三河—平谷地区地脉动H/V谱比法探测:场地响应、浅层沉积结构及其反映的断层活动

1679年三河—平谷8级地震,造成了包括北京在内的华北广大地区严重的人员和经济损失,但是该地区和地震灾害密切相关的沉积结构和地震场地响应特征研究,相对比较缺乏.本文利用1~4 km台间距的高密度单台地脉动观测、几个小孔径台阵观测和钻孔测井数据,研究了三河—平谷地区的浅层沉积的场地响应和沉积层三维起伏特征,以及构造运动对沉积厚度的影响.结果表明,研究区沉积层的场地共振频率和沉积厚度有明显的分区特征：东北部场地共振频率主要在1.0~7.0 Hz之间,具有相对较薄的沉积厚度,沉积界面起伏也相对平稳;西南部场地共振频率大部分区域小于1.0 Hz,可低至0.3 Hz,相应的沉积厚度起伏显著,其中在大厂凹陷厚度可达300~600 m.沉积起伏和隐伏断裂带的分布有较好的对应关系,反映了夏垫断裂、南苑—通县断裂第四纪以来较为活跃的正断活动.北东走向的夏垫等隐伏断裂在东北部山前的正断活动相对不发育,以及东北部和西南部沉积厚度的显著差异,都可能受到北西走向的二十里长山断裂带活动的影响.按照Vs30场地分类,研究区内沉积较厚的凹陷地区主要为E类软土,其他区域为D类中硬土.研究结果与区域地质调查、钻孔数据和浅层地震勘探剖面结果基本相符,说明地脉动探测方法能够高效、低成本的获取区域沉积厚度和场地作用,为地震小区划和抗震设防提供有意义的参考.     

三河—平谷地区地脉动H/V谱比法探测:场地响应、浅层沉积结构及其反映的断层活动

1679年三河—平谷8级地震,造成了包括北京在内的华北广大地区严重的人员和经济损失,但是该地区和地震灾害密切相关的沉积结构和地震场地响应特征研究,相对比较缺乏.本文利用1~4 km台间距的高密度单台地脉动观测、几个小孔径台阵观测和钻孔测井数据,研究了三河—平谷地区的浅层沉积的场地响应和沉积层三维起伏特征,以及构造运动对沉积厚度的影响.结果表明,研究区沉积层的场地共振频率和沉积厚度有明显的分区特征：东北部场地共振频率主要在1.0~7.0 Hz之间,具有相对较薄的沉积厚度,沉积界面起伏也相对平稳;西南部场地共振频率大部分区域小于1.0 Hz,可低至0.3 Hz,相应的沉积厚度起伏显著,其中在大厂凹陷厚度可达300~600 m.沉积起伏和隐伏断裂带的分布有较好的对应关系,反映了夏垫断裂、南苑—通县断裂第四纪以来较为活跃的正断活动.北东走向的夏垫等隐伏断裂在东北部山前的正断活动相对不发育,以及东北部和西南部沉积厚度的显著差异,都可能受到北西走向的二十里长山断裂带活动的影响.按照Vs30场地分类,研究区内沉积较厚的凹陷地区主要为E类软土,其他区域为D类中硬土.研究结果与区域地质调查、钻孔数据和浅层地震勘探剖面结果基本相符,说明地脉动探测方法能够高效、低成本的获取区域沉积厚度和场地作用,为地震小区划和抗震设防提供有意义的参考.

