Effects of random variations of soil properties on site amplification of seismic ground motions

This paper investigates the effects of random variations of soil properties on site amplification of seismic waves. First, based on attenuation laws and the filtered Tajimi–Kanai spectrum, seismic motion at the base rock of a soil site is stochastically generated according to an assumed earthquake with a given magnitude and epicentral distance. Motions on the surface of this layered random soil site are calculated by nonlinear wave propagation methods, and by assuming the incoming seismic wave consisting of SH wave or combined P and SV waves. Soil properties, including shear modulus, damping ratio and mass density, as well as ground water level are considered as random in the numerical calculation. The Rosenblueth method is used to solve the random dynamic responses of the soil site. Parametric calculations are performed to investigate the effects of various parameters on site amplification of seismic waves. The mean and maximum ground motions on surface of the site are estimated. Numerical results indicate that the estimated surface motions differ substantially if the random variations of soil properties and soil saturation level are taken into consideration in the analysis.     

卵石土场地地震反应特征振动台试验研究

为研究卵石土场地地震反应特征,基于四川成都典型卵石土场地,通过振动台模型试验研究卵石土场地在不同地震波、不同地震强度激励下的加速度峰值放大系数、加速度频谱反应及动土压力反应,并且对其场地地震反应非线性效应及土体动剪应力-动剪应变关系进行分析。结果表明：卵石土场地表层土层对地震波具有明显的放大效应,加速度峰值放大系数介于1～1.4之间,下部土层放大效应较小,加速度峰值放大系数介于0.9～1.2之间。卵石土场地对地震波具有低频放大,高频滤波的作用,滤波频率上、下限随激励强度的增大逐渐向低频方向移动。激励强度较小时,土体尚未破坏,动土压力在地震过程中逐渐增大;随着激励强度的增大,动土压力反应明显增大,表现出骤减后逐渐增大的现象。在激励强度较小时(SN1),中部土体最先进入非线性反应阶段,地震波在中部土层能量损耗最大;激励强度较大时(EL3),土体均发生了较大变形,土体最大动剪应变达到1.7%,此时卵石土场地对地震波的放大作用明显减弱。     

含峭壁V形峡谷对地震SH波散射的解析解

地表地形常引起地震动的局部放大,这是由于地震波传播至局部地形时产生了散射现象.本文利用波函数展开方法和区域匹配技术,提出了含峭壁V形峡谷对平面SH波散射问题的解析解,并进行了退化验证.通过频域内的参数分析,揭示了峭壁深度、入射波频率和角度等因素对峡谷场地地面运动的影响规律,发现上部峭壁会增强峡谷对地震动的地形放大效应....     

Slope topography effects on ground motion in the presence of deep soil layers

An extensive investigation has been made into the interaction between topographic amplification and soil layer amplification of seismic ground motion. This interaction is suggested in the literature as a possible cause for the differences between topographic amplification magnitudes observed in field studies and those obtained from numerical analysis. To investigate this issue a numerical finite element (FE) parametric study was performed for a slope in a homogeneous linear elastic soil layer over rigid bedrock subjected to vertically propagating in-plane shear waves (Sv waves). Analyses were carried out using two types of artificial time history as input excitation, one mimicking the build-up and decay of shaking in the time histories of real earthquake events, and the other to investigate the steady-state response. The study identified topographic effects as seen in previous numerical studies such as modification of the free-field horizontal motion, generation of parasitic vertical motion, zones of alternating amplification and de-amplification on the ground surface, and dependence of topographic amplification on the frequency of the input motion. For the considered cases, topographic amplification and soil layer amplification effects were found to interact, suggesting that in order to accurately predict topographic effects, the two effects should not be always handled separately.     

