共查询到20条相似文献，搜索用时 15 毫秒
1.
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. 相似文献
2.
Obtaining high-resolution images of the geology and hydrogeology of the subsurface in the depth range from ground level to 50 m is one of the major challenges of modern geophysics. The methods which are commonly used (such as compressional-wave surveys and ground-penetrating radar) often suffer from adverse effects caused by the near-surface conditions, changes in water saturation and various sources of noise. This paper demonstrates some of the advantages offered by the use of shear-wave seismology and by the combination of shear- and compressional-wave seismic methods in shallow subsurface investigations.
Multicomponent shallow seismic tests were carried out at four different sites to examine the effectiveness of different acquisition geometries under a variety of near-surface geological conditions. Near-surface conditions encountered at the sites included thick clays, clay/sand sequences overlying Chalk, mudstone overlying granodiorite bedrock and landfill material.
Under all conditions, shear-wave data acquisition was found to have advantages over compressional-wave acquisition for the investigation of the shallow subsurface. Shear head waves, being unaffected by water saturation, achieved penetration to greater depths at a site in Crewkerne, Dorset where compressional head-wave penetration was limited to the near-surface layers. Better vertical resolution was achieved at shallow depths using shear-wave reflection energy at a landfill site. Shear-wave reflections from shallow interfaces were in some cases less affected by noise compared with the equivalent compressional-wave reflections. Combinations of shear- and compressional-wave data recording allowed the measurement of a Poisson's ratio log and gave indications of seismic anisotropy at two sites where dipping clay layers were present. 相似文献
Multicomponent shallow seismic tests were carried out at four different sites to examine the effectiveness of different acquisition geometries under a variety of near-surface geological conditions. Near-surface conditions encountered at the sites included thick clays, clay/sand sequences overlying Chalk, mudstone overlying granodiorite bedrock and landfill material.
Under all conditions, shear-wave data acquisition was found to have advantages over compressional-wave acquisition for the investigation of the shallow subsurface. Shear head waves, being unaffected by water saturation, achieved penetration to greater depths at a site in Crewkerne, Dorset where compressional head-wave penetration was limited to the near-surface layers. Better vertical resolution was achieved at shallow depths using shear-wave reflection energy at a landfill site. Shear-wave reflections from shallow interfaces were in some cases less affected by noise compared with the equivalent compressional-wave reflections. Combinations of shear- and compressional-wave data recording allowed the measurement of a Poisson's ratio log and gave indications of seismic anisotropy at two sites where dipping clay layers were present. 相似文献
3.
Eric M. Thompson Laurie G. Baise Robert E. Kayen 《Soil Dynamics and Earthquake Engineering》2007,27(2):144-152
Ground motions recorded within sedimentary basins are variable over short distances. One important cause of the variability is that local soil properties are variable at all scales. Regional hazard maps developed for predicting site effects are generally derived from maps of surficial geology; however, recent studies have shown that mapped geologic units do not correlate well with the average shear-wave velocity of the upper 30 m, Vs(30). We model the horizontal variability of near-surface soil shear-wave velocity in the San Francisco Bay Area to estimate values in unsampled locations in order to account for site effects in a continuous manner. Previous geostatistical studies of soil properties have shown horizontal correlations at the scale of meters to tens of meters while the vertical correlations are on the order of centimeters. In this paper we analyze shear-wave velocity data over regional distances and find that surface shear-wave velocity is correlated at horizontal distances up to 4 km based on data from seismic cone penetration tests and the spectral analysis of surface waves. We propose a method to map site effects by using geostatistical methods based on the shear-wave velocity correlation structure within a sedimentary basin. If used in conjunction with densely spaced shear-wave velocity profiles in regions of high seismic risk, geostatistical methods can produce reliable continuous maps of site effects. 相似文献
4.
