Local Site Effects in the Town of Benevento (Italy) from Noise Measurements

— The study of ground motion amplification produced by surface geology is extremely interesting in the Benevento area, Southern Italy, as it is characterized by high seismic hazard. The present moderate-to-low seismicity makes the noise method appropriate to estimate the seismic site response in the area. The three components of seismic noise have been recorded in five sites in the Benevento metropolitan area characterized by different surface geology, in order to estimate the seismic site response. In evaluating site amplification effects we used the direct interpretation of amplitude spectra and standard spectral ratio techniques, evaluating sediment-to-bedrock, sediment-to-average and H/V spectral ratios. The temporal evolution of the noise spectra is analysed within one day, in order to assess the stationarity of the noise signal. The noise wavefield properties have been studied through polarization analyses in selected bands of frequency, where spectral peaks are observed to dominate, to better understand the real nature of those peaks. Results give evidence of low amplification levels, missing any correlation between spectral amplitudes and sediment thickness over the basement. We interpret this result as due to the poor impedance contrast between sediments and basement, which is characterized by low values of shear waves velocity. Moreover, sharp amplitude peaks are observed in the raw spectra of the sediment-sites, in the 2–4 Hz frequency band; a numerical simulation interprets this effect as possibly associated with a wide-scale structure, invoking the presence of a sharper impedance contrast at greater depth. At high frequencies the action of ambient noise sources, mainly active on horizontal components of motion, is retained dominant to generate the prominent peaks observed in the H/V spectral ratios; in some cases the presence of a near-surface low-velocity layer can contribute to amplify the seismic motion generated at these frequencies.     

HORIZONTAL-TO-VERTICAL SPECTRUM RATIO OF EARTHQUAKE GROUND MOTION FOR SITE CHARACTERIZATION

Nakamura's method, which uses a horizontal-to-vertical Fourier spectrum ratio of microtremor, has become popular to determine the predominant period and amplification of a site. In this study, this method is extended for earthquake ground motion recordings using new strong motion data recorded by JMA-87-type accelerometers. From the analysis of these accelerograms, horizontal-to-vertical Fourier spectrum ratios of a site for different earthquakes are also found to be stable irrespective of magnitude, distance and depth. To establish this fact, attenuation relations of velocity response spectra for horizontal and vertical components are derived for three damping ratios (0, 2 and 5 per cent) using the JMA data. Then the horizontal-to-vertical ratios of the velocity response spectra are obtained. The results show that the horizontal and vertical velocity response spectra are dependent on magnitude, distance and depth, but that their ratios are almost independent of magnitude, distance and depth. However, since the current data set consists of mostly intermediate to far field data, this observation should be limited to records of these distance ranges. Introducing station coefficients, representing site amplification, to this relation yields the value comparable to the horizontal-to-vertical Fourier spectrum ratio at a specific site. The stability of the spectrum ratio is explained by the transfer function between the ground surface and stiff-soil outcrop due to S-wave propagation. These results suggest that site amplification characteristics can be evaluated by one-point two-component surface recordings of earthquake ground motion, in a similar manner as proposed by Nakamura for microtremor. © 1997 by John Wiley & Sons, Ltd.     

A comparison of surface and underground array measurements of ambient noise recorded in Naples (Italy)

In this study, we describe two experiments of seismic noise measurements carried out in Naples, Italy. The site allowed measurements to be obtained both at the surface and in a tunnel that is 120-m-deep. The main goal was to compare the seismic response evaluated at the surface to the in-tunnel response, through spectral, polarization, and resonance directivity analyses. In the 1 to 20 Hz frequency band, the noise level was up to 15 dB higher at the surface than in the tunnel. The polarization properties and horizontal-to-vertical spectral ratios appear not to be influenced by the tunnel geometry or by the topography. Some preferential alignments were observed in the polarization azimuths computed at the surface, which are likely to be due to local sources, rather than morphological features. The absence of directivity effects and the low noise levels in the tunnel make this site suitable for installing seismic stations. We also studied how the subsoil structure affects the seismic motion at the surface. The dispersive properties of the Rayleigh waves were investigated using the spatial autocorrelation method. A joint inversion of the dispersion data and the horizontal-to-vertical spectral ratios provided the subsurface Vs profile. The derived model has a low velocity contrast at depth, such as to generate moderate and broad H/V spectral ratio peak amplitude. The normalized spectral ratio appears more appropriate to identify the soil-resonance frequencies.     

