基于OpenSees的地震动对软土震陷影响研究

软土具有高灵敏度、低强度等特性,在地震过程中极易产生震陷。基于OpenSees数值模拟方法对软土场地的震陷反应进行非线性动力有限元分析,通过改变地震动峰值加速度、频谱特性、输入方式来研究其对软土震陷的影响。结果表明,地震动峰值加速度对地基土的不均匀震陷有显著影响,地震动峰值加速度越大,震陷量显著增大,震陷影响深度更大,对水平地表造成的破坏范围也更大;地震动频谱特性对软土震陷有重要影响,当地震动卓越频率与场地自振频率相近时,其幅值越大,产生的震陷越严重;水平、竖向同时输入地震动的方式能更好地反映土体的振动及震陷响应。该研究成果对探索软土震陷的机理有一定的指导意义。     

Attenuation of P- and S-waves in the Chamoli Region, Himalaya, India

The attenuation properties of the crust in the Chamoli region of Himalaya have been examined by estimating the frequency-dependent relationships of quality factors for P waves (Q_α) and for S waves (Q_β) in the frequency range 1.5–24 Hz. The extended coda normalization method has been applied on the waveforms of 25 aftershocks of the 1999 Chamoli earthquake (M 6.4) recorded at five stations. The average value of Q_α is found to be varied from 68 at 1.5 Hz to 588 at 24 Hz while it varies from 126 at 1.5 Hz to 868 at 24 Hz for Q_β. The estimated frequency-dependent relations for quality factors are Q_α = (44 ± 1)f^(0.82±.04) and Q_β = (87 ± 3)f^(0.71±.03). The rate of increase of Q(f) for P and S waves in the Chamoli region is comparable with the other regions of the world. The ratio Q_β/Q_α is greater than one in the region which along with the frequency dependence of quality factors indicates that scattering is an important factor contributing to the attenuation of body waves in the region. A comparison of attenuation relation for S wave estimated here (Q_β = 87f^0.71) with that of coda waves (Q_c = 30f^1.21) obtained by Mandal et al. (2001) for the same region shows that Q_c > Q_β for higher frequencies (>8 Hz) in the region. This indicates a possible high frequency coda enrichment which suggests that the scattering attenuation significantly influences the attenuation of S waves at frequencies >8 Hz. This observation may be further investigated using multiple scattering models. The attenuation relations for quality factors obtained here may be used for the estimation of source parameters and near-source simulation of earthquake ground motion of the earthquakes, which in turn are required for the assessment of seismic hazard in the region.     

Intensity measures for the assessment of the seismic response of buried steel pipelines

To estimate the demand of structures, investigating the correlation between engineering demand parameters and intensity measures (IMs) is of prime importance in performance-based earthquake engineering. In the present paper, the efficiency and sufficiency of some IMs for evaluating the seismic response of buried steel pipelines are investigated. Six buried pipe models with different diameter to thickness and burial depth to diameter ratios, and different soil properties are subjected to an ensemble of 30 far-field earthquake ground motion records. The records are scaled to several intensity levels and a number of incremental dynamic analyses are performed. The approach used in the analyses is finite element modeling. Pipes are modeled using shell elements while equivalent springs and dashpots are used for modeling the soil. Several ground motion intensity measures are used to investigate their efficiency and sufficiency in assessing the seismic demand and capacity of the buried steel pipelines in terms of engineering demand parameter measured by the peak axial compressive strain at the critical section of the pipe. Using the regression analysis, efficient and sufficient IMs are proposed for two groups of buried pipelines separately. The first one is a group of pipes buried in soils with low stiffness and the second one is those buried in soils with higher stiffness. It is concluded that for the first group of pipes, \(\sqrt {{\text{VSI}}[\upomega_{1} ({\text{PGD}} + {\text{RMS}}_{\text{d}} )]}\) followed by root mean square of displacement (RMS_d) are the optimal IMs based on both efficiency and sufficiency; and for the second group, the only optimal IM is PGD²/RMS_d.     

