Intrinsic versus extrinsic seismic anisotropy: Surface wave phase velocity inversion

The precise determination and interpretation of anisotropy are relatively difficult because the apparent anisotropy is usually a mixture of intrinsic and extrinsic anisotropy, which might partly hide the true properties of the medium investigated. The artificial anisotropy can be due to the fact that seismic waves do not ‘see’ the real details of a medium, but a ‘filtered’ (or ‘upscaled’) version of the Earth model. This can be due to a bad quality of the data coverage, to limited frequency band effects, or to errors in the approximate theory. With the limitation to layered Earth models, through comparisons of the results of the homogenization method with those of the periodic isotropic two-layered model as an analytical solution, we illustrate that the Backus theory for the long wavelength equivalent effect can be extended to calculate the extrinsic anisotropy, due to upscaling effects at discontinuities for the general isotropic layered model, when its spatial scale is much less than or equal to the seismic wavelength. We find that the extrinsic radial S-wave anisotropy produced by the vertical heterogeneities in the upper mantle of the Earth can be as large as 3% (about 30% extrinsic anisotropy of the 10% radial anisotropy). To better recover information from seismic data, we propose a surface wave phase velocity inversion method based on the first-order perturbation theory. We show that resolution at discontinuities can be improved by adding overtone modes of surface wave data. For more general layered models, the homogenization method could be considered, which can flexibly adapt the scale of the model to seismic wavelengths. However, the periodic isotropic two-layered model can also help to analytically quantify the amount of extrinsic radial, and possibly azimuthal anisotropy produced by the tilted fine layering.     

S-wave velocity structure of the North China from inversion of Rayleigh wave phase velocity

We constructed the S-wave velocity structure of the crust and uppermost mantle (10–100 km) beneath the North China based on the teleseismic data recorded by 187 portable broadband stations deployed in this region. The traditional two-step inversion scheme was adopted. Firstly, we measured the interstation fundamental Rayleigh wave phase velocity of 10–60 s and imaged the phase velocity distributions using the Tarantola inversion method. Secondly, we inverted the 1-D S-wave velocity structure with a grid spacing of 0.25° × 0.25° and constructed the 3-D S-wave velocity structure of the North China. The 3-D S-wave velocity model provides valuable information about the destruction mechanism and geodynamics of the North China Craton (NCC). The S-wave velocity structures in the northwestern and southwestern sides of the North–South Gravity Lineament (NSGL) are obviously different. The southeastern side is high velocity (high-V) while the northeastern side is low velocity (low-V) at the depth of 60–80 km. The upwelling asthenosphere above the stagnated Pacific plate may cause the destruction of the Eastern Block and form the NSGL. A prominent low-V anomaly exists around Datong from 50 to 100 km, which may due to the upwelling asthenosphere originating from the mantle transition zone beneath the Western Block. The upwelling asthenosphere beneath the Datong may also contribute to the destruction of the Eastern Block. The Zhangjiakou-Penglai fault zone (ZPFZ) may cut through the lithosphere and act as a channel of the upwelling asthenosphere. A noticeable low-V zone also exists in the lower crust and upper mantle lid (30–50 km) beneath the Beijing–Tianjin–Tangshan (BTT) region, which may be caused by the upwelling asthenosphere through the ZPFZ.     

Surface wave studies for shear wave velocity and bedrock depth estimation over basalts

