Calibration of stochastic finite-fault ground motion simulations for the 1997 Umbria-Marche, Central Italy, earthquake sequence

The recent 1997 Umbria-Marche, Central Italy, earthquake sequence allowed us to model recorded ground motions using a method developed by Beresnev and Atkinson [Bull Seism Soc Am 87 (1997) 67–84; Seism Res Lett, 69 (1998) 27–32; Bull Seism Soc Am 88 (1998) 1392–1401]. The method generalizes the stochastic ground-motion simulation technique, developed for point sources, to the case of finite faults. It subdivides the fault plane into subfaults and assumes each subfault to be a point source with a ω² spectrum. Geometric spreading and regional anelastic attenuation are included in the model. The data include horizontal acceleration recordings from the SSN and ENEL databases of the 1997 Umbria-Marche events on 26 September, at 00:33 GMT, with M_w=5.7, and at 09:40 GMT, with M_w=6.0; and on 14 October at 15:23 GMT, with M_w=5.6. The strong motion simulations are performed using model parameters based on the results of previous studies, and adjusting the subfault size to calibrate the simulation model against recorded ground motions. Local site response is considered to account for observed amplification effects at specific recording sites (e.g. Nocera Umbra). A good agreement is found between the simulated response spectra and the recorded data, concluding that this method reproduces the salient ground-motion characteristics at different distances and azimuths.     

Calculation of free field response spectrum of a non-homogeneous soil deposit from bed rock response spectrum

In this paper, a method to determine free field response spectrum of a soil deposit from a specified bed rock response spectrum is presented. This method treats the earthquake motion as if it was a stationary Gaussian random process but to account for the non-stationary character, an approximate method is used. The soil deposit is assumed to have mechanical properties (strength, shear modulus, etc.) increasing with some power exponent of depth. This layer overlies either a compliant elastic half space or a layer with shear modulus increasing linearly with depth.To demonstrate the validity and usefulness of this approach, two examples are presented. The first one consists in the transfer, from bed rock to free surface of a given soil profile, of the response spectrum derived from an accelerogram used to generate the Eurocode 8 response spectrum. The obtained free field response spectrum is compared to the one obtained by using a wave propagation program. The second example consists in the validation of this method with experimental records.     

基于《电力设施抗震设计规范》的地震动随机模型参数研究

依据《电力设施抗震设计规范》（GB50260-96）中的设计反应谱,对随机地震动功率谱参数的取值进行了具体研究。首先,采用时间包络函数考虑地震的非平稳特性,给出了地震动持时的取值;然后,根据加速度峰值等效原则,迭代计算得到地面的加速度功率谱密度曲线;最后,选定Clough-Penzien修正过滤白噪声模型作为拟合函数,通过非线性拟合技术拟合了与《规范》中的地震烈度、场地类别相对应的谱参数。研究结果表明,Clough-Penzien修正过滤白噪声模型能较好地拟合其曲线形状。本文给出了相对于规范中的地面加速度功率谱参数值,可供这种模型作为地震地面运动输入时选用。     

九寨沟MS7.0地震强地震动模拟及漳扎镇地震动强度预测

破坏性地震强度预测可用于工程领域抗震设防以及地震危险性分析评估,是防震减灾中一项很重要的基础工作.为了再现九寨沟地震的地震动强度,评估缺失强震记录的九寨章扎台站的地震动强度,本文用经验格林函数法对九寨沟地震进行了数值模拟.选取了震源周边地震动峰值加速度超过10 Gal的10个强震台站进行模拟.因未得到九寨沟地震的余震,初次尝试将汶川地震和定西地震的余震作为格林函数模拟九寨沟地震.模拟结果整体上可以反映各台站地震动的强度特征,尤其是地震动高频成份拟合较好.模拟值的地震动峰值加速度、时程数据、反应谱等与观测值拟合较好.预测结果显示漳扎镇的地震动峰值加速度值约为180~200 Gal.预测结果也表明在缺少大震的余震记录时,经验格林函数法使用其他大震的余震同样可以再现目标地震的强度特征.本研究也为经验格林函数方法在缺乏小震记录地区的使用积累了经验.最后总结了格林函数的选取标准,为经验格林函数方法来预测未来强震动特征积累了经验.

     

Ambient noise as the new source for urban engineering seismology and earthquake engineering: a case study from Beijing metropolitan area

In highly populated urban centers, traditional seismic survey sources can no longer be properly applied due to restrictions in modern civilian life styles. The ambient vibration noise, including both microseisms and microtremor, though are generally weak but available anywhere and anytime, can be an ideal supplementary source for conducting seismic surveys for engineering seismology and earthquake engineering. This is fundamentally supported by advanced digital signal processing techniques for effectively extracting the useful information out from the noise. Thus, it can be essentially regarded as a passive seismic method. In this paper we first make a brief survey of the ambient vibration noise, followed by a quick summary of digital signal processing for passive seismic surveys. Then the applications of ambient noise in engineering seismology and earthquake engineering for urban settings are illustrated with examples from Beijing metropolitan area. For engineering seismology the example is the assessment of site effect in a large area via microtremor observations. For earthquake engineering the example is for structural characterization of a typical reinforced concrete high-rise building using background vibration noise. By showing these examples we argue that the ambient noise can be treated as a new source that is economical, practical, and particularly valuable to engineering seismology and earthquake engineering projects for seismic hazard mitigation in urban areas.     

超随机特性对地震动谱特性和时间历程的影响

本文从单个地震动样本的角度分析了超随机部分对地震动波形的影响,认为自相关函数噪声部分或功率谱的随机波动部分对地震动波形的影响不大,对地震动的波形起主要控制作用的是地震动的相位谱或相位差谱,而自相关函数的噪声部分（或功率谱的随机波动成分）只对地震动波形起一定程度的调整作用。文中基于平稳随机过程的理论,用蒙特卡罗方法分析了自相关函数噪声项对人工合成地震动振幅和反应谱的影响：由于自相关函数的随机噪声项对地震动总功率或平方和的贡献为零,因此随机噪声项对合成地震动幅值的影响不大,但是由于自相关函数随机噪声项明显改变了地震动功率谱的形状和分布特征,使其出现了明显的波动,也在一定程度上提高了平均反应谱的峰值,从而对平均反应谱也产生一些影响。考虑地震动自相关函数中的随机噪声部分有助于更真实地模拟和反映地震动的真实特性。     

强震地面运动超随机特性的几点注记

本文详细论述了功率谱与功率谱密度、平方和SS与平方平均值MS的适用范围及物理意义,比较深入地分析了平稳和非平稳加速度时间历程的统计特征,通过引入时域包线获取特定相位的方法,考虑了非平稳特性。本文基于非平稳时间历程,在不考虑频率非平稳性的基础上,提出了设计地震动三要素:(1)平方和SS;(2)归一化自相关函数或其对应的功率谱;(3)地震动时域包线.本文提出了在设计地震动合成中考虑超随机特性的实用方法,并指出地震动的超随机特性(即归一化功率谱的锯齿状波动部分,或归一化自相关函数的随机噪声部分)与相位的随机性是地震动随机特性的两个重要方面。本文将地震动的统计分析直接建立在非平稳时间历程的基础之上,从而避开了要对地震动进行平稳化处理的困难。这样提取出的地震动统计特征完全是以真实地震动记录为基础的。应用这些统计特征,可模拟出符合统计要求的基于非平稳时间历程的设计地震动。最后本文还讨论了工程上常用的将反应谱近似转换为功率谱的转换式的一些缺陷。