地震动对斜坡稳定性的影响具有很强的相关性. 与峰值加速度相比, 阿里亚斯烈度综合了地震动的多方面特征, 可以更好地表征地形对地震动的影响, 与地震动作用下斜坡稳定性的相关性更强.      

Modeling of spatially correlated,site-reflected,and nonstationary ground motions compatible with response spectrum

Seismic risk analysis and mitigation of spatially extended structures require the synthesis of spatially varying ground motions in the response history analysis of these structures. These synthetic motions are usually desired to be spatially correlated, site reflected, nonstationary, and compatible with target design response spectra. In this paper, a method is presented for simulating spatially varying ground motions considering the nonstationarity, local site effects, and compatibility of response spectra. The scheme for generating spatially varying and response spectra compatible ground motions is first established for spatial locations on the ground surface with varying site conditions. The design response spectrum is introduced as the “power” spectrum at the base rock. The site amplification approach is then derived based on the deterministic wave propagation theory, by assuming that the base rock motions consist of out-of-plane SH wave or in-plane combined P and SV waves propagating into the site with assumed incident angles, from which tri-directional spatial ground motions can be generated. The phase difference spectrum is employed to model ground motions exhibiting nonstationarity in both frequency and time domains with different site conditions. The proposed scheme is demonstrated with numerical examples.     

Effects of near-fault and far-fault ground motions on nonlinear dynamic response and seismic damage of concrete gravity dams

As the forward directivity and fling effect characteristics of the near-fault ground motions, seismic response of structures in the near field of a rupturing fault can be significantly different from those observed in the far field. The unique characteristics of the near-fault ground motions can cause considerable damage during an earthquake. This paper presents results of a study aimed at evaluating the near-fault and far-fault ground motion effects on nonlinear dynamic response and seismic damage of concrete gravity dams including dam-reservoir-foundation interaction. For this purpose, 10 as-recorded earthquake records which display ground motions with an apparent velocity pulse are selected to represent the near-fault ground motion characteristics. The earthquake ground motions recorded at the same site from other events that the epicenter far away from the site are employed as the far-fault ground motions. The Koyna gravity dam, which is selected as a numerical application, is subjected to a set of as-recorded near-fault and far-fault strong ground motion records. The Concrete Damaged Plasticity (CDP) model including the strain hardening or softening behavior is employed in nonlinear analysis. Nonlinear dynamic response and seismic damage analyses of the selected concrete dam subjected to both near-fault and far-fault ground motions are performed. Both local and global damage indices are established as the response parameters. The results obtained from the analyses of the dam subjected to each fault effect are compared with each other. It is seen from the analysis results that the near-fault ground motions, which have significant influence on the dynamic response of dam–reservoir–foundation systems, have the potential to cause more severe damage to the dam body than far-fault ground motions.