Centrifuge modeling of seismic response of layered soft clay

Centrifuge modeling is a valuable tool used to study the response of geotechnical structures to infrequent or extreme events such as earthquakes. A series of centrifuge model tests was conducted at 80g using an electro-hydraulic earthquake simulator mounted on the C-CORE geotechnical centrifuge to study the dynamic response of soft soils and seismic soil–structure interaction (SSI). The acceleration records at different locations within the soil bed and at its surface along with the settlement records at the surface were used to analyze the soft soil seismic response. In addition, the records of acceleration at the surface of a foundation model partially embedded in the soil were used to investigate the seismic SSI. Centrifuge data was used to evaluate the variation of shear modulus and damping ratio with shear strain amplitude and confining pressure, and to assess their effects on site response. Site response analysis using the measured shear wave velocity, estimated modulus reduction and damping ratio as input parameters produced good agreement with the measured site response. A spectral analysis of the results showed that the stiffness of the soil deposits had a significant effect on the characteristics of the input motions and the overall behavior of the structure. The peak surface acceleration measured in the centrifuge was significantly amplified, especially for low amplitude base acceleration. The amplification of the earthquake shaking as well as the frequency of the response spectra decreased with increasing earthquake intensity. The results clearly demonstrate that the layering system has to be considered, and not just the average shear wave velocity, when evaluating the local site effects.     

Directional dependence of site effects observed near a basin edge at Aegion,Greece

We study site effects using 520 weak motion earthquake records from a vertical array in Aegion, Greece. The array is inside a basin, has four stations in soil, and one in bedrock (178 m depth). The site is marked by high seismicity and complex surface geology. We first use the records to establish the downhole accelerometer orientations and their evolution with time. Then we estimate site effects using empirical spectral ratios with and without a reference site (standard and horizontal-to-vertical spectral ratio). We find significant site amplification which cannot be accounted for by 1D model predictions, along with a significant difference in the amplification level between the two horizontal components. These are indications of 2D effects, namely surface waves generated at the basin edge. The difference in amplification between the horizontal components is maximised when these are rotated with respect to the orientation of the basin edge. The strongest amplification takes place in the direction parallel to the basin edge (SH, or out-of-plane motion), and is up to 2 times higher than in the perpendicular direction (SV, or in-plane motion). This directional effect on the amplification is corroborated by numerical 2D modelling using incident SH and SV waves, with the former possibly generating strong Love waves. In the records, the directionality is clear for windows containing the largest amplitudes of the records (S waves and strong surface waves), while it tends to vanish for coda-wave windows. This directionality is also observed when using response spectral ratios rather than Fourier ratios. We compute soil-to-rock amplification factors for peak ground acceleration (PGA) and find it is significantly higher than what is predicted by current design codes. We attribute this difference to the basin edge amplification, linear soil behaviour, and to the inability of simple scalar values like PGA to describe complex amplification effects. Finally, we analyse the earthquake records at a surface station near the slope crest and do not observe significant topographic amplification.     

Response of a randomly inhomogeneous layered media to harmonic excitations

We present a unified formulation of an analytical method for evaluating the response of a random soil medium to surface or earthquake excitations. Specifically, we are interested in this study to the case of a horizontally stratified layered soil profile. Soil properties, mass density and shear modulus of each layer are modeled as spatial random fields. The soil profile is laying on a homogeneous half-space. Integro-differential equations are formulated and solved using the Laplace transform method. Numerical results are, firstly, obtained in terms of effects of soil properties on distributions of mean dimensionless displacement and stress with depth. Then, the amplification function of the surface layer of a soil profile is formulated and a parametric study is conducted to examine the effects of stochastic variations of mass density and shear modulus on the amplification function. We, lastly, computed the seismic response of a site located in the basin of Metidja in Algeria to base rock accelerations which were recorded at Keddara's station during the May 21, 2003 Boumerdes earthquake, in terms of free surface Fourier spectrum amplitudes and accelerations. Comparison between computed accelerations and recorded ones at Dar Elbeida site has proved the effectiveness of the approach.     