深厚软土场地基岩地震动反演——以天津波为例

根据天津波地表加速度记录和相应实测场地钻孔资料,采用等效线性化方法模拟土体非线性,进行了深厚软土场地基岩地震动反演,研究了频率截断对深厚软土场地基岩地震动反演的影响,为天津波地表加速度记录提供了相应的基岩地震动输入。研究表明:不同截断频率情况下,反演得到的基岩地震动峰值和反应谱曲线形状均相差悬殊,但反演后再正演所得到的地面运动与原地表记录相比,加速度时程曲线形状和峰值大小、反应谱曲线形状和峰值大小均非常接近;可根据工程需要对地表加速度记录进行不同频率的截断,反演得到所需强度的基岩地震动。     

Simulation of accelerograms from simplified deterministic approach for the 23rd October 2004 Niigata-ken Chuetsu,Japan earthquake

A simple hybrid approach for the simulation of strong ground motion is presented in this paper. This approach is based on the deterministic modelling of rupture plane initially started by Midorikawa, Tectonophysics 218:287–295, (1993) and further modified by Joshi, Pure Appl Geophys (PAGEOPH) 8:161, (2004). In this technique, the finite rupture plane of the target event is divided into several subfaults, which satisfy scaling relationship. In this paper, simulation of strong ground motion due to a rupture buried in a earth medium consisting of several layers of different velocities and thicknesses is made by considering (1) transmission of energy at each layer; (2) frequency filtering properties of medium and earthquake source; (3) correction factor for slip of large and small magnitude earthquakes and (4) site amplification ratio at various stations. To test the efficacy of the developed technique, strong motion records were simulated at different stations that have recorded the 2004 Niigata-ken Chuetsu, Japan earthquake (M _s 7.0). Comparison is made between the simulated and observed velocity and acceleration records and their response spectra. Distribution of peak ground acceleration, velocity and displacement surrounding the rupture plane is prepared from simulated and observed records and are compared with each other. The comparison of synthetic with the observed records over wide range of frequencies shows that the present technique is effective to predict various strong motion parameters from simple deterministic model which is based on simple regression relations and modelling parameters.     

Monofrequency in situ damping measurements in Ottawa area soft soils

In Ottawa, Canada, unusually high amplification ratios have recently been measured in clayey silts (called ‘Leda Clays’) at low levels of earthquake-induced ground shaking. However, the contribution of seismic Q, or material damping (ξ=1/2Q), to the overall ground motion at soft soil sites across the city is not well understood. This research investigates attenuation measurements in soft soils (V_s<250 m/s) for ongoing seismic hazard evaluation in the Ottawa area. The work focuses on in situ measurements of damping in two deep boreholes drilled into Leda Clay. To investigate the possibility of frequency-dependent dynamic properties of these materials at low strains, a new approach to the spectral ratio technique has been developed for the measurement of Q_s in the field using a mono-frequency vibratory source (generating signals between 10 and 100 Hz), and two identical downhole 3-component geophones. Monofrequency signals also allowed for the measurement of dispersion (variation of velocity with frequency). Analysis of the data show that dynamic properties are, for the most part, independent of frequency in the homogenous silty soils, yielding negligible variation in shear wave velocity (<2 m/s) across the frequency test band, and small strain Q_s's ranging from 170 to 200 (damping of 0.25–0.30%) over soil thickness intervals ranging from 10 to 60 m. At intervals within 20 m of the ground surface, laminated silt and clay beds of elevated porosity are found to have slight influence on the frequency dependence of damping for frequencies greater than 70 Hz (damping increase to 0.6%).     

