泉州盆地地震效应的二维非线性分析

基于ABAQUS显式有限元并行计算平台,采用大尺度二维精细化有限元非线性分析方法,研究泉州盆地地震效应特征.结果表明:(1)与基岩输入地震动相比,地表峰值加速度总体呈放大效应,且盆地不同位置的放大效应存在明显差异;(2)地表或基岩剧烈起伏及土层横向分布极不均匀处,地震动易产生局部聚集效应,相应地表地震动呈显著放大或缩小效应;地表加速度反应谱产生双峰甚至多峰现象.(3)在土层竖向、横向分布不均匀处,峰值加速度沿深度方向呈非单调递减,加速度反应谱沿深度存在较大差异.场地地震效应的二维非线性分析法能在一定程度上反映特殊场地条件对地震动效应的影响,该结果可为泉州盆地及类似盆地的场地设计地震动参数的确定提供参考依据.     

泉州盆地地震效应特征的一维等效线性和二维非线性分析的比较

采用等效线性动粘弹性模型描述土的动力非线性特性,基于一维等效线性波传法,对泉州盆地地震效应进行了分析;同时,采用修正Martin-Seed-Davidenkov动粘弹塑性模型描述土的动力非线性特性,对泉州盆地非线性地震效应进行了大尺度二维精细化有限元分析,研究了地形地貌和土层横向不均匀性对地震效应的影响。将两种分析结果进行对比,结果表明：①随着基岩输入地震动强度增大,地表峰值加速度PGA放大效应总体呈现减小趋势,中震与小震、大震与小震的地表PGA放大系数之比依次为0.83~0.99、0.72~0.97;②该盆地Ⅲ类场地处,基岩、地表起伏不大,且土层横向分布较均匀,两种方法计算得到的地震效应特征类似;基岩或地表起伏剧烈、土层横向分布明显不均匀的Ⅱ类场地上,二维非线性分析给出的地表PGA放大系数明显大于一维等效线性结果,两种方法得到的地表加速度反应谱及PGA随土层深度的变化特征存在显著差异,二维非线性分析给出的地表加速度反应谱大多呈现双峰甚至多峰现象,且PGA在土层特定深度处存在聚集效应,使PGA随土层深度的变化呈现非单调性。     

基于有限断层动力学模型的逆断层—三维层状沉积盆地地震动模拟

近断层效应使得沉积盆地对地震动放大效应更为复杂。本文针对逆断层发震下三维层状沉积盆地地震反应,基于波动谱元法,采用有限断层动力学模型,模拟断层动力破裂、地壳层地震波传播和层状沉积盆地对地震波散射全过程。在此基础上,对比分析了层状和均质沉积盆地对近断层地震动放大效应的影响,讨论了不同断层倾角下层状沉积盆地地震动加速度特性。结果表明：层状沉积盆地PGA空间分布与均质沉积盆地存在较大差异,由于近断层效应和盆地效应,层状沉积盆地地表局部范围竖向PGA大于水平向PGA;90°断层倾角下层状沉积盆地地表地震动放大范围与60°断层倾角结果明显不同,主要集中在盆地中心区域和断层附近,且幅值远小于60°断层倾角下结果;沿断层走向,盆地内地表地震动加速度峰值对应时刻较盆地外延后。     

北京地区中硬场地地震动效应研究

依据典型工程场地的地震安全性评价报告,研究北京地区中硬场地地震动效应。首先,统计分析了地震安全性评价结果中的场地地震动放大效应。再利用201个工程场地的钻孔资料进行土层地震反应计算,分析不同地震动输入下的场地地震动放大效应。结果表明,北京地区中硬场地地震动放大效应明显,得到的场地地震动峰值加速度转换系数Ka高于《中国地震动参数区划图》（GB18306-2001）中的值,大震水准下更为突出。因此,如在北京地区直接采用《中国地震动参数区划图》（GB18306-2001）中的调整系数,可能会低估北京地区中硬场地的地震动放大效应,降低抗震设防标准。最后指出,场地地震动峰值加速度转换系数明显具有区域性,应根据详细的场地地震动效应研究结果,合理地确定场地地震动转换系数。     

