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.     

应用微动H/V谱比法和台阵技术探测场地响应和浅层速度结构

为了快速而且廉价地获取北京市详细的场地响应和浅层速度结构,应用于地震动模拟和地震灾害预防,我们开展了微动观测技术和处理方法研究.本文利用2007年夏季北京五棵松地区进行的几个微动观测实验数据,使用单台H/V谱比法分析场地的卓越频率及其对应的放大系数,并对比了不同地震仪和观测时间对H/V曲线的影响;应用高分辨率F-K频谱分析方法从微动台阵数据中得到Rayleigh波的频散曲线并使用邻域算法反演出浅层速度结构.H/V结果表明该地区卓越频率在2.1～2.2 Hz之间,对应的放大系数下限约为3;利用微动H/V方法得到的场地卓越频率具有较高的稳定性.微动台阵反演结果给出了比较合理的波阻抗界面深度和层平均速度结构,认为地下80多米处的波阻抗界面是决定场地卓越频率和其场地放大系数的主要界面.本研究表明微动技术应用于评估城市地震场地响应和浅层速度结构是可行且易于实施的.     

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.     

Comparison of long-term and short-term uplift rates along an active blind reverse fault zone (Bajo Segura,Se Spain)

The seismic activity of the Guardamar-Torrevieja zone (Eastern Betic Cordillera, SE Spain) can be associated with the Bajo Segura fault zone, an E-W reverse blind fault with secondary NW-SE dextral faults. A high-precision levelling profile 30 kilometers long was set up and levelled in 1997 to monitor the vertical displacement of this active fault zone. This profile runs parallel to an older high-precision levelling line included in the Spanish first order levelling network measured by the Instituto Geográfico Nacional (IGN) in two different campaigns (1934 and 1976). The 1997 line was relevelled in 2003 and 10 new benchmarks were set up, both to increase benchmark density and to restore some of the 1997 benchmarks that had been destroyed. We have used the 1976 IGN and the 2003 measurement to construct a recent vertical movements profile with a significant time difference (27 years). This recent vertical movements profile shows that the vertical movements are very small, nearly equal to the error bars, with a 0.2 mm/year rise in the town of Guardamar, and a 0.2 mm/year subsidence of the southern part of the profile (Punta Prima) respect to the town of Torrevieja. These movements could be related to the activity of the Bajo Segura and the San Miguel faults respectively. Using geological markers we have deduced uplift rates of 0.1 mm/year during the last 3 million years. Therefore, these preliminary results indicate that geodetically (short-term deformation) determined uplift rates are similar to those estimated from geological markers (long term deformation).     

Determination of S-wave velocity structure using microtremors and spac method applied in Thessaloniki (Greece)

Array measurements of microtremors at 16 sites in the city of Thessaloniki were performed to estimate the V_s velocity of soil formations for site effect analysis. The spatial autocorrelation method was used to determine phase velocity dispersion curves in the frequency range from 0.8–1.5 to 6–7 Hz. A Rayleigh wave inversion technique (stochastic method) was subsequently applied to determine the Vs profiles at all the examined sites. The determination of V_s profiles reached a depth of 320 m. Comparisons with V_s values from cross-hole tests at the same sites proved the reliability of the SPAC method. The accuracy of the V_s profiles, the ability to reach large penetration depths in densely populated urban areas and its low cost compared to conventional geophysical prospecting, make Mictrotremor Exploration Method very attractive and useful for microzonation and site effects studies. An example of its application for the site characterization in Thessaloniki is presented herein.     

A stochastic based approach for a new site classification method: application to the Algerian seismic code

Building codes have widely considered the shear wave velocity to make a reliable subsoil seismic classification, based on the knowledge of the mechanical properties of material deposits down to bedrock. This approach has limitations because geophysical data are often very expensive to obtain. Recently, other alternatives have been proposed based on measurements of background noise and estimation of the H/V amplification curve. However, the use of this technique needs a regulatory framework before it can become a realistic site classification procedure. This paper proposes a new formulation for characterizing design sites in accordance with the Algerian seismic building code(RPA99/ver.2003), through transfer functions, by following a stochastic approach combined to a statistical study. For each soil type, the deterministic calculation of the average transfer function is performed over a wide sample of 1-D soil profiles, where the average shear wave(S-W) velocity, V s, in soil layers is simulated using random field theory. Average transfer functions are also used to calculate average site factors and normalized acceleration response spectra to highlight the amplification potential of each site type, since frequency content of the transfer function is significantly similar to that of the H/V amplification curve. Comparison is done with the RPA99/ver.2003 and Eurocode8(EC8) design response spectra, respectively. In the absence of geophysical data, the proposed classification approach together with micro-tremor measures can be used toward a better soil classification.     

