用数值模拟方法研究北京盆地对3～10s地震动的放大效应

以北京盆地为例,针对不同的设定地震,研究了不同震源条件对盆地放大效应的影响。结果表明:尽管震源因素可对计算结果带来一些不确定性,但盆地内沉积物的厚度是控制3~10 s周期段地震动加速度反应谱的主要因素。因此,计算了盆地对3~10 s周期段地震动加速度反应谱的平均放大系数,并通过回归分析得到了平均放大系数与盆地等效沉积物厚度之间的函数关系。最后,结合北京盆地内高层建筑的分布,初步探讨了高层建筑地震致灾风险水平与盆地结构的关系。     

兰州黄土场地地震动参数量化研究

为了进一步了解土体性质(Q3和Q4黄土)和土层厚度对黄土场地地震动效应的定量影响规律,利用等效线性化波动分析方法,在总结了兰州地区不同年代黄土波速值随土层变化的基础上,结合具体的黄土动力学参数及现场波速测试成果,开展黄土场地在不同性质黄土覆盖条件下地表峰值加速度PGA和黄土场地特征周期Ts等指标变化特征的场地地震反应计算。本文在考虑50年超越概率分别为63%,10%,2%的兰州人造地震波荷载作用下,探讨了不同性质黄土覆盖层的影响,并进一步建立了不同条件下黄土场地地震动反应谱特征周期计算公式。研究结果表明:黄土的性质和厚度对地表放大系数影响较大,随着土层厚度的增加,地表峰值加速度放大系数具有非线性的特点,整体上随土层厚度的增大而增大。在两种性质黄土覆盖条件下,反应谱特征周期变化规律相同,呈对数变化特点。随着土层厚度增大,反应谱特征周期不断增大,增长速率逐渐减小。     

用位移反应谱研究长周期设计地震反应谱

利用美国太平洋地震工程研究中心NGA数据库,分析了滤波低频截止周期(Tc)对位移反应谱的影响。选择较小的Tc将严重抑制长周期位移反应谱幅值。本文选用可靠使用周期不小于10 s的强地面运动加速度记录,研究了位移反应谱的长周期特性。结果表明,位移反应谱峰点周期是长周期地震反应谱的关键控制参数,受场地条件和震级的影响相对较小,但与震中距基本呈线性增长关系,而动力放大系数最大值波动范围较小,设计中可视为常数。建议在现行抗震设计规范谱的基础上,在速度控制段与位移控制段之间增加一个过渡段,以体现位移反应谱的有峰特征,并给出了控制参数的建议值。     

基于汶川地震远场强震动记录的厚覆盖土层对长周期地震动影响分析

利用2008年5月12日汶川8．0级地震的远场强震动记录,研究太原盆地厚覆盖土层对长周期地震动的影响,并将其结果与地震安全性评价工作中的土层反应分析结果进行比较。结果表明：厚覆盖土层对长周期地震动有很强的放大作用,覆盖层厚度越厚,峰值出现的周期越大,而相对于基岩最大放大可达9倍左右。另外,对于厚覆盖层的场地,观测结果与场地地震安全性评价计算结果在一定的长周期范围内差别相当大,相对于基岩,观测结果的放大系数至少高于场地地震安全性评价计算结果的2倍以上,最大达到近9倍。     

集集地震地震动频谱参数研究

地震动的频谱特征是影响结构动力反应的重要因素。以集集地震动记录为数据基础,对反映地震动频谱特征的4个参数周期(反应谱卓越周期Tp,平滑化反应谱卓越周期To,傅氏幅值谱平均周期Tm和反应谱特征周期Tg)进行了研究。依据理论模型,经非线性回归分析,给出了不同场地上地震动各周期分量随断层距变化的经验公式。结果表明,不同的频谱参数周期反映地震动不同的频谱特征;同一场地上各周期随断层距的增大而增大;To和Tm具有较好的可估性。     

邢台地区长周期地震动特征初步分析

沉积平原中存在地表低速沉积层是长周期地震动产生的关键因素,而长周期地震动会对长周期建筑物造成严重威胁和震害。本文基于邢台地震的历史地震震源模型和含地表沉积层的地壳速度模型,采用离散波数有限元法,对邢台地区进行了长周期地震动的计算分析。研究表明,邢台平原地区在大震中会产生长周期地震波,邢台地震中反应谱峰值区域位于宁晋南,与极震区基本重合。震中距小于55km区域内的周期1.5—4s的放大系数谱超过了抗震设计规范谱,反应谱放大系数值在周期2s附近达到峰值。因此,邢台地区因存在地表软弱沉积层,该区抗震设防可能对长周期地震动估计不足,建议应予以特别考虑。     