     

On the use of microtremor recordings in seismic microzonation

Experimental methods involving microtremor recordings are useful for determining site effects in regions of moderate seismic activity where ground motion records are few, and in urban or industrial contexts where the noise level is high. The aim of this study is to establish a microzonation by using the Nogoshi–Nakamura method,[1, 2] a simple experimental technique based on microtremor recordings. Since the physical phenomena underlying the method are only partially understood, the spectral responses obtained cannot be used alone. We, therefore, complete our experimental results by comparing them with the solutions of a one-dimensional numerical simulation (SHAKE91).[3, 4] The experimental programme was carried out on a plain near the Rhone Delta (South of France). H/V spectral ratios were calculated at 137 noise measurement points. In addition, we were able to compute the numerical transfer functions from soil columns defined by geotechnical characteristics inside the studied region. A comparison of the results obtained by the experimental and numerical methods showed that the fundamental frequencies are in good agreement, but that the amplitudes obtained by the two techniques are sometimes different. The analysis of H/V spectral ratios enabled us to establish maps to characterize the region: a resonance frequency map and maps of amplification levels as a function of frequency range, leading to a seismic microzonation for the whole of the region.     

Use of microtremor measurement for assessing site effects in Northern Belgium – interpretation of the observed intensity during the MS = 5.0 June 11 1938 earthquake

To provide quantitative information on site effects in the northern part of Belgium, forty-seven H/V microtremor measurements were performed with 5 second seismometers over an area of about 15.000 km². Most of the results show a northward regular increase of the fundamental period in agreement with the augmentation of the Mesozoic and Cainozoic soft sedimentsthickness from a few meters 40 km south of Brussels to 900 m at the Netherlands-Belgium border. The measured resonance frequency values wereconsistent with theoretical computations performed at different sites onthe basis of existing information and shallow seismic experiments. At oneparticular site (Uccle) where borehole data were available, microtremor measurements using an array of four seismological stations with differentapertures allowed to obtain the low frequency part of the Rayleigh wave dispersion curve, extending the range covered by the analysis of surfacewaves artificially generated. The Vs profile derived from the surface waveinversion corroborates the 1 Hz natural frequency of the site. Comparison of these results with the macroseismic information concerning the M_S = 5.0 1938 earthquake which occurred 50 km west of Brussels,confirmed the hypothesis that the geological structure of the Brabant massifis likely to control damage distribution during such an earthquake. Comparisonbetween the intensity map of the 1938 earthquake and the resonance period ofsediments obtained by our microtremor study shows a clear relation betweenthe two parameters. During the 1938 earthquake, site effects played a prominent role due to the dimension of the source whose corner frequency wasabout 1 Hz.     

Evaluation of site effects in some localities of ‘Alta Val Tiberina Umbra’ (Italy) by numerical analysis

In the paper the application of a methodology for the evaluation of site effects in an area of Umbria Region (Italy) is shown. In the project 20 localities were analyzed. In this paper the results of some representative localities are presented. The objective of the project was to develop a methodology that may be easily repeated, in order to take into account site effects in urban planning. Therefore a series of geologic, geomorphologic (1:5.000 scale) and geotechnical analyses have been carried out, to identify the areas affected by site effects and to characterize the lithotechnical units. The expected seismic inputs have been calculated and the numerical analyses 1D and 2D have been done. The results, in terms of elastic pseudo-acceleration spectra and amplification coefficients, as a ratio between spectral intensity of output and input, give elements for urban planning, both for urban general choice and for buildings design.     

Seismicity, seismic input and site effects in the Sahel—Algiers region (North Algeria)

Algiers city is located in a seismogenic zone. To reduce the impact of seismic risk in this Capital city, a realistic modelling of the seismic ground motion (SGM) is conducted by using the hybrid method that combines the finite differences method and the modal summation. For this purpose, a complete database of geological, geophysical and earthquake data is constructed. A critical re-appraisal of the seismicity of the zone [2.25°E–3.50°E, 36.50°N–37.00°N] is performed and an earthquake list, for the period 1359–2002, is compiled. The analysis of existing and newly retrieved macroseismic information allowed the definition of earthquake parameters of macroseismic events for which a degree of reliability is assigned. Geological cross sections have been built up to model the SGM in the city, caused by the 1989 Mont-Chenoua and the 1924 Douéra earthquakes. Synthetic seismograms and response spectral ratio is produced for Algiers, and they show that the soft sediments in Algiers centre are responsible of the noticed amplification of the SGM.     