Shear wave velocity (V _S) estimation is of paramount importance in earthquake hazard assessment and other geotechnical/geo engineering studies. In our study, the shear wave velocity was estimated from ground roll using multichannel analysis of surface wave (MASW) technique making use of dispersive characteristics of Rayleigh type surface waves followed by imaging the shallow subsurface basaltic layers in an earthquake-prone region near Jabalpur, India. The reliability of MASW depends on the accurate determination of phase velocities for horizontally traveling fundamental mode Rayleigh waves. Inversion of data from surface waves resulted in a shear wave velocity (V _S) in the range of 200–1,200 m/s covering the top soil to weathering and up to bedrock corresponding to a depth of 10–30 m. The P-wave velocity (V _P) obtained from refraction seismic studies at these locations found to be comparable with V _S at an assumed specific Poisson’s ratio. A pair of selected set of V _S profiles over basalt which did not result in a hazardous situation in an earthquake of moderate magnitude are presented here as a case study; in other words, the shear wave velocity range of more than 200 m/s indicate that the area is highly unlikely prone to liquefaction during a moderate or strong earthquake. The estimated depth to basalt is found to be 10–12 m in both the cases which is also supported by refraction studies.     

青藏高原东部基于噪声的面波群速度分布特征

通过收集青海、甘肃、四川三省的76个地震台记录的2008年1—12月三分量的连续噪声数据,利用噪声面波层析成像的方法获得了青藏高原东部的面波群速度分布特征。首先采用多重滤波方法提取了1 000多条台站对5～50 s的三分量面波群速度频散曲线,然后将研究区域划分为0.2°×0.2°的网格,利用O ccam方法反演了瑞利波(R-R)和勒夫波(T-T)的群速度分布。反演得到的群速度分布特征与地表地质和构造特征表现出较好的相关性,清晰地揭示了地壳内部的横向速度变化。层析成像的结果显示在短周期(8～20 s)内,拥有较厚的沉积层的四川盆地表现为明显的低速特征,而青藏高原东部则表现为较高的群速度分布特征;随着周期的增加(>20 s),群速度的分布特征呈现出与短周期相反的特性,青藏高原东部下方的速度远远低于四川盆地,这可能与青藏高原东部中、下地壳低速层相关联,同时也意味着研究区域的地壳结构具有明显的横向不均匀性。在群速度分布图上,龙门山不仅是四川盆地与青藏高原的地形和构造分界带,同时也对应着高群速度与低群速度的过渡带。     

长白山火山区高精度Rayleigh面波相速度结构与岩浆系统

近年来,有关长白山火山是否存在潜在喷发危险的讨论引起了国内外地学研究者的广泛关注,但人们目前对其壳幔岩浆系统的了解却十分不足,已有的研究对长白山火山壳内岩浆房存在的深度位置、形态规模及其部分熔融程度的认识仍存在较大分歧。本研究通过汇集长白山火山及邻区（包括朝鲜境内）多个密集流动地震台阵和区域固定地震台网的观测资料,采用背景噪声成像方法获得了长白山火山区高精度的Rayleigh面波相速度模型。成像结果表明：长白山火山中-下地壳深度位置存在显著低波速异常,可能指示了岩浆房的存在。上地壳局部低速区可能反映了壳内深部岩浆向上运移的通道或者是区域小尺度的岩浆聚集体。长白山火山上地幔顶部的低速可能揭示了软流圈热物质上涌,其减压熔融为壳内岩浆房提供了幔源岩浆补给。

     

Soil liquefaction evaluation using shear wave velocity

A reasonably good relationship between shear wave velocity (SWV) and standard penetration resistance (SPT) of granular soils in agreement with previous studies was obtained from field tests. A similar correlation between SWV and cone penetration resistance of granular soils was also obtained. Using Seed's Standard Penetration Test (SPT)-based soil liquefaction charts, new charts of soil liquefaction evaluation based on SWV data were developed for various magnitude earthquakes.     

3D mantle structure of Central Asia from Rayleigh wave group velocity dispersion

The mantle structure in Central Asia was investigated by surface-wave tomography from dispersion of the fundamental mode of the Rayleigh wave group velocities along more than 3200 earthquake-station paths within 40° N to 60° N and 80° E to 132° E. The velocities were processed by the frequency-time analysis at periods from 10 to 250 s to obtain their dispersion curves. Then group velocity maps were computed separately for each period, at different sampling intervals: at every 5 s for the short periods from 10 to 30 s, at 10 s for periods between 30 and 100 s, and at 25 s for the longest periods of 100 to 250 s. Resolution was estimated according to the effective averaging radius (R) and presented likewise in the form of maps. To estimate the depths of the revealed inhomogeneties, locally averaged dispersion curves were calculated using the group velocity maps, with reference to the radius R, and were then inverted to S-wave velocity-depth profiles. The resulting three-dimensional S-wave velocity structure to depths of about 700 km revealed large lateral inhomogeneties through the entire depth range. This pattern may be due to the history of the major tectonic structures, as well as to ongoing processes in the mantle.     