不均匀软弱夹层对沉积盆地地震动放大的影响研究

含有倾斜软弱夹层的盆地将引起地震波复杂散射,这较大程度改变地表地震动的强度和分布。目前,盆地内倾斜夹层对地震动放大影响较少。通过自编ABAQUS插件程序实现对黏弹性边界快速准确施加,并基于黏弹性和修正Drucker-Prager理想弹塑性本构,研究了SV波垂直入射下横向不均匀软弱夹层对沉积盆地地震动放大的影响。研究结果表明：横向不均匀软弱夹层降低了地表PGA与Arias烈度,使二者空间分布显著不均匀,软弱夹层较厚一侧下降程度更高。考虑土层非线性时,软弱夹层隔震效应增强;存在横向不均匀软弱夹层的二维沉积盆地的基频接近夹层较厚一侧一维土层基频,显著低于夹层较薄一侧的一维土层基频。随着平均阻抗比越大,横向不均匀软弱夹层对二维盆地基频、f_1d/f_2d及放大系数的影响越小;随着软弱夹层倾角增大,传递函数主要放大区域均向夹层较厚一侧移动,且主要放大频带向低频移动。随着地震动幅值增大,横向不均匀软弱夹层非线性增强,地表PGA放大系数与传递函数下降,同时地表地震动不均匀分布程度降低。     

2020年7月12日唐山古冶5.1级地震揭示的北京城区地震动场地效应分析

局部场地条件对地震动特性影响显著,深厚的软弱覆盖层引起的地震动场地效应会显著放大中长周期反应谱。采用谱比法,对2020年7月12日唐山古冶5.1级地震中获得的部分强震动记录进行统计,发现在本次地震中北京城区的地震动场地效应显著,深厚覆盖层明显放大了加速度反应谱,在T=1.2 s左右反应谱放大倍数可达4.0,说明北京地区的场地和盆地效应使得远场地震动的中长周期成分显著放大。此外,发现参考基岩场地记录是否与土层场地处遭受的基岩地震动一致,仍然是制约统计结果可靠性的关键因素。      

SV波斜入射下成层盆地地震动时-频域放大特征研究

基于二维成层盆地模型,以盆地不同场点对应的一维等效土层模型的结果为参考,采用有限元法与人工透射边界相结合的方法,研究了SV波入射时入射角θ对盆地地表地震动放大特征的影响.结果表明入射角度和盆地分层构造对地表地震动放大特征均有显著影响:(1)盆地地表地震动峰值受θ影响显著,不同分量、不同区域(盆地边缘和内部区域)的位移峰...     

圆弧状沉积盆地与软土单覆盖层出平面地表运动对比

采用解析方法，通过圆弧状沉积盆地与同样参数的软土均匀单覆盖层的对比，研究沉积盆地对平面SH波二维散射效应和其出平面地表运动。数值对比结果表明：（1）波垂直入射时盆地中央地表位移幅值反应，其随频率变化形式与软土单覆盖层的地表运动有较强的对应性，且此时单覆盖层地表位移幅值为其下降；（2）盆地的二维散射效应特点显著波的汇聚作用使盆地中的地表运动显著放大，总体上较软土单覆盖层的地表位移幅值要大，其频域内的最大反应则明显大于单覆盖层情况，但最大反应在盆地表面出现的位置依赖于入射波角度、盆地深宽比和盆地的软硬程度；（3）沉积盆地中地表位移运动有强烈扭转效应，且随盆地变软和入射波频率增高而增强。     

地震动残差分析盆地附加放大效应:日本关东盆地为例

我国地震动预测及地震危险性分析通常仅考虑局部场地浅层岩土层对地震动的放大效应,不能考虑较大范围的地质条件影响,如沉积盆地厚沉积层对地震动的附加放大效应通常被忽略,造成盆地内地震动及地震危险性预测结果普遍被低估。本文以地震动观测记录数据充足的日本关东盆地为例,采用地震动残差分析方法评估盆地附加放大效应,分析覆盖层厚度、盆地内空间位置、震级、震源距对地震动放大效应的影响,建立关东盆地附加放大效应经验评估模型。分析表明:关东盆地附加放大效应与反应谱周期相关,整体上从短周期的1.0逐渐增大至周期为5s时的1.5,附加放大效应与覆盖层厚度相关性较小,主要受盆地空间位置和震源距的影响;盆地北部边缘及西北部地区附加放大效应更强烈,盆地南部附加放大效应较小,这可能与盆地边缘效应密切相关。本文建立的关东盆地附加放大效应经验模型略高于BSSA14和ASK14模型的放大效应预测。相关研究结果可用于我国地震动预测、下一代地震动区划图修订等。     

基于广义反演法的不同位置地震引起盆地放大效应的研究——以仙台盆地为例

基于强震观测记录,采用广义反演法和HVSR法分析日本仙台盆地48个强震台站以及场地v_S30和盆地深度对放大效应的影响。研究表明,相比于广义反演法,HVSR法能够较好地给出场地主频,但会明显低估放大效应的幅值;处于盆地外不同位置的浅源地震引起的盆地放大效应差异明显,仙台盆地南部海域地震引起的放大效应最大,盆地北部陆地地震引起的放大效应最小;盆地南部和东部地震引起的S波的放大效应与v_S30的相关性较强,北部地震的放大效应与v_S30基本不相关;盆地S波的放大效应与盆地深度在0.5～5 Hz频段内的相关性较强,在0.25～0.5 Hz和5～10 Hz频段内基本不相关。     