Site characterization in the Vega Baja, SE Spain, using ambient-noise H/V analysis

New available geological, geotechnical, and geophysical data and new acquired ambient-noise surveys allowed for a detailed characterization of the soils in the Vega Baja region, located in the Lower Segura River basin, SE of the Iberian Peninsula. Single-station measurements were carried out at 90 sites, mainly over the central area of the basin, checking reproducibility by repeating some measurements under different conditions, on different dates, and through different acquisition systems, following well established standard procedures. The detailed data processing and analysis of the new acquired ambient-noise records provided the identification of multiple peaks in some of the horizontal-to-vertical spectral ratio curves, representing different impedance contrasts at depth that were 1D modelled to characterize the Vega Baja soils in terms of resonant frequencies, average shear-wave velocity, thickness of the different soil units, depth to the engineering bedrock and depth to the rock units when the model includes it. The depth to the engineering bedrock is around 30 m in the central part of the region, reaching maximum values of about 50 m in the north and to the east. Maximum values of the shear-wave velocity averaged over the soil units are close to 200 m/s while that averaged in the top 30 m reach values around 300 m/s south of the river bank. Contour mapping and cross-sections of the obtained model parameters illustrate the soil characteristics over the region, providing relevant information for the generation of representative sets of seismic hazard scenarios at different probability levels in future studies for their application in risk mitigation and emergency planning in the Vega Baja.     

辽宁及邻区背景噪声面波群速度结构研究

收集辽宁及邻区59个宽频带地震仪记录的自2012年1月1日至12月31日的背景噪声连续波形垂向记录, 以背景噪声的方法获取辽宁及邻区面波群速度图像。 过程采用互相关的方法提取瑞利面波格林函数, 利用CPS330提取了群速度的频散曲线, 共从1655条频散曲线中筛选出了1233条信噪比较高的频散曲线。 将研究区划分为0.5°×0.5°的网格, 应用的层析成像方法得到了周期为8~40 s的瑞利面波群速度结构分布。 结果表明: 辽宁地区地壳及上地幔存在明显的横向不均匀性。 短周期群速度分布与研究区内断裂带及地质构造地貌形态表现出良好的相关性, 其中8~15 s周期内群速度分布特征与盆地坳陷、 山区隆起对应性较好, 呈“两垒高, 一堑低”的群速度分布特点, 基本与地质构造相吻合, 地震多位于高低速过渡带内。 较长周期20~30 s的群速度在渤海湾-辽东湾中存在低速异常, 显示了渤海湾盆地和下辽河盆地具有较厚的沉积层覆盖。 35~40 s与莫霍面的深度有明显关联性,莫霍面埋深大体呈西厚东薄的特点。 38~40 s周期内郯庐断裂带东侧的低速异常可能说明渤海内存在局部的热物质上涌现象。 本文结果较好地反映了研究区内地貌地质构造情况, 与区域内地壳及上地幔结构的相关研究成果相吻合, 为辽宁及邻区的地震活动构造背景及地震孕育机理提供重要参考资料。     

S-wave Velocity Profiles for Earthquake Engineering Purposes for the Cologne Area (Germany)

Local S-wave velocity-depth profiles are a key factor in seismic hazard assessment, as they allow the amplification potential of the sedimentary cover to be evaluated. Ambient seismic noise is mainly composed of surface waves, and therefore contains vital information about the S-wave velocity structure, allowing polarization or dispersion curves to be obtained from single station or array noise recordings. At two sites in the area of Cologne, Germany, the extended spatial correlation method was applied to such recordings and apparent phase velocity curves in the frequency range of interest for earthquake engineering were obtained. Using this data, a linearized inversion, the simplex downhill method, and a genetic algorithm yielded similar S-wave profiles. However, the latter method is recommended since it is less dependent upon a good starting model. Importantly, the presence of low-velocity layers in the Cologne area made it necessary to consider in the frequency range of interest higher modes in the inversion procedures. Finally, independent information on the total thickness of the sedimentary cover permitted the estimation of a 2D S-wave velocity profile crossing the Cologne area. Here, the H/V ratio inversion using 20 single-station noise recordings was used, with the results in good agreement with a geological profile.     