R. Maresca L. Nardone G. Pasquale F. Pinto F. Bianco 《Pure and Applied Geophysics》2012,169(7):1173-1188
The effects of surface geology on ground motion provide an important tool in seismic hazard studies. It is well known that the presence of soft sediments can cause amplification of the ground motion at the surface, particularly when there is a sharp impedance contrast at shallow depth. The town of Avellino is located in an area characterised by high seismicity in Italy, about 30?km from the epicentre of the 23 November 1980, Irpinia earthquake (M?=?6.9). No earthquake recordings are available in the area. The local geology is characterised by strong heterogeneity, with impedance contrasts at depth. We present the results from seismic noise measurements carried out in the urban area of Avellino to evaluate the effects of local geology on the seismic ground motion. We computed the horizontal-to-vertical (H/V) noise spectral ratios at 16 selected sites in this urban area for which drilling data are available within the first 40?m of depth. A Rayleigh wave inversion technique using the peak frequencies of the noise H/V spectral ratios is then presented for estimating Vs models, assuming that the thicknesses of the shallow soil layers are known. The results show a good correspondence between experimental and theoretical peak frequencies, which are interpreted in terms of sediment resonance. For one site, which is characterised by a broad peak in the horizontal-to-vertical spectral-ratio curve, simple one-dimensional modelling is not representative of the resonance effects. Consistent variations in peak amplitudes are seen among the sites. A site classification based on shear-wave velocity characteristics, in terms of Vs30, cannot explain these data. The differences observed are better correlated to the impedance contrast between the sediments and basement. A more detailed investigation of the physical parameters of the subsoil structure, together with earthquake data, are desirable for future research, to confirm these data in terms of site response. 相似文献
5.
In order to propose a seismic design spectrum that includes the effect of rupture directivity in the near-fault region, this study investigates the application of equivalent pulses to the parameter attenuation relationships developed for near-fault, forward-directivity motions. Near-fault ground motions are represented by equivalent pulses with different waveforms defined by a small number of parameters （peak acceleration, A, and velocity V; and pulse period, Tv）. Dimensionless ratios between these parameters （e.g., ATv/V, VTv/D） and response spectral shapes and amplitudes are examined for different pulses to gain insight on their dependence on basic pulse waveforms. Ratios of ATv/V, VTv/D, and the ratio of pulse period to the period for peak spectral velocity （Tv-p） are utilized to quantify the difference between rock and soil sites for near-fault forward-directivity ground motions. The ATv/Vratio of recorded near-fault motions is substantially larger for rock sites than that for soil sites, while Tvp/Tv ratios are smaller at rock sites than at soil sites. Furthermore, using simple pulses and available predictive relationships for the pulse parameters, a preliminary model for the design acceleration response spectra for the near-fault region that includes the dependence on magnitude, rupture distance, and local site conditions are developed. 相似文献
6.
R. Maresca M. Castellano R. De Matteis G. Saccorotti P. Vaccariello 《Pure and Applied Geophysics》2003,160(9):1745-1764
7.
本文采用喜马拉雅科学探测台阵四川省境内10个台站,计算了所有地震信号0.5—15 Hz的H/V谱比,分析了震级、震源距对H/V谱比的影响,并与各台站场地地脉动H/V谱比进行了比较. 结果表明: 震级对各台站地震信号的H/V谱比振幅值以及峰值频率几乎没有影响;震源距对谱峰明显的H/V谱比曲线的谱形和峰值频率影响不大,但对谱峰不明显的H/V谱比曲线的峰值频率有一定影响;H/V谱峰明显的台站,地震信号与地脉动信号的H/V谱比曲线的峰值频率具有很高的一致性,H/V谱峰不明显的台站,二者的峰值频率有一定差异. 相似文献
8.
9.