北京市区小区域地震影响

文中研究了北京及其周围地区的历史地震和地震地质资料,并在此基础上估计了基岩运动的参数。众所周知,局部地质、地形和土质条件等许多因素对给定场地的地面运动特性都是有影响的。本文着重考虑了土层的特性对地面运动的影响.作为分析工作的基础,我们研究了北京地区典型土层的动力特性。为了分析市区各场地上的地震反应,我们将它分成面积大体相等的许多网格,对于每一网格都分析了相应土层柱状在指定输入运动下的地震反应,通过电子计算和绘图装置将结果用许多图件表示,从而给出了不同场地上地震反应谱和强震地面运动的地区分布。这些图件可用来作为制定土地利用和防震规划的参考。我们根据市区每一地点的计算反应谱和典型结构的平均周期确定了相应结构的地震系数,结果已绘成等值线,这些图件可作为典型结构的震害预测和抗震设计的参考。      

Influence of ground motion spatial variation,site condition and SSI on the required separation distances of bridge structures to avoid seismic pounding

It is commonly understood that earthquake ground excitations at multiple supports of large dimensional structures are not the same. These ground motion spatial variations may significantly influence the structural responses. Similarly, the interaction between the foundation and the surrounding soil during earthquake shaking also affects the dynamic response of the structure. Most previous studies on ground motion spatial variation effects on structural responses neglected soil–structure interaction (SSI) effect. This paper studies the combined effects of ground motion spatial variation, local site amplification and SSI on bridge responses, and estimates the required separation distances that modular expansion joints must provide to avoid seismic pounding. It is an extension of a previous study (Earthquake Engng Struct. Dyn. 2010; 39 (3):303–323), in which combined ground motion spatial variation and local site amplification effects on bridge responses were investigated. The present paper focuses on the simultaneous effect of SSI and ground motion spatial variation on structural responses. The soil surrounding the pile foundation is modelled by frequency‐dependent springs and dashpots in the horizontal and rotational directions. The peak structural responses are estimated by using the standard random vibration method. The minimum total gap between two adjacent bridge decks or between bridge deck and adjacent abutment to prevent seismic pounding is estimated. Numerical results show that SSI significantly affects the structural responses, and cannot be neglected. Copyright © 2010 John Wiley & Sons, Ltd.     

Strong ground motion simulation by the Empirical Green��s Function method for Bursa-Yalova Region, Turkey

In this study near field strong ground motion generation of Mw 6.9 scenario events on Gemlik Bay was presented at broadband frequency (0.5–10 Hz) ground motion at 9 stations. In the first stage of the study, focal mechanism of a small earthquake, which was used as the Empirical Green’s Function (EGF) throughout the scenario simulation, was decided by simulating it with a smaller magnitude event. The best waveform fitting was judged with the smallest misfit value. In the second stage, near field ground motion simulation of scenario events was performed. Calculations were achieved by considering three different rupture processes which have the same magnitude but different asperity locations. Fault and asperity parameters for each scenario were determined from empirical scaling laws. It has been found that the peak ground acceleration and peak ground velocities reach maximum values of 1,440 cm/s² and 125 cm/s, respectively for the worst case scenario. Rupture directivity effect is observed with clear peaks at a forward station. The design spectra for Turkish seismic design code (TSDC 2007) were either nearly or actually exceeded by the scenario earthquakes at periods lower than 0.6  s at all near field stations. Majority of structures in the area were built to lower design spectra before the 1998 code was implemented. The strength of many structures would have been insufficient to resist the forces that may be generated by an earthquake that is similar to Scenario I and Scenario II in this study.     

Simulation of orthogonal horizontal ground motion components for specified earthquake and site characteristics

A method for generating an ensemble of orthogonal horizontal ground motion components with correlated parameters for specified earthquake and site characteristics is presented. The method employs a parameterized stochastic model that is based on a time‐modulated filtered white‐noise process with the filter having time‐varying characteristics. Whereas the input white‐noise excitation describes the stochastic nature of the ground motion, the forms of the modulating function and the filter and their parameters characterize the evolutionary intensity and nonstationary frequency content of the ground motion. The stochastic model is fitted to a database of recorded horizontal ground motion component pairs that are rotated into their principal axes, a set of orthogonal axes along which the components are statistically uncorrelated. Model parameters are identified for each ground motion component in the database. Using these data, predictive equations are developed for the model parameters in terms of earthquake and site characteristics and correlation coefficients between parameters of the two components are estimated. Given a design scenario specified in terms of earthquake and site characteristics, the results of this study allow one to generate realizations of correlated model parameters and use them along with simulated white‐noise processes to generate synthetic pairs of horizontal ground motion components along the principal axes. The proposed simulation method does not require any seed recorded ground motion and is ideal for use in performance‐based earthquake engineering. Copyright © 2011 John Wiley & Sons, Ltd.     