河北廊坊地区的设定地震灾害评估

本文对1700年以来的历史地震烈度资料进行了分析,并结合地震地质资料来确定设定地震(Scenario Earthquake),然后利用模拟强地面运动确定了廊坊地区的强地面运动灾害.研究结果表明,按照目前的地震活动趋势,廊坊地区出现大于或等于地震烈度8度的周期大约是600年.也就是说,在未来50年廊坊地区出现地震烈度8度的超越概率为8％.为此,本文建议廊坊地区应该考虑0.3g峰值加速度(Ⅷ度)作为基本地震工程设计参数.     

保定市设计地震动参数的研究

本文根据保定市及周围地区的地震地质环境，在地震危险性分析的基础上，采用等效线性一维波动方程进行土层的地震反应分析。给出50年超越概率63％、10％、2％基岩和地面的水平向峰值加速度、反应谱（场址基本烈度Ⅶ度）和地震影响系数最大值。该结果为抗震设计提供了可靠依据，具有应用价值。     

场地地震动水平/竖向谱比与地表/基底谱比差异及修正水平/竖向谱比法研究

局部场地条件是决定场地地震动强度和频谱的重要因素,基于强震动和脉动记录的统计分析,获取表征场地条件影响的特征参数已成为确定工程场地设计地震动的较经济和实用方法,特别是对于大范围或难以开展现场勘测的工程场地.利用日本KiK-net台网强震动记录计算分析了台站场地地震动水平/竖向谱比(HVSR)与地表/基底谱比(SBSR)...     

Evaluation of local site effect from microtremor measurements in Babol City,Iran

Every year, numerous casualties and a large deal of financial losses are incurred due to earthquake events. The losses incurred by an earthquake vary depending on local site effect. Therefore, in order to conquer drastic effects of an earthquake, one should evaluate urban districts in terms of the local site effect. One of the methods for evaluating the local site effect is microtremor measurement and analysis. Aiming at evaluation of local site effect across the city of Babol, the study area was gridded and microtremor measurements were performed with an appropriate distribution. The acquired data was analyzed through the horizontal-to-vertical noise ratio (HVNR) method, and fundamental frequency and associated amplitude of the H/V peak were obtained. The results indicate that fundamental frequency of the study area is generally lower than 1.25 Hz, which is acceptably in agreement with the findings of previous studies. Also, in order to constrain and validate the seismostratigraphic model obtained with this method, the results were compared with geotechnical, geological, and seismic data. Comparing the results of different methods, it was observed that the presented geophysical method can successfully determine the values of fundamental frequency across the study area as well as local site effect. Using the data obtained from the analysis of microtremor, a microzonation map of fundamental frequency across the city of Babol was prepared. This map has numerous applications in designing high-rise building and urban development plans.     

Variations of local seismic response in Benevento (Southern Italy) using earthquakes and ambient noise recordings

The city of Benevento (Southern Italy) has been repeatedly struck by large historical earthquakes. A heterogeneous geologic structure and widespread soft soil conditions make the estimation of site effects crucial for the seismic hazard assessment of the city. From 2000 until 2004, we installed seismic stations to collect earthquake data over zones with different geological conditions. Despite the high level of urban noise, we recorded more than 150 earthquakes at twelve sites. This data set yields the first, well documented experimental evidence for weak to moderate local amplifications. We investigated site effects primarily by the classical spectral ratio technique (CSR) using a rock station placed on the Benevento hill as reference. All sites in the Calore river valley and in the eastern part of the Benevento hill show a moderate high-frequency (f > 4 Hz) amplification peak. Conversely, sites in the Sabato river valley share weak-to-moderate amplification in a wide frequency band (from 1–2 to 7–10 Hz), without evident frequency peaks. Application of no-reference-site techniques to earthquake and noise data confirms the results of the CSRs in the sites of the Calore river valley and of the eastern part of the Benevento hill, but fails in providing indications for site effects in the Sabato river valley, being the H/V ratios nearly flat. One-dimensional modeling indicates that the ground motion amplification can be essentially explained in terms of a vertically varying geologic structure. High-frequency narrow peaks are caused by the strong impedance contrast existing between near-surface soft deposits and stiff cemented conglomerates. Conversely, broad-band amplifications in the Sabato river valley are likely due to a more complex layering with weak impedance contrasts both in the shallow and deep structure of the valley.     