本文以玉溪盆地为例,提出了一种包含数据预处理、模型建立、模型修正和模型检验的建模方法.基于各类数据间不同的可信度,给出了消除各类数据间速度偏差的折减函数.为避免以往模型修正过程中对地震波形数据的依赖以及对地脉动H/V谱进行模拟等复杂问题,本文提出了一种改进的模型修正方法,即根据基阶瑞雷波H/V谱与实测地脉动H/V谱形状变化相似的原则,对模型进行修正.修正依据为:在玉溪盆地中,单个地脉动测点所在位置处的地下速度结构中各沉积层面的深度均增加约15 m时,由该点的地下速度结构得到的基阶瑞雷波H/V谱的波峰周期和波谷周期均增加约0.1 s,且二者分别由盆地内沉积层的深层和浅层的速度结构所控制.由于地脉动数据的获取较方便,因此该模型修正方法具有广泛的适用性,由该方法修正后的玉溪盆地三维速度结构模型经检验具有较高的准确度. 相似文献
10.
In recent years, numerous induced seismic events have occurred in the proximity of the natural gas field in Northern Germany. To monitor the seismicity and to asses the seismic hazard potential, a local monitoring network was installed in the area. Focusing on the seismicity hazard assessment, a major challenge is the characterisation of potential site effects due to local soil characteristics. This is quantitatively performed by estimating the shear-wave velocity (V s) variation with subsurface layer thickness. Such local effects can only be covered with a coarse spatial resolution due to the limited number of monitoring stations. Profiles were determined at three test sites (Langwedel, Walle and Bomlitz) by using a combined approach of small aperture 2D array ambient noise and small-scale active 1D measurements. The high-resolution frequency-wavenumber (HRFK), spatial autocorrelation (SPAC) and multichannel analysis of surface waves (MASW) methods were applied to the recorded ambient noise and active seismic data using various array sizes supplemented by the active measurements. This jointly allowed obtaining phase velocity dispersion curves covering a wide frequency range from 2 up to 32 Hz. The inversion of the obtained dispersion curves results in average S-wave velocity profiles down to depths of 70 m, identifying thin near-surface layers of a few meters as well as thicker layers of tens of meters in greater depth. A comparison with available borehole data shows a good correlation with the layering. Additionally, to asses the impact of a seismic event at the test sites, PGV estimations for various seismic events were performed. The final results of the test surveys demonstrate that the combined approach represents a suitable tool for near-surface characterisation, which can be used to improve the seismic hazard assessment in the area of the natural gas fields in Northern Germany. 相似文献
11.
通过布置于龙门山断裂带中段、龙门山山前玉皇观区域的地震观测台站阵列接收地震数据,研究该区域的地震动放大效应和地下地质结构.观测阵列共10台宽频带地震仪,分布在玉皇观河口冲积扇区域.分别采用参考场址谱比法(RSSR)和HV谱比法(HVSR)计算64个高信噪比近震数据的振幅谱比函数,结果显示在玉皇观区域具有较明显的地震动放大效应,并且局部场址效应显著.以S06场址为例,建立近地表地震地质模型,通过SH波放大效应正演模拟研究该场址的地震动放大模式.RSSR与HVSR的结果表明,两者所计算的场址放大效应主频一致,但是HVSR的放大峰值却比RSSR的放大峰值大一倍左右,表明HVSR的结果可能包含了波场在近地表低速层之下传播路径的改造作用.另外,采用27个远震P波的接收函数计算了该区域地壳上地幔S波速度结构.接收函数研究结果显示玉皇观地区的莫霍面深度为44 km,沉积盖层、结晶地壳和上地幔的S波速度分别为2.5 km·s~(-1)、3.5 km·s~(-1)和4.5 km·s~(-1).观测阵列台站之间的接收函数反演结果一致性较好,说明本研究区域范围内地形地貌等近地表结构因素的相对变化对接收函数的影响不大. 相似文献
12.