Reconnaissance report and preliminary ground motion simulation of the 12 May 2008 Wenchuan earthquake

The M _w = 8.0 Wenchuan earthquake of May 12, 2008, caused destruction over a wide area. The earthquake cost more than 69,000 lives and the damage is reported to have left more than 5 million people homeless. It is estimated that 5.36 million buildings were destroyed and 21 million buildings were damaged in Sichuan and the nearby provinces. Economic losses due to the event are estimated to be 124 billion USD. From a field reconnaissance trip conducted in October 2008, it is evident that the combination of several factors, including mountainous landscape, strong ground shaking, extensive landslides and rock-falls, has exacerbated the human and economic consequences of this earthquake. Extensive damage occurred over a wide area due to the shear size of the earthquake rupture combined with poor quality building construction. In order to investigate the ground shaking during the earthquake, we have conducted a strong ground motion simulation study, applying a hybrid broadband frequency technique. The preliminary results show large spatial variation in the ground shaking, with the strongest ground motions along the fault plane. The simulation results have been calibrated against the recorded ground motion from several near-field stations in the area, and acceleration values of the order of 1 g are obtained, similar to what was recorded during the event. Comparison with the damage distribution observed in the field confirms that the effect of fault rupture complexity on the resulting ground motion distribution also controls to a large extent the damage distribution. The applied simulation technique provides a promising platform for predictive studies.     

The Northridge, California, earthquake of 1994: fire ignition by strong shaking

The Northridge earthquake contributed unprecedented detail and quality of data on strong ground motion and on its effects on man-made structures. About 110 fires have been attributed directly to the effects of this earthquake. Two hypotheses for the principal causative agents leading to fire ignition were examined: differential motion and strains in the soil, and inertial forces. The fire-ignition frequency is described with respect to: (1) simple measures of strain in the soil (via density of water pipe breaks, n), (2) occurrence of severely damaged buildings (via density of red-tagged buildings, N), (3) site intensity of shaking, (I_MM), and (4) inertial forces (via peak horizontal ground velocity, v_m). It is shown that the rate of fires (per unit area) ignited by earthquake shaking can be predicted by several empirical equations of comparable accuracy and in terms of common scaling parameters of strong ground motion.     

A simplified method for simulation of strong ground motion using finite rupture model of the earthquake source

We present a simplified method to simulate strong ground motion for a realistic representation of a finite earthquake source burried in a layered earth. This method is based on the stochastic simulation method of Boore (Boore, D. M., 1983, Bull. Seism. Soc. Am. 73, 1865–1894) and the Empirical Greens Function (EFG) method of Irikura (Irikura, K., 1986, Proceedings of the 7th Japan Earthquake symposium, pp. 151–156). The rupture responsible for an earthquake is represented by several subfaults. The geometry of subfaults and their number is decided by the similarity relationships. For simulation of ground motion using the stochastic simulation technique we used the shapping window based on the kinetic source model of the rupture plane. The shaping window deepens on the geometry of the earthquake source and the propagation characteristics of the energy released by various subfaults. The division of large fault into small subfaults and the method for accounting their contribution at the surface is identical to the EGF. The shapping window has been modified to take into account the effect of the transmission of energy released form the finite fault at various boundaries of the layered earth model above the source. In the present method we have applied the correction factor to adjust slip time function of small and large earthquakes. The correction factor is used to simulate strong motion records having basic spectral shape of ² source model in broad frequency range. To test this method we have used the strong motion data of the Geiyo earthquake of 24th March 2001, Japan recorded by KiK network. The source of this earthquake is modelled by a simple rectangular rupture of size 24 × 15 km, burried at a depth of 31 km in a multilayered earth model. This rupture plane is divided into 16 rectangular subfaults of size 6.0 × 3.75 km each. Strong motion records at eight selected near-field stations were simulated and compared with the observed records in terms of the acceleration and velocity records and their response spectrum. The comparison confirms the suitability of proposed rupture model responsible for this earthquake and the efficacy of the approach in predicting the strong motion scenario of earthquakes in the subduction zone. Using the same rupture model of the Geiyo earthquake, we compared the simulated records from our and the EGF techniques at one near-field station. The comparison shows that this technique gives records which matches in a wide frequency range and that too from simple and easily accessible parameters of burried rupture.     