Applicability of the H/V spectral ratio of microtremors in assessing site effects on seismic motion

The authors examine the reliability of site response estimations obtained by the horizontal to vertical (H/V) spectral ratios of microtremors by means of cross‐validation with the ratio of the horizontal spectra of earthquake motion with respect to reference sites. The data comprise microtremor and ground motion records recorded at 150 sites of Yokohama strong motion array. The use of non‐supervised pattern recognition techniques aims to group the sites with more objectivity. Attributes defining the overall shape of the amplification spectra serve as input in the computation of Euclidean distance similarity coefficients amongst sites. The implementation of the Ward clustering scheme leads to the attainment of a meaningful tree diagram. Its analysis shows the possibility of summarizing the results into six general patterns. A good coincidence of site effects estimates at 80 per cent of the sites becomes apparent. However, this coincidence appears poor for sites characterized by H/V amplification ratios around 2 or smaller and predominant periods longer than 0.5 s. In such cases, the presence of stiff, sandy sediments in the soil profile proves common. To proscribe H/V estimations, relying solely on the small spectral ratios criterion seems inadequate. Copyright © 2001 John Wiley & Sons, Ltd.     

Soil characteristics in Doon Valley (north west Himalaya,India) by inversion of H/V spectral ratios from ambient noise measurements

Past and recent observations have shown that the local site conditions significantly affect the behavior of seismic waves and its potential to cause destructive earthquakes. Thus, seismic microzonation studies have become crucial for seismic hazard assessment, providing local soil characteristics that can help to evaluate the possible seismic effects. Among the different methods used for estimating the soil characteristics, the ones based on ambient noise measurements, such as the H/V technique, become a cheap, non-invasive and successful way for evaluating the soil properties along a studied area.In this work, ambient noise measurements were taken at 240 sites around the Doon Valley, India, in order to characterize the sediment deposits. First, the H/V analysis has been carried out to estimate the resonant frequencies along the valley. Subsequently, some of this H/V results have been inverted, using the neighborhood algorithm and the available geotechnical information, in order to provide an estimation of the S-wave velocity profiles at the studied sites.Using all these information, we have characterized the sedimentary deposits in different areas of the Doon Valley, providing the resonant frequency, the soil thickness, the mean S-wave velocity of the sediments, and the mean S-wave velocity in the uppermost 30 m.     

Site characterizations for the Iranian strong motion network

Twenty-six sites of the Iranian strong motion network, having provided numerous records of good quality, were selected for a site effect study with the objective of obtaining a reliable site categorization for later statistical work on Iranian strong motion data. For each site, superficial Vp and Vs profiles were measured with refraction techniques, microtremor recordings were obtained and analysed with the H/V technique and the available three-component accelerograms by the receiver function technique. The aggregation of these results allows the proposition of a four-class categorization based on the H/V spectral ratio of strong ground motions, which demonstrate a satisfactory correlation with the S-wave velocity profile. Iran has a particular geological and meteorological situation compared to other seismic countries such as Japan or California, a mountainous country with dry weather conditions and a low water table in most areas. These conditions result in a relatively small number of sites with low frequency amplification, while many sites exhibit moderate amplifications in the intermediate and high frequency range.     

Effects of local site and building parameters on damage distribution in Angra do Heroísmo—Azores

During the M_w=6.9 January 1, 1980 earthquake in the Azores, the damage distribution in the town of Angra do Heroísmo, in Terceira Island, had a non-homogeneous spatial character. In order to understand the observed pattern of damage a microtremor survey was carried out, using ambient vibrations, and the data were analysed with the J-Sesame software, following the H/V Nakamura methodology. The results are in good agreement with the surface geology and they show a good correlation with the observed damages. Two building surveys were performed, in 1980 on damage evaluation and in 2000 on characterization of few building parameters. With all these elements (surface geology type, soil peak frequency, building type, number of stories and damage level) for 2111 buildings it was possible to perform several interparameter analysis, which enabled the identification of indicators that could be used to improve the damage estimation for future events.     