基于最佳小波基的地震面波插值方法

在利用实际地震数据中的面波反演近地表横波速度的过程中,若道间距较大、空间采样率不足,则会产生空间假频现象,从而降低频率速度谱的信噪比,影响频散曲线提取的精度以及反演效果,因此需要针对面波进行插值处理。文中提出了一种基于最佳小波基的地震面波插值方法,通过理论分析和实验误差对比在地震数据处理常用的众多小波基中选出适用于插值处理的最佳小波基bior6.8,提高了插值精度。针对面波同向轴为线性且斜率较大的特点,文中首先采用线性动校正的方法对面波进行拉平处理,再进行小波变换插值,最后进行反线性动校正恢复面波。通过对理论模型与实际资料进行插值处理验证了本文方法的有效性,插值后的面波记录波形恢复较好,显著提高了频率速度谱的信噪比,有效解决了面波数据空间采样率不足引起的假频问题。     

Adaptive generalized multiscale approximation of a mixed finite element method with velocity elimination

Computational Geosciences - In this paper, we propose offline and online adaptive enrichment algorithms for the generalized multiscale approximation of a mixed finite element method with velocity...     

The upper crust low velocity layer; a Rayleigh (Rg) phase velocity study from SE Norway

A recent surge of interest in short-period fundamental mode Rayleigh waves (Rg) propagation is motivated by their usefulness for structural mapping and estimation of attenuation (Q) properties of the uppermost crust, their importance in scattering studies and as a nuisance (false alarms) in detection seismology. We have investigated Rg phase velocity dispersion characteristics, using recordings from 16 out of 22 subarrays of the NORSAR array of aperture 100 km. The observed phase velocity in combination with a one-layer and two-layers model over halfspace models were inverted using a maximum likelihood scheme. In view of the smoothness and simplicity of the dispersion curves, the number of unknowns were limited to three parameters, namely shear velocities and thickness, using a Poisson's ratio of 0.25 for P-S velocity coupling and density fixed. In the general array siting area the derived low velocity layer has an average thickness of 1.02 km, and the associated shear velocity is 2.82 km s^-1. The underlying model half space appears to be uniform, and its derived shear velocity is 3.55 km s^-1, which is typical of the upper crust. In contrast to other similar studies, no obvious correlation was found with local geology, which mainly consists of Precambrian crystalline rock sequences. Group velocities were calculated from the structural models and compared to the observational ones. The agreement between calculated and observed group velocities was reasonable.     

致密岩石纵横波波速各向异性的比较研究

通过对板岩、千枚岩、糜棱岩和变质砂岩等4种岩石在平行和垂直层理、板理的3个正交方向上的纵、横波波速试验,提出了致密岩石纵波波速的各向异性与横波波速的各向异性普遍存在着一致性和差异性两种特征。一致性特征主要表现为致密岩石的横波各向异性随纵波各向异性的增强而增强;差异性特征主要表现为在平行和垂直层理、板理的两个方向上,纵波波速的各向异性指数大于横波,亦即纵波波速的各向异性比横波更为显著。从横观各向同性弹性介质的波速方程出发,通过具体的算例对这两种特征进行了较好的理论分析。     