基于小田原试验场地数据的不同基岩输入模型对预测地震动特征的影响研究

随着强震台网的密布及观测记录的增加,为研究各类局部场地地震反应预测模型的合理性提供了有效的参考依据,也使利用强震记录及场地条件研究地震动特征成为可能。选取场地地质参数资料和地震记录数据齐全的日本小田原（Ashigara Valley）盲测试验场地,通过对比不同地震动输入方式及场地反应分析模型,研究地震动特征,分析现有模型的优劣。基于1990年8月5日M5.1强震事件的地表基岩记录和地下基岩地震记录,采用地下台强震记录直接输入、地表基岩台强震记录减半为基底地震动输入、地表基岩台强震记录反演为基底地震动输入作为3种基岩地震动输入。基于局部场地条件分别建立一维等效线性模型、二维黏弹性模型及二维时域等效线性化模型等工程中常用的场地数值分析模型,进行局部场地地震反应分析,预测该盲测场地的地表地震动特征,并与对应的实测强震记录结果进行对比,分析不同基岩地震动输入方式对预测地震动特征及地表土层反应谱特征的影响,重点分析地震动输入、土体非线性、场地横向不均匀性及几何与非线性特征共同作用等因素对地表地震动特征的影响,以期为地表地震动的合理预测提供参考。     

Prediction of response spectra via real-time earthquake measurements

The development and implementation of an earthquake early warning system (EEWS), both in regional or on-site configurations can help to mitigate the losses due to the occurrence of moderate-to-large earthquakes in densely populated and/or industrialized areas. The capability of an EEWS to provide real-time estimates of source parameters (location and magnitude) can be used to take some countermeasures during the earthquake occurrence and before the arriving of the most destructive waves at the site of interest. However, some critical issues are peculiar of EEWS and need further investigation: (1) the uncertainties on earthquake magnitude and location estimates based on the measurements of some observed quantities in the very early portion of the recorded signals; (2) the selection of the most appropriate parameter to be used to predict the ground motion amplitude both in near- and far-source ranges; (3) the use of the estimates provided by the EEWS for structural engineering and risk mitigation applications.In the present study, the issues above are discussed using the Campania–Lucania region (Southern Apennines) in Italy, as test-site area. In this region a prototype system for earthquake early warning, and more generally for seismic alert management, is under development. The system is based on a dense, wide dynamic accelerometric network deployed in the area where the moderate-to-large earthquake causative fault systems are located.The uncertainty analysis is performed through a real-time probabilistic seismic hazard analysis by using two different approaches. The first is the Bayesian approach that implicitly integrate both the time evolving estimate of earthquake parameters, the probability density functions and the variability of ground motion propagation providing the most complete information. The second is a classical point estimate approach which does not account for the probability density function of the magnitude and only uses the average of the estimates performed at each seismic station.Both the approaches are applied to two main towns located in the area of interest, Napoli and Avellino, for which a missed and false alarm analysis is presented by means of a scenario earthquake: an M 7.0 seismic event located at the centre of the seismic network.Concerning the ground motion prediction, attention is focused on the response spectra as the most appropriate function to characterize the ground motion for earthquake engineering applications of EEWS.     

Seismic design spectra and mapping procedures using hazard analysis based directly on oscillator response

The calculation of design spectra for building sites threatened by seismic ground motion is approached by considering the maximum responses of linearly elastic oscillators as indicators of ground motion intensity. Attenuation functions describing the distribution of response as a function of earthquake magnitude and distance are derived using 68 components of recorded ground motion as data. With a seismic hazard analysis for several hypothetical building sites, the distributions of maximum oscillator responses to earthquakes of random magnitude and location are calculated, and spectra are drawn to indicate the maximum responses associated with specified probability levels. These spectra are compared to design spectra calculated from published methods of amplifying peak ground motion parameters. The latter spectra are found to be inconsistent in terms of risk for building sites very close and very far from faults. A ground motion parameter defined to be proportional to the maximum response of a 1 Hz, 2 per cent damped linearly elastic oscillator is investigated; this parameter, in conjunction with peak ground acceleration, is found to lead to risk-consistent design spectra. Through these two parameters, a design earthquake magnitude and design hypocentral distance are defined, for a specified building site and risk level. The use of these parameters in the seismic hazard mapping of a region is illustrated.     