输入界面对反应谱特征周期的影响

为了研究输入界面对反应谱特征周期的影响,在研究我国数百个工程场地钻孔资料的基础上,选取了5个有代表性的土层剖面,利用目前工程上广泛应用的场地地震反应分析方法(一维等效线性化波动方法),计算了这些场地在6种不同强度地震动时程输入下的反应谱特征周期。结果表明,当覆盖土层厚度薄时,特征周期随输入界面剪切波速的增大而减小;当覆盖土层厚度一般时,特征周期随输入界面剪切波速的增大没有一定的规律;当覆盖土层厚度很厚时,特征周期随输入界面剪切波速的增大而增大,特别是输入地震强度较大时,增速较快;改变基岩剪切波速,对特征周期影响很小。     

考虑近断层方向性效应及土层非线性影响的场地概率危险性分析

针对一典型砂土场地基于等效线性分析方法,研究了土层对输入近断层地震动脉冲特性的影响。选取两组近断层脉冲和非脉冲地震动对地表加速度反应谱放大系数进行了对比研究。结果表明：基岩非脉冲地震动经过土层的高频滤波作用传至地表后有转换成脉冲地震动的可能;基岩脉冲地震动经土层传播至地表后仍具有脉冲特性,部分含有潜在第二脉冲的地震动由于共振作用会产生附加脉冲。与非脉冲地震动相比,脉冲地震动会引起土层较强的非线性响应,导致加速度反应谱放大系数随基岩加速度值的增大衰减较快;而对于长周期加速度反应谱（3 s）,脉冲地震动引起的放大系数在基岩加速度大于0.1 g时远大于非脉冲地震动引起的放大系数。最后通过回归分析得到场地放大系数方程,建立了一种考虑近断层脉冲特性及土层非线性相互影响的场地概率地震危险性分析方法。     

考虑近断层方向性效应及土层非线性影响的场地概率危险性分析

针对一典型砂土场地基于等效线性分析方法,研究了土层对输入近断层地震动脉冲特性的影响。选取两组近断层脉冲和非脉冲地震动对地表加速度反应谱放大系数进行了对比研究。结果表明：基岩非脉冲地震动经过土层的高频滤波作用传至地表后有转换成脉冲地震动的可能;基岩脉冲地震动经土层传播至地表后仍具有脉冲特性,部分含有潜在第二脉冲的地震动由于共振作用会产生附加脉冲。与非脉冲地震动相比,脉冲地震动会引起土层较强的非线性响应,导致加速度反应谱放大系数随基岩加速度值的增大衰减较快;而对于长周期加速度反应谱（3 s）,脉冲地震动引起的放大系数在基岩加速度大于0.1 g时远大于非脉冲地震动引起的放大系数。最后通过回归分析得到场地放大系数方程,建立了一种考虑近断层脉冲特性及土层非线性相互影响的场地概率地震危险性分析方法。     

基于等损伤的主余震序列型地震动PGA放大系数谱研究

大地震发生后往往会伴随强余震发生,强烈余震会加重结构的破坏程度,在抗震设计及结构损伤评估中考虑余震的影响是一重要课题。采用NGA-West2数据库建立主余震序列型地震动记录数据集,基于损伤等效思想定义主余震作用下PGA放大系数α,针对单自由度体系结构,通过动力时程分析建立不同相对强度、不同场地条件下平均PGA放大系数谱,进一步通过回归分析构建PGA放大系数谱的预测方程,分析统计结果的离散性。结果表明：PGA放大系数谱受场地条件影响较小,受主余震相对强度影响显著;放大系数谱值随周期增大而减小。谱预测方程能够提供目标损伤下结构的主震PGA放大需求,并可以作为设计谱调整系数使用,以实现在结构抗震设计及结构损伤评估中考虑余震的影响。     

Prediction of ground motion in the Osaka sedimentary basin associated with the hypothetical Nankai earthquake