Application of horizontal-to-vertical (H/V) Fourier spectral ratio for analysis of site effect on rock (NEHRP-class B) sites in Taiwan

The frequency-dependent amplification for rock (NEHRP-class B) sites was studied using earthquake ground-motion database collected in Taiwan during implementation of the Taiwan Strong Motion Instrumentation Program. The database used includes several hundred records from earthquakes of M_L 4.0–7.3 occurred between 1993 and 2004. The characteristics of amplification were evaluated using the well-known technique of horizontal-to-vertical Fourier spectral ratio (H/V) of the S-wave phase [Lermo J, Chavez-Garcia FJ. Site effect evaluation using spectral ratios with only one station. Bull Seism Soc Am 1993;83:1574–94]. The study allows us to analyze peculiarities of rock sites amplification in Northern and Eastern Taiwan. It was suggested to divide the NEHRP-class B site amplification into four types based on frequency of maximum amplification and the shape of amplification function. The applicability of the technique was also checked for a few stiff and soft soil sites (NEHRP-classes D and E).     

Empirical evaluation of microtremor H/V spectral ratio

The objective of this work is to perform a purely empirical assessment of the actual capabilities of the horizontal-to-vertical (H/V) spectral ratio technique to provide reliable and relevant information concerning site conditions and/or site amplification. This objective has been tackled through the homogeneous (re)processing of a large volume of earthquakes and ambient noise data recorded by different research teams in more than 200 sites located mainly in Europe, but also in the Caribbean and in Tehran. The original recordings were first gathered in a specific database with information on both the sites and recorded events. Then, for all sites close to an instrumented reference, average site-to-reference spectral ratios (“spectral ratio method” (SSR)) were derived in a homogeneous way (window selection, smoothing, signal-to-noise ratio threshold, averaging), as well as H/V ratios (“HVSRE–RF”) on earthquake recordings. H/V ratios were also obtained from noise recordings at each site (either specific measurements, or extracted from pre- or post-event noise windows). The spectral curves resulting from these three techniques were estimated reliable for a subset of 104 sites, and were thus compared in terms of fundamental frequency, amplitude and amplification bandwidth, exhibiting agreements and disagreements, for which interpretations are looked for in relation with characteristics of site conditions. The first important result consists in the very good agreement between fundamental frequencies obtained with either technique, observed for 81% of the analyzed sites. A significant part of the disagreements correspond to thick, low frequency, continental sites where natural noise level is often very low and H/V noise ratios do not exhibit any clear peak. The second important result is the absence of correlation between H/V peak amplitude and the actual site amplification measured on site-to-reference spectral ratios. There are, however, two statistically significant results about the amplitude of the H/V curve: the peak amplitude may be considered as a lower bound estimate of the actual amplification indicated by SSR (it is smaller for 79% of the 104 investigated sites), and, from another point of view, the difference in amplitude exhibits a questioning correlation with the geometrical characteristics of the sediment/basement interface: large SSR/HV differences might thus help to detect the existence of significant 2D or 3D effects.     

Improvement of HVSR technique by wavelet analysis

In this work we investigate the application of the wavelet analysis to improve the well-known horizontal to vertical spectral ratio (HVSR) technique for site effect estimation. Wavelet analysis has been widely used in the last decade for its capacity to localise the signal in both the time and frequency domains. The wavelet packet transform and the evaluation of a cost function for every node permit one to remove from the signal the least important components and lead to significant improvements in the identification of the fundamental frequency. In this paper a new algorithm is presented based on wavelet analysis, that allows, in some cases, to improve the identification of the peak in the QTS ratio. A number of examples of application are also presented.     