通过成都平原砾性土场地勘察测试,研究汶川地震中大量砾性土液化场地的基本特性,找出一般规律,对砾性土场地液化发生主客观原因提出解释,并修正以往若干认识偏差.分析表明:汶川地震液化砾性土层粒径范围宽,含砾量5%~85%甚至更大,同时其实测剪切波速140~270 m·s~(-1),修正剪切波速160~314 m·s~(-1),都远超历史记录;液化砾性土场地1/2集中在Ⅷ度区内,表明如砂土层液化一样,砾性土场地大规模液化需要较强地震动触发,但超过触发强度后液化规模增长均有限;成都平原浅表地层二元基本结构是汶川地震中出现大量砾性土场地的客观条件之一,该结构可使饱和砾性土层处于封闭状态,构成了砾性土液化的基本条件;虽然液化砾性土层剪切波速很高,但实际上大多松散状态,是此次地震大量砾性土场地发生液化的客观条件之二;地震中地表(井中)喷出物与地下实际液化土类大相径庭,且液化层埋深大多小于6.0 m,以往以地表喷出物反推地下液化层土性类型的做法不再成立;认为砾性土层波速大、透水性好而不会液化的传统认识也不再成立,但砾性土层液化条件与砂土层液化条件不同,前者要求更高. 相似文献
13.
Shear-waves have complicated interactions with the free surface, particularly in the presence of low-velocity surface layers, topographic irregularities, and the expected near-surface crack and stress anomalies. Consequently, it has been suggested that shear-waves should be recorded subsurface in vertical seismic profiles (VSPs), in order to extract accurate information about the in situ crack and stress geometry contained in shear-wave splitting. This paper compares the information in synthetic shear-waves in reflection gathers and VSPs, in order to assess the relative merits of the two techniques for investigating shear-wave splitting. Synthetic seismograms demonstrate that in the presence of even very simple surface layers, shear-waves recorded in reflection surveys at the surface have polarizations which may not indicate crack and stress geometry at depth. In contrast, shear-waves recorded in VSPs are relatively unaffected by surface layers and near-surface stress and crack anomalies, and the behaviour of shear-wave splitting is dominated by the structure of the rock mass in the vicinity of subsurface geophones. Matrix rotations of multicomponent-multisource shear-wave reflection data to extract the information contained in the split shear-waves, are found to be directly meaningful only in situations where crack orientations do not change with depth. 相似文献
14.
We use records from the East Thuringian Seismic Network (OTSN, Ostthüringer Seismisches Netzwerk) to characterize the site response for each station, and to analyze the scope and limits of the Rayleigh modeling for H/V spectral ratio. The stations considered in this work can be classified by their seismic response as hard rock sites or as sites with some site effect. From these results we propose velocity models based on Rayleigh modeling (theoretical Rayleigh wave ellipticity).Our results show that for locations affected by site effects the H/V spectral ratio can be modeled by the theoretical ellipticity of layered velocity models. For hard rock sites the spectral ratio is rather flat and the modeling with the theoretical ellipticity was not very clear. This may be explained by the fact that for hard rock sites the conditions for a clear fundamental frequency associated with S-wave resonance, and therefore with Rayleigh wave ellipticity, are not fulfilled. 相似文献
15.
Simple straightforward methods are applied to testtheir ability to detect the non-linear response of thesoil. Recordings of the main shock and aftershocks ofthe 1995, Hyogo-ken Nanbu (Kobe) earthquake are used.Non-linear effects are investigated using twodifferent techniques, on a collection of data for 12sites situated on different geological structures inthe Kobe and Osaka areas. The first method used is theso-called receiver functions technique (Langston,1979), which consists of computing the spectral ratiobetween horizontal and vertical components of motion.This ratio has been shown to reveal the fundamentalfrequency of a site (Lachet and Bard, 1994; Lachet etal., 1996; Theodulidis et al., 1995, 1996). For eachsite, recordings of the main shock and a set ofaftershocks are considered. The variation of thisspectral ratio for different values of the maximumacceleration recorded at a site is investigated. Bothvariations of the amplitude of the H/V ratio (due tonon-linear behavior, on the horizontal components inparticular) and of the frequency position of theamplified band-width are observed. The secondtechnique used in this study is related to thevariation of the high frequency content of therecordings during the main-shock and its aftershocks.The high frequency spectral decay of the motion,characterized by parameter, is assumed to berelated mainly to the near-surface attenuation. Itshould then increase with increasing peak velocity, incase of non-linearity. The value of kappa iscalculated for the 12 sites in the Kobe area, fordifferent types of soil conditions, and againdifferent values of peak ground acceleration.Variations of kappa are then related to non-linearbehavior of the soil during the Kobe earthquake. 相似文献
16.