Flexible multimethod approach for seismic site characterization

Journal of Seismology - We describe the flexible multimethod seismic site characterization technique for obtaining shear-wave velocity (VS) profiles and derivative information, such as the...     

利用随机噪声研究华南地区瑞利波相速度特征

基于2009~2010年国家、区域测震台网的387个宽频带台站数据,通过互相关方法提取到可靠的瑞利波经验格林函数,利用相位匹配滤波时频分析技术测量瑞利波相速度频散曲线,最后采用噪声层析成像方法获得了华南地区不同周期的瑞利波相速度分布图。结果显示,华南地区速度结构横向变化幅度较小,反映了华南地区作为一个整体较为稳定,与华南地区自晚中生代以来未发生过强烈构造活动的特征基本一致;虽然华南地区整个岩石圈速度结构较为均匀,但扬子块体西部、四川盆地与扬子块体东部、华夏块体间存在明显的速度差异,体现在周期为8~10s时华夏块体相速度大于扬子块体西部、川滇块体以及四川盆地,由于沉积层较厚四川盆地速度最低;周期为10~30s时华夏块体面波相速度大于扬子块体西部和四川盆地,而川滇块体速度最低;周期为35s时扬子块体、华夏块体、四川盆地的速度基本一致,且高于川滇块体,这与华南地区地壳厚度明显小于川滇块体的特征相符。     

青藏高原东南缘瑞利波群速度分布特征及其构造意义探讨

青藏高原东南缘是研究印度—欧亚板块碰撞过程、块体间相互作用和壳幔变形机制的重要地区.本文利用川滇地区流动地震台阵和固定地震台网共557个台站的连续波形数据,基于改进的背景噪声处理流程和分析方法得到了6023条瑞利波群速度频散曲线,反演获得了6~48 s的瑞利波群速度分布图像.结果显示在四川盆地内部短周期群速度分布较好地揭示了盆地内沉积层厚度的横向变化.在30~48 s周期,四川盆地西部群速度存在南低、北高的特征,推测是南部中下地壳和上地幔温度较高引起的.温度的增高降低了地壳的力学强度,在青藏高原东向挤压作用下盆地西南部地壳更易发生变形,并导致脆性上地壳在新生代产生地壳缩短和褶皱、断裂等地质活动.攀枝花及其周边地区从地壳浅部至上地幔深度的高速异常体,可能与基性和超基性岩的侵入有关.该高速体具有较大的介质强度,在一定程度上阻碍了青藏高原物质东南向的运移,这可能是造成丽江—小金河断裂两侧巨大高程差异的重要因素.自24 s开始,南盘江盆地表现为明显的高速异常,与华南块体西南部其他区域的深部结构存在明显差异.反演的S波速度结构揭示,自中上地壳至上地幔,南盘江盆地的速度一直高于北侧其他区域.结合此地区的地壳运动模式,推测介质S波速度较高、力学强度较大的南盘江盆地对青藏高原物质的东南向逃逸具有一定的阻挡作用.     

3D near-surface soil response from H/V ambient-noise ratios

The applicability of the horizontal-to-vertical (H/V) ambient-noise spectral ratio for characterizing earthquake site effects caused by near-surface topography and velocity structures was evaluated at sites underlain by thick (i.e. >100 m) sediment deposits near the southern-end of the New Madrid seismic zone in the central United States. Three-component ambient-noise and velocity models derived from seismic (shear-wave) refraction/reflection surveys showed that a relatively horizontal, sharp shear-wave velocity interface in the soil column resulted in an H/V spectral ratio with a single well-defined peak. Observations at sites with more than one sharp shear-wave velocity contrast and horizontally arranged soil layers resulted in at least two well-defined H/V spectral ratio peaks. Furthermore, at sites where there were sharp shear-wave velocity contrasts in nonhorizontal, near-surface soil layers, the H/V spectra exhibited a broad-bandwidth, relatively low-amplitude signal instead of a single well-defined peak.     