We studied the long-period ground motions in the Osaka sedimentary basin, Japan, which contains a 1- to 3-km thickness of sediments and is the site of many buildings or construction structures with long-natural period. We simulated the broadband ground motions likely to be produced by the hypothetical Nankai earthquake: the earthquake expected to give rise to the most severe long-period ground motion within the basin. For the simulation, we constructed multiscale heterogeneous source models based on the Central Disaster Management Council of Japan (CDMC) source model and adopted a hybrid computation method in which long-period motion and short-period motion are computed using a 3-D finite difference method and the stochastic Green’s function method, respectively. In computing long-period motions, we used a 3-D structure model of the crust and the Osaka sedimentary basin. The ground motions are estimated to have peak velocities of 50–90 cm/s, prolonged durations exceeding 300 s, and long predominant periods of 5–10 s in the area with great thickness of sediments. The predominant periods are in agreement with an approximate evaluation by 4 H/V _s where H and V _s are the thickness of the sediment and the average S wave velocity, respectively.     

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。     

松辽盆地地壳精细结构研究

基于东北地区已有的宽频带流动台阵远震数据,利用波场延拓和分解的H-β网格搜索法,对松辽盆地的沉积层及地壳结构进行了深入分析。结果显示:松辽盆地的沉积层厚度为0.2—2.5 km,整体呈现中央坳陷区厚、边缘薄且西南地区最薄的分布特征;研究区地壳较薄,厚度介于24—34 km之间,其横向变化特征与沉积层厚度分布具有一定的对应性。依据沉积层和地壳的厚度计算了地壳伸展系数,其平均值接近于以往接收函数研究估测的岩石圈伸展因子。因此,本文推测松辽盆地在伸展构造过程中,其地壳和岩石圈的减薄以纯剪切模式为主。此外,松辽盆地具有较高的地壳平均波速比v_P/v_S,暗示盆地下方岩石圈地幔的减薄过程中可能存在岩浆的底侵作用。      

Validation of 3-D basin structure models for long-period ground motion simulation in the Osaka basin, western Japan

We studied the applicability of two types of existing three-dimensional (3-D) basin velocity structure models of the Osaka basin, western Japan for long-period ground motion simulations. We synthesized long-period (3–20 s) ground motions in the Osaka basin during a M6.5 earthquake that occurred near the hypothetical Tonankai earthquake source area, approximately 200 km from Osaka. The simulations were performed using a 3-D finite-difference method with nonuniform staggered grids using the two basin velocity structure models. To study the ground motion characteristics inside the basin, we evaluated the wave field inside the basin using the transfer functions derived from the synthetics at the basin and a reference rock site outside the basin. The synthetic waveforms at the basin site were obtained by a convolution of the calculated transfer function and the observed waveform at the reference rock site. First, we estimated the appropriate Q values for the sediment layers. Assuming that the Q value depends on the S wave velocity V _S and period T, it was set to Q = (1/3V _S)(T ₀/T) where V _S is in m/s and the reference period T ₀ is 3.0 s. Second, we compared the synthetics and the observations using waveforms and pseudovelocity response spectra, together with a comparison of the velocity structures of the two basin models. We also introduced a goodness-of-fit factor to the pseudovelocity response spectra as an objective index. The synthetics of both the models reproduced the observations reasonably well at most of the stations in the central part the basin. At some stations, however, especially where the bedrock depth varies sharply, there were noticeable discrepancies in the simulation results of the models, and the synthetics did not accurately reproduce the observation. Our results indicate that the superiority of one model over the other cannot be determined and that an improvement in the basin velocity structure models based on simulation studies is required, especially along the basin edges. We also conclude that our transfer function method can be used to examine the applicability of the basin velocity structure models for long-period ground motion simulations.     

Validation of the Deep Velocity Structure of the Tokachi Basin Based on 3-D Simulation of Long-Period Ground Motions

The construction of 3-D basin velocity structures is ongoing in many regions of Japan. The structure models are constructed mainly for the prediction of long-period ground motions from future large earthquakes. In this paper, we validate the 3-D velocity structure model of the Tokachi basin, a deep sedimentary basin located in eastern Hokkaido, Japan, based on 3-D simulation of long-period (2–20 s) ground motions from three nearby intermediate-depth earthquakes; this model was constructed by the National Research Institute for Earth Science and Disaster Prevention (NIED). We make comparisons between the observed and synthetic long-period ground motions for the basin-induced surface waves as well as the direct S-wave. We also try to revise the 3-D velocity structure in the western part of the Tokachi basin based on 1-D velocity structures estimated using long-period S-wave modeling and the microtremor survey method. We then perform the 3-D simulation again to validate the revised model. Based on quantitative comparisons of the long-period ground motions from these simulations with those observed, we conclude that the NIED and revised velocity structure models are generally good at the central basin sites, but that both models require modification at the basin edges to explain the details of the observed basin-induced surface waves.     