Site engineering seismic survey provides basic data for seismic effect analysis. As an important parameter of soil, shear-wave velocity is usually obtained through wave velocity testing in borehole. In this paper, the passive source surface-wave method is introduced into the site engineering seismic survey and practically applied in an engineering site of Shijingshan District. By recording the ubiquitous weak vibration on the earth surface, extract the dispersion curve from the surface-wave components using the SPAC method and obtain the shear-wave velocity structure from inversion. Over the depth of 42 m underground, it totally consists of five layers with interface depth of 3.31, 4.50, 7.23, 17.41, and 42.00 m; and shear-wave velocity of 144.0, 198.3, 339.4, 744.2, and 903.7 m/s, respectively. The inversion result is used to evaluate site classification, determine the maximum shear modulus of soil, provide basis for further seismic hazard analysis and site assessment or site zoning, etc. The result shows that the passive source surface-wave method is feasible in the site engineering seismic survey and can replace boreholes, shorten survey period, and reduce engineering cost to some extent. 相似文献
17.
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. 相似文献
18.
Site characterizations for the Iranian strong motion network 总被引：2，自引：0，他引：2
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. 相似文献
19.
The reference spectral noise ratio method to evaluate the seismic response of a site 总被引：2，自引：0，他引：2
The seismic response of several sites in the Durban area of South Africa has been investigated using a refined version of the spectral ratio (H/V) generated by cultural seismic noise. Two samples of noise separated by an interval of several minutes for the vertical and the two orthogonal horizontal components were taken at each site. Though the two samples differed noticeably in the time as well as in the frequency domain, when the components are considered separately, the ratios of the horizontal to the vertical spectra were very similar for most of the pairs. This similitude was obvious not only in the frequencies of the peaks, but also in the range of amplifications involved. These observations have been used to introduce a refinement to the Nakamura Method. This refinement consists in using as a reference the H/V spectral ratio of a well-known, hard rock site. All the other soft sites of interest are then compared with this standard reference site. This comparison provides valuable information on the frequencies and amplification factors introduced by the critical soft sediments. This refined method will be referred here as the ‘Reference spectral noise ratio’.This method, is compared to a similar one devised to investigate the response of horizontal layers to longitudinal seismic waves, using the ratio of the spectra generated by long- period waves. 相似文献
20.
The Effect of Velocity Inversions on H/V 总被引：2，自引：0，他引：2
We analyzed the phenomenology of microtremor H/V curves under inversions in the shear-wave velocity (Vs) profile in the subsoil. Under no Vs inversion the spectral signature of the H/V peaks is found to be ‘eye-shaped’ with the horizontal components higher than
the vertical. Conversely, under negative velocity gradients, numerous of differences emerge. I) A H/V ratio below 1 is observed
for a wide range of frequencies, due to the decrease of the horizontal components below the vertical one. II) In the presence
of persistent H/V < 1, small bumps in the H/V ratio given by local minima in the vertical spectral component may represent
the relics of the peaks indicating resonances and stratigraphic discontinuities. As a consequence, in the presence of velocity
inversions the H/V > 2 SESAME (2004) criterion fails but a stratigraphic interpretation may still be possible. III) The H/V
curves should always be interpreted together with the single component spectra. IV) Microtremor H/V measurements for stratigraphic/microzonation purposes on stiff artificial
soils, (asphalt, concrete, cement, pavements) should always be avoided since the latter often produce velocity inversions.
This may have consequences in the intermediate to high frequency domain ( > 1 Hz) also in the application of reference site
methods, like Hsite/Hbedrock, to microtremor. Theoretical modeling confirms these experimental findings. 相似文献