P波入射下断层参数对地表地震动的影响

基于应用透射人工边界条件的显式有限元法计算断层破碎带宽度及力学参数变化、地震动入射角变化时二维断层场地模型P波入射下地表地震动场的分布。结果表明:(1)低速度破碎带的存在导致整个场地都有P波转换为SV波的分量,且在断层破碎带的区域出现断层陷波;(2)低速度破碎带的存在使输入场地恒定的能量向破碎带集聚放大,随着破碎带宽度增大或其介质波速降低集聚放大效应增大;(3)场地放大效应是频率相关的,宽度较宽或介质波速较低的断层破碎带对输入地震动中较低的频率成份放大显著;(4)竖向断层破碎带能阻隔斜入射地震P波,随着入射角增加隔震效应更显著。     

Seismic rotational displacement of gravity walls by pseudo-dynamic method: Passive case

Prediction of the seismic rotational displacements of retaining wall under passive condition is an important aspect of design in earthquake prone region. In this paper, the pseudo-dynamic method is used to compute the rotational displacements of rigid retaining wall supporting cohesionless backfill under seismic loading for the passive earth pressure condition. The proposed method considers time, phase difference and effect of amplification in shear and primary waves propagating through both the backfill and the retaining wall. The influence of ground motion characteristics on rotational displacement of the wall is evaluated. Also the effects of variation of parameters like wall friction angle, soil friction angle, amplification factor, shear wave velocity, primary wave velocity, period of lateral shaking, horizontal and vertical seismic accelerations on the rotational displacements are studied. The rotational displacement of the wall increases substantially with increase in amplification of both shear and primary waves, time of input motion, period of lateral shaking and decreases with increase in soil friction angle, wall friction angle. The rotational displacements of the wall also increase when the effect of wall inertia is taken into account. Results are provided in graphical form.     

Topographic effects on the Monte Po Hill in Catania (Italy)

The study regards an evaluation of site effects on and near the Monte Po hill, located in the north-eastern part of the city of Catania (Italy), an area at high seismic risk. At the beginning of 2007 a seismic station was located in a school building, situated at the slope toe, but no seismic events have been recorded as yet. Therefore, synthetic seismograms have been used to evaluate the ground response analysis at the surface. Because the average slope is moderate (less than 15°), 1-D computer codes have been used to model the equivalent-linear earthquake site response analyses of layered hill deposits, as generally performed by professionals. However, the slope response has also been analysed in greater detail, using a 2-D computer code and the soil characterisation has been evaluated accurately by means of borings, Down-Hole tests, SDMT tests and laboratory tests. Comparing 1-D with 2-D results the stratigraphic site amplification and the Topographic Aggravation Factor (TAF) have also been computed. The aim of the study is that it will form a basis for the design of works to remediate the damage caused by a landslide reactivated by the earthquake in Eastern Sicily on December 13, 1990 (M_L=5.4).     

土层平均剪切波速确定及其可靠性验证——基于钻井台阵强震动记录

震害资料显示,场地条件对地震动特性以及工程结构破坏程度影响显著。为减少因场地效应而造成的经济损失和社会影响,在进行场地地震反应分析时,需最大限度地减小因场地土层模型参数的不确定性引起的地震动评估偏差,为工程结构地震反应分析选取并生成适当的地震动输入。随着强震动观测技术的逐渐发展,大量可靠的钻井台阵记录为地震过程中场地观测点的动力反应提供了直接数据。以美国加州地区La Cienega钻井台阵强震动观测数据为基础,利用互相关函数,对不同强度地震作用下场地土层的平均剪切波速进行分析,并在此基础上,以Cyclic 1D为模拟平台,建立一维自由场地地震反应有限元分析模型。分析结果表明：通过钻井台阵地震动观测数据识别,得到场地平均剪切波速,能够反映该场地的动力特性,数值模拟计算结果和台阵地震动记录基本吻合,可为数值模型参数选取提供依据。     

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。     

Seismic microzonation of Dehradun City using geophysical and geotechnical characteristics in the upper 30 m of soil column

The understanding of geotechnical characteristics of near-surface material is of fundamental interest in seismic microzonation. Shear wave velocity (Vs), one of the most important soil properties for soil response modeling, has been evaluated through seismic profiling using the multichannel analysis of surface waves in the city of Dehradun situated along the foothills of northwest Himalaya. Fifty sites in the city have been investigated with survey lines between 72 and 96 m in length. Multiple 1-D and interpolated 2-D profiles have been generated up to a depth of 30–40 m. The Vs were used in the SHAKE2000 software in combination with seismic input motion of the recent Chamoli earthquake to obtain site response and amplification spectra. The estimated Vs are higher in the northern part of the study area (i.e., 200–700 m/s from the surface to a depth of about 30 m) as compared to the south and southwestern parts of the city (i.e., 180–400 m/s for the same depth range). The response spectra suggest that spectral acceleration values for two-story structures are three to eight times higher than peak ground acceleration at bedrock. The analysis also suggests peak amplification at 3–4, 2–2.5, and 1–1.5 Hz in the northern, central, and south-southwestern parts of the city, respectively. The spatial distributions of Vs and spectral accelerations provide valuable information for the seismic microzonation in different parts of the urban area of Dehradun.     