关于中村(Nakamura)方法分析结构响应有效性的讨论

日本学者中村(Nakamura)1989年提出了一种基于同一地表测点地脉动水平分量与竖向分量傅里叶振幅谱比值来估计场地特征的方法, 即所谓的中村方法. 该方法被广泛应用于场地特征的估计, 并已取得大量的成果. 目前, 这一方法也被国内外的研究人员用来进行结构响应特征的研究. 但中村方法的合理性, 国内外尚存在较大争议. 通过对北京城区的一栋钢筋混凝土框架结构建筑的脉动观测, 采用中村方法对获取的速度记录进行了计算分析. 结果表明, 该方法能有效地得到结构的自振频率, 但是不能给出结构对振动的真实放大倍数, 因此, 在使用此方法分析结构响应方面时应谨慎, 建议采用多种方法进行比较分析.     

Site characterizations for the Tehran network (TDMMO) in Tehran Region Using Micro-Earthquake,Microtremor and Quarry Blast Data

We did a Study of Horizontal-to-Vertical Component Spectral Ratio in the Tehran seismic zone. Micro-earthquakes, microtremors and quarry blasts data were used as an estimation of the site response in the Tehran zone. Site effects were studied based on horizontal to vertical ratios by the Nakamura׳s technique. Also, we used the spectra of signals for three components with the lowest noise levels for spectral slope studies. The analysis used seismic events from a network of 13 seismic stations by the permanent local seismological network of the Tehran Disaster Mitigation and Management Organization (TDMMO) from 2004 to 2007. The number of events used were different for each station. Quarry blast events were with 1.2≤M_L≤2.2 and micro-earthquakes were with 1.1≤M_L≤4.1.By comparing results for earthquake, microtremor and quarry blast, we could see that there is a significant difference between them. The data showed clear observations, especially in high-frequencies. The H/V spectral ratios indicate dominant frequency for rock/soft site with a higher ratio level for quarry blast ratios, which are comparable to the earthquake results due to their difference sources. The results derived by spectral H/V ratios and spectral analysis may be used to distinguish between local earthquakes and quarry blasts.     

Strong motion dataset of Turkey: data processing and site classification

Strong motion records of Turkey are studied in order to prepare a catalog to be used as a database for further studies (for instance empirical attenuation laws). The network started to be installed in 1973, and the first record was obtained in 1976. The instruments are of SMA-1 analog recorders and SIG SM-2 and GeoSys GSR-16 digital types. Out of a total of 426 records released on the web sites of the General Directorate of the Disaster Affairs and of Kandili Observatory, a set of 210 records was selected with a satisfactory quality, for which it was possible to associate correctly determined source parameters (source magnitudes and epicentral distances). Most of the records are obtained from around North and East Anatolian Fault zones, as well as from western and southwestern parts of Turkey. The main outcome of this paper is a strong motion catalog of Turkey, with the indication of site conditions, of the frequency band of the reliability of the records, peak values of acceleration, velocity and displacements, source parameters (magnitude, epicentral and macroseismic distances), intensity and finally the fault plane solutions whenever possible. The aim is to have, with other regional dataset, a homogenous and good quality dataset.     

Improvement of Nakamura technique by singular spectrum analysis

In this work we investigate the application of the Singular Spectrum Analysis to improve Nakamura Technique. The SSA has a wide and multidisciplinary range of applications; it allows a time series to be decomposed into different components, e.g. the signal itself, as well as various noise components, which can be subsequently removed from the data. Removal of the minor component of the data can lead to significant improvements in the identification of the system. In this paper the use of SSA Technique allows to optimize the signal to noise ratio before computing the classical Nakamura spectral ratios. A number of typical applications are also presented.     

安徽强震动台站场地响应研究

采用H/V谱比法,利用安徽强震台网记录的波形数据,计算了3个强震台的场地响应。结果表明,当台站场地为基岩或覆盖层较薄时,由地脉动和S波数据分别计算出的谱比曲线一致性很好。当覆盖层大于30.5 m的时候,两者的放大因子有差异,即用S波计算的场地放大响应要明显大于用脉动计算的场地响应。     

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.