基于H/V谱比法的乌恰地区场地分类研究

场地条件对于地震动特性影响显著,在工程地震领域中强震记录与之对应的场地条件在研究中十分重要。利用H/V谱比法对新疆乌恰地区12个强震台站场地进行场地分类计算,并与实际钻孔剪切波速的场地分类结果对比分析,H/V谱比法和实际钻孔波速分类结果还是有很好的对应关系,为研究区域内缺乏钻孔剪切波速测试资料的场地进行场地分类提供了参考依据。     

Characterization of site conditions (soil class,V<Subscript>S30</Subscript>, velocity profiles) for 33 stations from the French permanent accelerometric network (RAP) using surface-wave methods

Data provided by accelerometric networks are important for seismic hazard assessment. The correct use of accelerometric signals is conditioned by the station site metadata quality (i.e., soil class, V_S30, velocity profiles, and other relevant information that can help to quantify site effects). In France, the permanent accelerometric network consists of about 150 stations. Thirty-three of these stations in the southern half of France have been characterized, using surface-wave-based methods that allow derivation of velocity profiles from dispersion curves of surface waves. The computation of dispersion curves and their subsequent inversion in terms of shear-wave velocity profiles has allowed estimation of V_S30 values and designation of soil classes, which include the corresponding uncertainties. From a methodological point of view, this survey leads to the following recommendations: (1) perform both active (multi-analysis surface waves) and passive (ambient vibration arrays) measurements to derive dispersion curves in a broadband frequency range; (2) perform active acquisitions for both vertical (Rayleigh wave) and horizontal (Love wave) polarities. Even when the logistic contexts are sometimes difficult, the use of surface-wave-based methods is suitable for station-site characterization, even on rock sites. In comparison with previous studies that have mainly estimated V_S30 indirectly, the new values here are globally lower, but the EC8-A class sites remain numerous. However, even on rock sites, high frequency amplifications may affect accelerometric records, due to the shallow relatively softer layers.     

Evaluation of site conditions for the Ankara Basin of Turkey based on seismic site characterization of near-surface geologic materials

Regional site conditions relevant for seismic hazard studies can be derived from various geologic, seismologic, and geotechnical sources. In this study, site conditions are derived for the Ankara Basin in Turkey by merging in-situ seismic measurements of dynamic properties, geologic information, and some geotechnical boring information. Field seismic refraction surveys were performed at 259 sites in the project area to classify and characterize Plio–Pleistocene fluvial deposits and Quaternary alluvial and terrace deposits. The shear-wave velocity profiles of the near-surface geologic units are used to characterize site classes according to the International Building Code [International Code Council, ICC. International Building Code. Structural and fire- and life-safety provisions (seismic, wind, accessibility, egress, occupancy and roof codes), 2006. Whittier, CA.] and the Turkish Seismic Code [Ministry of Public Works and Settlement, 1998. Turkish Seismic Code, Specification for Structures to be Built in Disaster Areas, Ankara, Turkey], and to develop a regional model for the average shear-wave velocity in the top 30 m. The resulting maps of site class indicate that the classification system for the Turkish Seismic Code results in a significant portion of the Ankara Basin being classified as Z4, the softest site class. The International Building Code site classification system results in most of the Ankara Basin being classified as D, stiff soil. These differences are caused by the Turkish Seismic Code incorporating information from only the surface layer, while the International Building Code incorporates information from the top 30 m.     