基于衰减关系的城镇烈度发生概率快速估计

震后快速产出的震动烈度分布是地震应急救援非常有效的依据, 通常由烈度与地震动参数的经验关系给出. 有台站的场点, 地震动参数可以直接由台站数据给出确定性的结果; 而无台站的场点, 地震动参数只能由衰减关系给出估计值. 目前我国台站覆盖有限, 且难于实时获取, 快速生成的地震动参数主要依赖于地震动衰减关系, 再依据烈度与地震动参数的经验关系, 输出确定性的震动烈度分布. 由于衰减关系本身存在着不确定性, 将其估计值用于生成确定性的震动烈度分布是不准确的. 而且实践证明, 震动烈度与实际调查烈度存在差异. 鉴于此, 从衰减关系模型中的ε出发, 提出了场点(城镇)遭遇不同烈度的概率计算方法: 利用衰减关系的估计值与衰减关系的标准差, 构造峰值加速度(PGA)变化的对数正态分布, 然后以烈度分档对应的PGA范围, 计算震区各城镇遭遇不同烈度的概率及各城镇抗震设防烈度被超越的概率. 具体以1966年3月8日河北邢台M_S6.8地震为例, 说明了此方法的可行性, 认为以概率形式给出城镇可能遭遇的烈度在表述上更为合理, 并建议将场点(城镇)遭遇烈度的概率表达方法用于震害快速评估.      

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。     

Seismic microzonation of Chania,Crete (Greece) based on SASW measurements and non-linear site response analyses

Following a brief overview of past applications of, and more recent advances on seismic microzonation, the results of a seismic microzonation study for the city of Chania, Greece, are presented. The study was based on V_s vs. depth profiles obtained at 19 sites of the urban area by performing SASW measurements. The spatial distribution of V_s values was utilized in estimating V_s30 values, depth to bedrock and the fundamental ground period variation across the area of the city as well as for conducting 1-D finite element non-linear inelastic site response analyses. The input earthquake excitations employed in the response analyses were based on the results of an available seismic hazard study for the Chania Area. The results of analyses were utilized for establishing the spatial distribution of rock motion amplification, the expected ground motions and spectral values in the area of the city. Contour maps providing values of the expected ground motion in the urban area are given which may become a practical tool in assessing the seismic risk and expected damage in the Chania area. The maps can also be used in the design of new earthquake resistant structures or the seismic retrofitting of existing ones. Finally, the results were utilized to demonstrate the inadequacy of using V_s,30 values for classifying the soil conditions in the Chania area.     

Local site effect microzonation of Lorca town (SE Spain)

Local site effect assessment based on subsurface ground conditions is often the key to evaluate urban seismic hazard. The site effect evaluation in Lorca town (south-eastern Spain) started with a classification of urban geology through the geological mapping at scale 1:10,000 and the use of geotechnical data and geophysical surveys. The 17 geological formations identified were classified into 5 geological/seismic formations according to their seismic amplification capacity obtained from ambient vibration measurements as well as from simultaneous strong motion records. The shear-wave velocity structure of each geological/seismic formation was evaluated by means of inversion of Rayleigh wave dispersion data obtained from vertical-component array records of ambient noise. Nakamura’s method was applied to determine a predominant period distribution map. The spectral amplification factors were fourfold the values recorded in a reference hard-rock site. Finally, the capability of this study for explaining the damage distribution caused by the May 11th, 2011 Lorca destructive earthquake (Mw \(=\) 5.2) was examined. The methods used in this work are of assistance to evaluate ground amplification phenomena in urban areas of complex geology as Lorca town due to future earthquakes with applicability on urban seismic risk management.     