Response spectral amplification ratios from 1- and 2-dimensional nonlinear soil site models

In order to determine the effect of geometry on the ground response of 2-dimensional (2-D) basins filled with soils that can develop nonlinear response, we use three basin models with width/depth ratios 3, 6 and 10. The three basins are subjected to a suite of rock site records with various magnitudes and source distances. We compute response spectral amplification ratios at four locations on the surface of the 2-D basins, and determine the average variation of the amplification ratios with respect to excitation spectra, for peak ground acceleration (PGA) and 3 spectral periods of 0.2, 0.5, 1 s. Similarly, we compute the average response spectral amplification ratios for two 1-dimensional (1-D) nonlinear models, one having the soil profile at the basin centre and the other having a soil profile at half the depth of the basin. From the relationship between the average amplification ratios and excitation spectra, we determine the cross-over point in terms of excitation spectral values that separate the amplification range from the deamplification range. Our results show that the cross-over point varies significantly from one location to another on the ground surface and from one basin to another, in a range of 0.3–1.1g for PGA. The effects of basin geometry are very strong at weak and moderate excitation, but decrease with increasing excitation spectra in a significant portion around the basin centre. Our results provide some justification for using 1-D models for 2-D basins with a width/depth ratio ?6 if the soil site is subjected to strong ground shaking.     

A numerical coupling scheme for nonlinear time history analysis of buildings on a regional scale considering site‐city interaction effects

Seismic damage simulation of buildings on a regional scale is important for loss estimation and disaster mitigation of cities. However, the interaction among densely distributed buildings in a city and the site, ie, the “site‐city interaction (SCI) effects,” is often neglected in most regional simulations. Yet, many studies have found that the SCI effects are very important in regional simulations containing a large number of tall buildings and underground structures. Therefore, this work proposed a numerical coupling scheme for nonlinear time history analysis of buildings on a regional scale considering the SCI effects. In this study, multiple‐degree‐of‐freedom models are used to represent different buildings above the ground, while an open source spectral element program, SPEED, is used for simulating wave propagation in underlying soil layers. The proposed numerical scheme is firstly validated through a shaking table test. Then, a detailed discussion on the SCI effects in a 3D basin is performed. Finally, a nonlinear time history analysis of buildings on a regional scale is performed using the Tsinghua University campus in Beijing as a case study. The Tsinghua University campus case results show that the SCI effects will reduce the seismic responses of most buildings. However, some buildings will suffer much more severe damage when the SCI effects are considered, which may depend on the input motions, site characteristics, and building configurations.     

Site effects in 3D basins using 1D and 2D models: an evaluation of the differences based on simulations of the seismic response of Euroseistest

Site effects are one of the most predictable factors of destructive earthquake ground motion but results depend on the type of model chosen. We compare simulations of ground motion for a 3D model of the Mygdonian basin in northern Greece (Euroseistest) using different approximation for this basin. Site effects predicted using simple 1D models at many points inside the basin are compared to site effects predicted using four different 2D cross sections across the basin and with results for a full 3D simulation. Surface topography was neglected but anelastic attenuation was included in the simulations. We show that lateral heterogeneity may increase ground motion amplification by 100 %. Larger amplification is distributed in a wide frequency range, and amplification may occur at frequencies different from the expected resonant frequencies for the soil column. In contrast, on a different cross section, smaller conversion of incident energy into surface waves and larger dispersion leads to similar amplitudes of ground motion for 2D and 1D models. In general, results from 2D simulations are similar to those from a complete 3D model. 2D models may overestimate local surface wave amplitudes, especially when the boundaries of the basin are oblique to the selected cross section. However, the differences between 2D and 3D site effects are small, especially in regard of the difficulties and uncertainties associated to building a reliable 3D model for a large basin.     

Non-linear soil response in ground motions

From both theoretical and empirical studies, we know that a soft soil layer will amplify seismic waves of certain frequencies and cause damage to structures, depending on the physical properties and the thickness of the layer. Most developed cities are situated on a plane or a basin with soft geological strata. Thus, understanding the characteristics of soft soil response to seismic loading is important. Seismologists and engineers are interested in questions of linear versus non-linear soil response and isotropy versus anisotropy as a soil property. The records of the downhole accelerographs of an array in Lotung provide an opportunity to study these problems. The results show that the soil response in Lotung does not have an anisotropy effect, and significant non-linear soil response occurred during the strong ground motions in which peak ground acceleration values at ground surface are larger than 150 gal.     