基于台阵记录的土层山体场地效应分析

选取2008年5月25日至8月7日期间由甘肃省文县上城山地形效应台阵获取的12次汶川地震余震事件(M_S≥4.0),在分析其地震动基本参数的基础上,采用参考场地谱比(RSSR)法和水平-竖向谱比(HVSR)法,研究了不同地震作用下上城山地形台阵的场地效应.分析结果显示:随着高程和覆盖层的增加,记录台站地震动的PGA呈增大趋势,地震频谱形状由宽变窄;上城山台阵记录到的地震波在地形基阶频段(2—4 Hz)和浅部土层频段(7—9 Hz)的幅值明显放大,RSSR曲线显示山顶NS向的土层频段谱比大于山体地形频段谱比;由于土层山体竖向地震动在中高频段放大,使得HVSR方法谱比结果在中高频段较RSSR方法所得结果明显偏低,而在山体基阶频段附近两种方法的谱比值接近.松散土层山体的台阵记录特征体现了地形和土层对地震动的联合作用,揭示了强震区起伏地形场地震害加重及地震滑坡集中发生的原因所在.      

The plurality effect of topographical irregularities on site seismic response

Topography can have significant effects on seismic ground response during an earthquake because topographic irregularities cause considerable differences between the seismic waves emitted by the source and the waves reaching the ground surface. When a seismic motion happens in a topographically irregular area, seismic waves are trapped and reflected between the topographic features. Therefore, the interaction between topographies can amplify seismic ground response. In order to reveal how interaction between topographies influences seismic response, several numerical finite element studies have been performed by using the ABAQUS program. The results show that topographic features a greater distance between the seismic source and the site would cause greater seismic motion amplification and is perceptible for the hills far away from the source and the ridges. Also, site acceleration response is impacted by surrounding topography further than site velocity and displacement response.     

Long period microtremors, microseisms and earthquake damage: Northridge, CA, earthquake of 17 January 1994

Three studies of site amplification factors, based on the recorded aftershocks, and one study based on strong motion data, are compared one with another and with the observed distribution of damage from the Northridge, CA, earthquake of 17 January 1994 (M_L=6.4). In the epicentral area, when the peak ground velocities are larger than v_m≈15 cm/s, nonlinear response of soil begins to distort the amplification factors determined from small amplitude (linear) wave motion. Moving into the area of near-field and strong ground motion (v_m>30 cm/s), the site response becomes progressively more affected by the nonlinear soil response. Based on the published results, it is concluded that site amplification factors determined from small amplitude waves (aftershocks, small earthquakes, coda waves) and their transfer-function representation may be useful for small and distant earthquake motions, where soils and structures respond to earthquake waves in a linear manner. However in San Fernando Valley, during the Northridge earthquake, the observed distribution of damage did not correlate with site amplification determined from spectra of recorded weak motions. Mapping geographical distribution of site amplification using other than very strong motion data, therefore appears to be of little use for seismic hazard analyses.     

Side effect of using response spectral amplification ratios for soil sites—variability and earthquake-magnitude and source-distance dependent amplification ratios for soil sites