2D inversion of refraction traveltime curves using homogeneous functions

A method using simple inversion of refraction traveltimes for the determination of 2D velocity and interface structure is presented. The method is applicable to data obtained from engineering seismics and from deep seismic investigations. The advantage of simple inversion, as opposed to ray‐tracing methods, is that it enables direct calculation of a 2D velocity distribution, including information about interfaces, thus eliminating the calculation of seismic rays at every step of the iteration process. The inversion method is based on a local approximation of the real velocity cross‐section by homogeneous functions of two coordinates. Homogeneous functions are very useful for the approximation of real geological media. Homogeneous velocity functions can include straight‐line seismic boundaries. The contour lines of homogeneous functions are arbitrary curves that are similar to one another. The traveltime curves recorded at the surface of media with homogeneous velocity functions are also similar to one another. This is true for both refraction and reflection traveltime curves. For two reverse traveltime curves, non‐linear transformations exist which continuously convert the direct traveltime curve to the reverse one and vice versa. This fact has enabled us to develop an automatic procedure for the identification of waves refracted at different seismic boundaries using reverse traveltime curves. Homogeneous functions of two coordinates can describe media where the velocity depends significantly on two coordinates. However, the rays and the traveltime fields corresponding to these velocity functions can be transformed to those for media where the velocity depends on one coordinate. The 2D inverse kinematic problem, i.e. the computation of an approximate homogeneous velocity function using the data from two reverse traveltime curves of the refracted first arrival, is thus resolved. Since the solution algorithm is stable, in the case of complex shooting geometry, the common‐velocity cross‐section can be constructed by applying a local approximation. This method enables the reconstruction of practically any arbitrary velocity function of two coordinates. The computer program, known as godograf , which is based on this theory, is a universal program for the interpretation of any system of refraction traveltime curves for any refraction method for both shallow and deep seismic studies of crust and mantle. Examples using synthetic data demonstrate the accuracy of the algorithm and its sensitivity to realistic noise levels. Inversions of the refraction traveltimes from the Salair ore deposit, the Moscow region and the Kamchatka volcano seismic profiles illustrate the methodology, practical considerations and capability of seismic imaging with the inversion method.     

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.     

Seismic damage evaluation in urban areas using the capacity spectrum method: Application to Barcelona

Conceptual aspects related to seismic vulnerability, damage and risk evaluation are discussed first, together with a short review of the most widely used possibilities for seismic evaluation of structures. The capacity spectrum method and the way of obtaining seismic damage scenarios for urban areas starting from capacity and fragility curves are then discussed. The determination of capacity curves for buildings using non-linear structural analysis tools is then explained, together with a simplified expeditious procedure allowing the development of fragility curves. The seismic risk of the buildings of Barcelona, Spain, is analyzed in the paper, based on the application of the capacity spectrum method. The seismic hazard in the area of the city is described by means of the reduced 5% damped elastic response spectrum. The information on the buildings was obtained by collecting, arranging, improving and completing a broad database of the dwellings and current buildings. The buildings existing in Barcelona are mainly of two types: unreinforced masonry structures and reinforced concrete buildings with waffled-slab floors. The ArcView software was used to create a GIS tool for managing the collected information in order to develop seismic risk scenarios. This study shows that the vulnerability of the buildings is significant in Barcelona and, therefore, in spite of the low-to-moderate seismic hazard in the region, the expected seismic risk is considerable.     

Shallow geology characterization using Rayleigh and Love wave dispersion curves derived from seismic noise array measurements

The local geology and shallow S-wave velocity structure of a site are recognized to be key factors for the increase in the damaging potential of seismic waves. Indeed, seismic amplitudes may be amplified in frequency ranges unfavorable for building stock by the presence of soft sedimentary covers over lying hard bedrock. Hence, microzonation activities, which aim at assessing the site response as accurately as possible, have become a fundamental task for the seismic risk reduction of urbanized areas. Methods based on the measurement of seismic noise, which typically are fast, non-invasive, and low cost, have become a very attractive option in microzonation studies.Using observations derived from seismic noise recordings collected by two-dimensional arrays of seismic stations, we present a novel joint inversion scheme for surface wave curves. In particular, the Love wave, the Rayleigh wave dispersion and the HVSR curves are innovatively combined in a joint inversion procedure carried out following a global search approach (i.e., the Genetic Algorithm).The procedure is tested using a data set of seismic noise recordings collected at the Bevagna (Italy) test-site. The results of the novel inversion scheme are compared with the inversion scheme proposed by Parolai et al. (2005), where only Rayleigh wave dispersion and HVSR curves are used, and with a cross-hole survey.