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.     

汶川Ms8.0级地震前后天气温度的异常变化

根据国内12个大中城市的12个气象站实际测量的2008年5月份31天的天气最低温度和最高温度共744个数据,结合曲线图对其特征进行对比研究,表明:①南宁市和广州市的低温,北京市和济南市的高温、低温,该月最低值都出现在5月12日汶川地震当天;②其他城市该月低温最低值出现的时间为:哈尔滨市9日,西宁市10日,西安市、郑州市...     

A semi-physical sediment yield model for estimation of suspended sediment in loess region

Sediment yield is a complex function of many environmental factors including climate,hydrology,vegetation,basin topography,soil types,and land cover.We present a new semi-physical watershed sediment yield model for the estimation of suspended sediment in loess region.This model is composed by three modules in slope,gully,and stream phases.For slope sediment yield,a balance equation is established based on the concept of hydraulic erosion capacity and soil erosion resistance capacity.According to the statistical analysis of watershed characteristics,we use an exponential curve to approximately describe the spatial variability of watershed soil erosion resistance capacity.In gully phase,the relationship between gully sediment concentration and flow velocity is established based on the Bagnold'stream power function.In the stream phase,we assume a linear dependence of the sediment volume in the reach on the weighted sediment input and output.The proposed sediment yield model is operated in conjunction with a conceptual hydrologic model,and is tested over 16 regions including testing grounds,and small,medium and large watersheds in the loess plateau region in the mid-reach of Yellow River.Our results indicate that the model is reasonable in structure and is able to provide a good simulation of sediment generation and transportation processes at both flood event scale and inter-annual time scale.The proposed model is generally applicable to the watersheds with soil texture similar to that of the loess plateau region in the Yellow River basin in China.     

Simulation of Near-Fault Strong Ground Motion in the Beijing Region

On the basis of previous study of the 1679 Sanhe-Pinggu(M8.0) earthquake,the biggest event in history ever recorded in Beijing and its adjacent area,we made a 3-D strong ground motion simulation utilizing the staggered-grid finite differences method to study the distributions of peak ground velocity with different earthquake source models in the Beijing region.In the paper,earthquake source models and a transmission medium velocity model are established and the corresponding parameters are given in accordance to the results from a related previous study.Then,using a three-dimensional finite difference computing program of near-fault strong ground motion developed by Graves,the peak ground velocity caused by a destructive earthquake in the Beijing area is simulated.In our computation model,the earthquake source is 3km in depth,and a total number of 21,679 observation points on the ground surface are figured out.The transmission medium velocity model is composed of four stratums which are the Quaternary deposit,the upper crust,the upper part of the middle crust and the lower part of the middle crust.With the minimum grid spacing of 0.15km,a total of 2.28×106 grids are generated.Using a time step of 0.02 seconds we calculated the peak ground velocity for a duration of 8 seconds.After the analysis of the simulation results,we observed some basic characteristics of near-fault strong ground motion such as the concentration effect of near-fault peak ground velocity,rupture directivity effect,hanging wall effect,and basin effect.The results from our simulation and analysis suggest that the source and transmitting medium parameters in our model are suitable and the finite difference method is applicable to estimate the distribution of strong ground motion in the study region.     

Regional seismic responses of shallow basins incorporating site‐city interaction analyses on high‐rise building clusters

This study assesses the 3D amplification effects in shallow basins and quantifies the effects of site‐city interaction (SCI) on high‐rise buildings. A regional‐scale 3D spectral element simulation is conducted on the Tuen Mun‐Yuen Long basin, which contains multiple subbasins with heterogeneous and nonlinear soil profiles, while 3D city models with various building layouts are fully integrated into the basin model for our SCI study. We found a good correlation between spectral amplification factors and soil depths. Site response is significantly amplified at basin edges and centers due to surface waves generated at basin edges and the focusing effects stemming from 3D basin geometry. Transfer functions of 3D basins can be up to fourfold at fundamental frequencies as compared to 1D response, and further amplifications occur at high frequencies due to surface waves. In the SCI simulations, we observe wave trapping in the open space amid buildings resulting in energy concentration and up to twofold PGA amplifications. The wave trapping effect diminishes as the space between buildings increase beyond their range of influence (～100 m). The SCI analyses show that destructive kinetic energy in superstructures increases 28% in one horizontal direction but decreases 22% in the other. Our study concluded that, 1D site response analysis can significantly underestimate the seismic demand in shallow basins. Site‐city interaction of high‐rise buildings increases the short‐period spectra of ground motions, leading to an increase in their story accelerations by up to 50% and to a substantial decrease in the seismic safety of short structures in their vicinity.