Strain-threshold- and frequency-dependent seismic simulation of nonlinear soils

A one-dimensional equivalent linear method (EQL) is widely used in estimating seismic ground response. For this method, the shear modulus and damping ratio of inelastic soil are supposed to be frequency independent. However, historical earthquake records and laboratory test results indicate that nonlinear soil behavior is frequency-dependent. Several frequency-dependent equivalent linear methods (FDEQL) related to the Fourier amplitude of shear strain time history have been developed to take into account the frequency-dependent soil behavior. Furthermore, the shear strain threshold plays an important role in soil behavior. For shear strains below the elastic shear strain threshold, soil behaves essentially as a linear elastic material. To consider the effect of elastic-shear-strain-threshold- and frequency-dependent soil behavior on wave propagation, the shear-strain-threshold- and frequency-dependent equivalent linear method (TFDEQL) is proposed. A series of analyses is implemented for EQL, FDEQL, and TFDEQL methods. Results show that elastic-shear-strain-threshold- and frequency-dependent soil behavior plays a great influence on the computed site response, especially for the high-frequency band. Also, the effect of elastic-strain-threshold- and frequency-dependent soil behavior on the site response is analyzed from relatively weak to strong input motion, and results show that the effect is more pronounced as input motion goes from weak to strong.     

Analysis of the spatial variation of seismic waves and ground movements from smart-1 array data

Specially designed arrays of strong motion seismographs located near earthquake sources are required for engineering studies of the near-source properties and the spatial variation of seismic waves. The SMART-1 array in Taiwan provides good records for this type of study. Careful study of the observed strong motion data permits the identification of wave types, directions and apparent wave velocities. In this paper, a principal direction ratio R (f,α) is defined; this indicates the principal direction of the motion (along a nearly straight line) within the range 0 < R < 1. Vertical motion of the ground is also included in this study. Orbit spectrum analysis is used to verify the identification of wave directions and wave types. The spatial variation of seismic waves along the principal direction is studied. From frequency-domain analysis, mathematical models of the spatial variation of ground displacement are developed using a wave-number spectrum and the cross-spectral density function between two spatial coordinates; these models in turn can provide two alternative models for the random vibration analysis of extensive structures subject to multiple point seismic excitation. The SMART-1 array data gathered during the January 29, 1981 earthquake also are used to demonstrate calculation of the ground strains and differential movements of the array site. From time-domain analysis, the spatial variation of seismic waves is defined for ground motion along the identified principal direction. The time variation of evolutionary spectra characterized by frequency-dependent parameters is used for this formulation. The SMART-1 array data again form the basis for discussion of the spatial variation of model parameters.     

Stochastic finite-fault simulation of ground motion from the August 11, 2012, <Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">w</Emphasis></Subscript> 6.4 Ahar earthquake,northwestern Iran

In this study, the 11 August 2012 M _w 6.4 Ahar earthquake is investigated using the ground motion simulation based on the stochastic finite-fault model. The earthquake occurred in northwestern Iran and causing extensive damage in the city of Ahar and surrounding areas. A network consisting of 58 acceleration stations recorded the earthquake within 8–217 km of the epicenter. Strong ground motion records from six significant well-recorded stations close to the epicenter have been simulated. These stations are installed in areas which experienced significant structural damage and humanity loss during the earthquake. The simulation is carried out using the dynamic corner frequency model of rupture propagation by extended fault simulation program (EXSIM). For this purpose, the propagation features of shear-wave including \( {Q}_s \) value, kappa value \( {k}_0 \), and soil amplification coefficients at each site are required. The kappa values are obtained from the slope of smoothed amplitude of Fourier spectra of acceleration at higher frequencies. The determined kappa values for vertical and horizontal components are 0.02 and 0.05 s, respectively. Furthermore, an anelastic attenuation parameter is derived from energy decay of a seismic wave by using continuous wavelet transform (CWT) for each station. The average frequency-dependent relation estimated for the region is \( Q=\left(122\pm 38\right){f}^{\left(1.40\pm 0.16\right)}. \) Moreover, the horizontal to vertical spectral ratio \( H/V \) is applied to estimate the site effects at stations. Spectral analysis of the data indicates that the best match between the observed and simulated spectra occurs for an average stress drop of 70 bars. Finally, the simulated and observed results are compared with pseudo acceleration spectra and peak ground motions. The comparison of time series spectra shows good agreement between the observed and the simulated waveforms at frequencies of engineering interest.     