A PRELIMINARY STUDY ON THE CORRELATIVITY OF SEISMIC HAZARD BETWEEN BEIJING AREA AND XIONG'AN NEW AREA

Xiong'an New Area is established on April 1, 2017 and some non-capital functions from Beijing would be transferred to this new area. As a political, economic, and cultural center of China, Beijing has a highly dense population, buildings, and transportation. With the rapid development of the urban economy, the population and assets exposed to dangerous areas with M ≥ 7 earthquakes have accumulated in an exponential manner, leading to a continuous surge in seismic risk in Beijing. Studying on the correlativity of seismic hazard between Beijing area and Xiong'an New Area is of great significance to judge whether Xiong'an can play a role in the dispersal of seismic risk of Beijing. Using Monte Carlo method to simulate synthetic earthquake sequences, and for each simulated earthquake, the peak ground acceleration data sets on each site of Beijing and Xiong'an can be calculated through the attenuation relationship. Based on the statistical analysis of the ground motion peak acceleration data sets, this paper holds that the correlativity of the ground motion between Beijing area and Xiong'an New Area is not high; the probability of Beijing and Xiong'an suffering at the same time from an exceeded fortification level of ground motion effect is very low; the probability of Beijing and Xiong'an suffering at the same time from a rare ground motion effect is extremely low. Through defibering population, assets, and setting up a remote backup of earthquake emergency equipment and supplies, Xiong'an New Area can disperse the high seismic risk of Beijing to some extent.     

汶川地震强余震场地非线性反应

采用汶川地震强余震26个强震动台站记录,基于H/V谱比法,计算台站场地的强、弱震作用下场地卓越频率之比R_(fp)和体现强、弱震作用下场地H/V谱比曲线差异程度的DNL,进而识别并分析场地非线性反应特征。结果显示,R_(fp)、DNL与PGA之间存在显著相关性;其中10个台站出现明显的场地非线性反应特征;大部分台站发生场地非线性反应的PGA阈值为100 cm/s~2,部分为50 cm/s~2;实例证明,因某些台站不易通过H/V谱比法识别场地卓越频率,因此采用R_(fp)识别场地非线性反应具有一定局限性。     

Application of Microtremor Horizontal-to-Vertical Spectral Ratio (MHVSR) Analysis for Site Characterization: State of the Art

Nakamura (Q Rep Railway Tech Res Inst 30:25–33, 1989) popularized the application of the horizontal-to-vertical spectral ratio (HVSR) analysis of microtremor (seismic noise or ambient vibration) recordings to estimate the predominant frequency and amplification factor of earthquake shaking. During the following quarter century, popularity in the microtremor HVSR (MHVSR) method grew; studies have verified the stability of a site’s MHVSR response over time and validated the MHVSR response with that of earthquake HVSR response. Today, MHVSR analysis is a popular reconnaissance tool used worldwide for seismic microzonation and earthquake site characterization in numerous regions, specifically, in the mapping of site period or fundamental frequency and inverted for shear-wave velocity depth profiles, respectively. However, the ubiquity of MHVSR analysis is predominantly a consequence of its ease in application rather than our full understanding of its theory. We present the state of the art in MHVSR analyses in terms of the development of its theoretical basis, current state of practice, and we comment on its future for applications in earthquake site characterization.     

2D numerical simulations of earthquake ground motion: examples from the Marche Region,Italy

A detailed numerical simulation of the ground motion and a site response analysis for two towns in the Marche Region (Treia and Cagli) is carried out on the basis of structural models deduced from available geological and geophysical data. In both cases, the reference event is an M = 5.7 earthquake associated with a normal fault located beneath each town. The ground motion is computed using the 2D spectral element method (SPEM 2D). The method solves the propagation of the seismic field through complex geological structures and enables an estimate of the effects of deep crustal structure, superficial geology, and topography on ground motion. Numerical simulations of the seismic field are performed along 2D vertical planes containing the seismic source. Strong ground motion has not been yet recorded in the two towns; therefore, the numerical simulation of ground motion represents a way to overcome the lack of instrumental data. The simulations carried out for Treia show that ground motion is influenced by both source mechanism and effects due to propagation through the geological structure, while ground motion in Cagli features strong local effects, caused by the presence of alluvial deposits under a large area of the town.