SH波入射下圆弧状沉积盆地地震反应瞬态解析解计算

地震波散射问题的解析解是研究局部场地、地形、盆地等不规则地层结构对地震动参数放大效应影响的重要理论工具。现有解析解大部分在频域内给出，无法直接用于研究不规则地层结构对地震动峰值、反应谱等参数的放大效应。本文基于平面SH波入射下圆弧状沉积盆地动力响应宽频带稳态解析解，通过Fourier变换，获取瞬态响应解析解。基于此，研究El Centro波入射下，沉积盆地对地震动峰值加速度、峰值速度、峰值位移及不同周期反应谱的放大效应。研究结果表明，盆地宽度和深度、沉积介质波速、入射波角度等对盆地放大效应具有显著影响，地震动反应谱谱比最大值超过2.0，且宽度达10 km的较大型盆地对长周期地震动参数具有显著放大效应，对于位于该类盆地的超高层建筑、大型储液罐、大跨度桥梁等长周期结构，应充分考虑盆地对抗震设防参数的影响。     

A method to extract successive velocity pulses governing structural response from long-period ground motion

A series of relatively long-period velocity pulses appearing in the later part of ground motion, which is the characterization of far-source long-period ground motions in basin (“long-period ground motion” for short), is mainly influenced by focal mechanism, basin effect, and dispersion. It was supposed that the successive low-frequency velocity pulses in long-period ground motion caused the resonance of long-period structures in basin, which are of special concern to designers of super high-rise buildings. The authors proposed a wavelet-based successive frequency-dependent pulse extraction (WSFPE) method to identify and extract these pulses with dominant period of interest from long-period ground motions. The pulses extracted by using two frequently used methods (zero-crossing analysis, empirical mode decomposition) were compared to the pulses extracted by using WSFPE. The results demonstrate that the WSFPE provides higher resolution in time–frequency domain than the other two methods do. The velocity pulses extracted by using WSFPE are responsible for the resonance and maximum response of structure subjected to long-period ground motions. WSFPE can be used to make a better understanding of long-period ground motions and to promote the formation of long-period ground motion model which will help the seismic design of long-period structures built in sedimentary basin.     

云南玉溪盆地电性结构MT测深研究

利用大地电磁测深技术探测了云南玉溪盆地的电性结构及可能的断裂构造分布。在与地质构造近垂直的方向,布设了两条测线进行大地电磁观测,应用Rebocc二维反演方法获得了玉溪盆地两条测线的视电阻率剖面。结果显示,低阻体厚度的变化趋势与盆地第四系沉积层平均厚度的变化类似,在盆地西侧,厚度较深,高阻和低阻界面变化较陡。自西向东,低阻体厚度逐渐减小,高低阻界面变化趋势趋缓。总体来说,低阻体厚度比已有资料给出的沉积层厚度要深,导致这一结果的原因一方面可能是盆地沉积层厚度在不同的区域变化较大,另一方面也可能和沉积层下的岩层富含水资源有关。     

Site effects observed in alluvial basins: the case of Norcia (Central Italy)

In this work, we investigate the site amplification effects observed in the Norcia plain, Central Italy. Data from 30 selected local earthquakes (2 ≤ Ml ≤ 4.1) recorded by a temporary seismic network composed by 15 stations, are analyzed to determine the spatial variability of site effects. Both the Horizontal-to-Vertical spectral ratio and the Standard Spectral Ratio techniques are applied to estimate the site amplification effects. The results show that most of the sites in the valley are affected by strong amplifications (up to a factor of 20) in the frequency range 0.5–5 Hz. The value of the fundamental frequency of resonance is strictly dependent on the location within the basin and on the sediment thickness. Strong amplifications also affect the vertical components. The time-frequency analysis performed on a station located inside the basin shows the presence of a large spectral amplitudes after the S-wave phase, not observed on a station located on the bedrock, suggesting the presence of locally generated wave trains. Then, in agreement with earlier observations for other alluvial basins in Central Italy, 2D–3D effects play an important role in determining the site amplification effects in Norcia.