We investigate a special type of variability in response spectral amplification ratios computed from numerical “engineering” models for a soft soil site. The engineering models are defined by shallow soil layers over “engineering” bedrock with a shear-wave velocity over 600–700 m/s and the model is subjected to vertical propagating shear waves. The variability, perhaps unique in earthquake engineering, is a result of the “perfectly accurate” computational procedure. For example, an engineering soil site model, subjected to two rock site records or the two horizontal components of a rock site record, produces different response spectral amplification ratios. We use a large number of strong-motion records from “engineering” rock sites, with a reasonably balanced distribution with respect to magnitude and source distance, generated by subduction earthquakes in Japan, to investigate the nature of the variability. In order to avoid any approximation in removing the effect of soil nonlinear response, we use a simple model, a single horizontal soil layer over a bedrock, modelled as elastic. We then demonstrate that a similar type of variability observed in the one- or two-dimensional nonlinear soil models is caused by the nature of response spectral amplification ratios, not a direct result of soil nonlinear response. Examination of variability reveals that the average of response spectral amplification ratios systematically depends on both earthquake magnitude and source distance. We find that, at periods much longer than the site natural periods of the soil sites, the scatter of the amplification ratios decreases with increasing magnitude and source distance. These findings may have a potential impact in establishing design spectra for soft soil sites using strong-motion attenuation models or dynamic numerical modelling.     

Spatial zonations for regional assessment of seismic site effects in the Seoul metropolitan area

Earthquake-induced hazards are profoundly affected by site effects related to the amplification of ground motions, which are strongly influenced by local geologic conditions such as soil thickness or bedrock depth and soil stiffness. In this study, an integrated geographic information system (GIS)-based system for geotechnical data, called the geotechnical information system (GTIS), was developed to establish a regional counterplan against earthquake ground motions in the Seoul metropolitan area. In particular, to reliably predict spatial geotechnical information, a procedural methodology for building the GTIS within a GIS framework was developed and applied to the Seoul area in Korea. To build the GTIS, pre-existing geotechnical data were collected in and around the study area, and then a walk-over site survey was conducted to acquire surface geo-knowledge data. In addition, the representative shear wave velocities for geotechnical layers were derived by statistically analyzing many seismic test data in Korea. The GTIS was used in a practical application to estimate site effects in the study area; seismic zoning maps of geotechnical earthquake parameters, such as the depth to bedrock and the site period, were created and presented as a regional synthetic strategy for earthquake risk assessment. Furthermore, seismic zonation of site classification was also performed to determine the site amplification coefficients for seismic design and seismic performance evaluation at any site and administrative sub-unit in the study area. The methodology and results of the case study of seismic zonations in the Seoul area verified that the GIS-based GTIS can be very useful for the regional estimation of seismic risk and also to support decisions regarding seismic hazard mitigation, particularly in the metropolitan area.     

Earthquake site effect modeling in the Granada basin using a 3-D indirect boundary element method

In this work, we develop the indirect boundary element method (IBEM) to simulate the seismic site response in a realistic, large-scale 3-D sedimentary basin. Most previous applications of boundary element method have used full-space Green’s functions for wave propagation between element points. We use half-space Green’s functions, which include the seismic wavefield interactions at the free surface and require only the boundary elements of the basin interface. In this way, the size of the matrix equation for solution in the IBEM can be reduced to approximately a quarter of that using full-space Green’s functions. The site response modeling of the Granada basin in southern Spain using the IBEM shows that the basin-induced scattering waves were identified as propagating back and forth inside the basin. The scattered waves also generate surface waves that are weakly propagated outside of the basin. The wave propagation inside and outside of the basin shows different patterns. We observe that the scattered wave is locally amplified, and its propagation direction deviates from that of the incident waves propagation direction. Therefore, the computed seismic response in the basin could provide us with good estimates of the seismic motion.     

New criteria on the filling ratio and impedance ratio effects in seismic response evaluation of the partial filled alluvial valleys

An extensive numerical analysis on the seismic site effects due to local topographical and geotechnical characteristics is carried out. 2D configurations under incidence of vertically propagating SV waves is modeled with the aid of HYBRID program, combining finite elements in the near field and boundary elements in the far field. The filling ratio and the impedance ratio effects on the modification of the seismic response of alluvial valleys are underlined. Parametric analysis is done on the central point of alluvial valleys where the critical point of response under existence of sediments stands. Specifying the amplification pattern under filling ratio effects, effective geometrical parameters are introduced. Subsequently, to assess the effect of the type of sediments on the seismic response, mechanical properties of materials are changed and impedance contrast coefficient is considered in combination with geometrical parameters. Finally, practical curves are presented for engineering applications.