强震动作用下土体非线性动力特征研究发展与展望

浅地表覆盖土层动力特性对地震动影响显著,软厚土层会明显改变地震动强度及频谱特性。由于观测数据匮乏,强震动作用下土体非线性动力特征研究长期以来均以室内试验为主,但在实验室中难以可靠地模拟实际地震历程中土体承受的加载路径、边界条件、排水条件等复杂因素。近二十年来竖向台阵（至少包含一个地表测点和一个井下基岩测点）记录数据大量增加,为土体非线性动力学研究提供了新的基础数据与发展机遇,使基于现场观测的土体非线性动力特征实证研究成为可能。     

Accuracy of three-dimensional seismic ground response analysis in time domain using nonlinear numerical simulations

To provide appropriate uses of nonlinear ground response analysis for engineering practice, a three-dimensional soil column with a distributed mass system and a time domain numerical analysis were implemented on the OpenSees simulation platform. The standard mesh of a three-dimensional soil column was suggested to be satisfied with the specified maximum frequency. The layered soil column was divided into multiple sub-soils with a different viscous damping matrix according to the shear velocities as the soil properties were significantly different. It was necessary to use a combination of other one-dimensional or three-dimensional nonlinear seismic ground analysis programs to confirm the applicability of nonlinear seismic ground motion response analysis procedures in soft soil or for strong earthquakes. The accuracy of the three-dimensional soil column finite element method was verified by dynamic centrifuge model testing under different peak accelerations of the earthquake. As a result, nonlinear seismic ground motion response analysis procedures were improved in this study. The accuracy and efficiency of the three-dimensional seismic ground response analysis can be adapted to the requirements of engineering practice.     

Generation of artificial strong motion accelerograms

A method for generating synthetic strong motion accelerograms for use in engineering design is presented. This method utilizes the model proposed by Trifunac in 1971²⁷ in conjunction with the recent empirical scaling functions for characterization of amplitudes and duration of strong shaking in terms of (i) earthquake magnitude, M, and epicentral distance, R, or (ii) Modified Mercalli Intensity (MMI) at the recording station. The method also enables one to consider the desired levels of confidence that the synthetic motion will not be exceeded, direction of ground motion (horizontal or vertical) and the dispersive properties of geologic environment beneath and surrounding the station. The principal features of this approach are that the resulting accelerograms have non-stationary frequency and amplitude characteristics which are in full agreement with known principles of wave propagation through a stratified medium, and that the Fourier amplitudes and the frequency-dependent duration are scaled in accordance with known trends as in recorded accelerograms.     

An evaluation of Güldürcek dam response during 6 June 2000 Orta earthquake

Güldürcek dam located in the central Anatolia was subjected to longitudinal cracks of not wider than 2 cm during 6 June 2000 Orta earthquake. In this study, a brief account of the 6 June 2000 Orta earthquake is given and the results of dynamic FEM analyses of the dam, incorporating the non-linear soil properties under six selected base motions, are presented. Of the six base motions used in the analyses, three have been modified from the European strong motion database having similar ground motion parameters with that of Orta earthquake; the other have been artificially generated using the same parameters. Pore pressure developed during shaking is also considered in the analyses carried out by the program TELDYN and the results are compared with those computed by QUAD4M without considering the excess pore pressures. It is found that New mark displacement analysis predicts the measured displacements after the earthquake in a reasonable accuracy.