波速测试在岩土工程中的应用

通过波速测试技术进行岩土体弹性波速度原位测试,利用岩土体的纵、横波速度值能够进行场地类别划分,确定场地卓越周期,以及计算地基土的动参数,为岩土工程提供必要的设计参数。通过计算获得了徐州苏宁广场场地土的纵、横波速度以及场地土的动参数,根据《建筑抗震设计规范》确定徐州苏宁广场场地为二类场地,利用波速值计算场地卓越周期为0. 415 s与规范中查表获得的特征周期一致。为该工程场地工程地质评价和抗震设计提供了丰富的参数。     

S-wave velocity profiling from refraction microtremor Rayleigh wave dispersion curves via PSO inversion algorithm

The refraction microtremor method has been increasingly used as an appealing tool for investigating near surface S-wave structure. However, inversion, as a main stage in processing refraction microtremor data, is challenging for most local search methods due to its high nonlinearity. With the development of data optimization approaches, fast and easier techniques can be employed for processing geophysical data. Recently, particle swarm optimization algorithm has been used in many fields of studies. Use of particle swarm optimization in geophysical inverse problems is a relatively recent development which offers many advantages in dealing with the nonlinearity inherent in such applications. In this study, the reliability and efficiency of particle swarm optimization algorithm in the inversion of refraction microtremor data were investigated. A new framework was also proposed for the inversion of refraction microtremor Rayleigh wave dispersion curves. First, particle swarm optimization code in MATLAB was developed; then, in order to evaluate the efficiency and stability of proposed algorithm, two noise-free and two noise-corrupted synthetic datasets were inverted. Finally, particle swarm optimization inversion algorithm in refraction microtremor data was applied for geotechnical assessment in a case study in the area in city of Tabriz in northwest of Iran. The S-wave structure in the study area successfully delineated. Then, for evaluation, the estimated Vs profile was compared with downhole data available around of the considered area. It could be concluded that particle swarm optimization inversion algorithm is a suitable technique for inverting microtremor waves.     

Generation of isoseismal maps for Mexico using velocity and acceleration data

Acceleration and velocity data of large earthquakes recorded by the Mexican National Broadband Seismological Network were used to generate isoseismal maps for Mexico. The seismic data consist of 99 events recorded at 27 seismic stations located in the southern and northern regions of Mexico from 2004 to 2009. The magnitude (Mw) of these events ranged from 4.1 to 7.1. Peak ground velocity values (PGV) and peak ground accelerations (PGA) were estimated, and velocity-derived peak ground accelerations (PGAv) were calculated. No important differences between PGAv and PGA values were found; thus, both parameters were used in the isoseismal determination. The generated synthetic isoseismal maps were compared with those from an existing catalog for large earthquakes in Mexico. Using empiric relations between PGA and MMI (modified Mercalli intensity) and PGV and MMI, the obtained simulated intensity maps showed similar behavior to those reported in the catalog. The results indicate that the PGAv values can be used to determine intensities when acceleration records (PGA) are unavailable.     

Shear-wave velocity structure of the south-eastern part of the Iberian Peninsula from Rayleigh wave analysis

In this study, we present the lithospheric structure of the south-eastern part of the Iberian Peninsula by means of a set of 2D images of shear velocity, for depths ranging from 0 to 50 km. This goal will be attained by means of the inversion of the Rayleigh wave dispersion. For it, the traces of 25 earthquakes occurred on the neighbouring of the study area, from 2001 to 2003, will be considered. These earthquakes have been registered by 11 broadband stations located on Iberia. All seismic events have been grouped in source zones to get an average dispersion curve for each source-station path. The dispersion curves have been measured for periods between 2 and 45 s, by combination of two digital filtering techniques: Multiple Filter Technique and Time Variable Filtering. The resulting set of source-station averaged dispersion curves has been inverted according to the generalized inversion theory, to get S-wave velocity models for each source-station path. Later, these models have been interpolated using the method of kriging, to obtain a 2D mapping of the S-wave velocity structure for the south-eastern part of Iberia. The results presented in this paper show that the techniques used here are a powerful tool to investigate the crust and upper mantle structure, through the dispersion analysis and its inversion to obtain shear velocity distributions with depth. By means of this analysis, principal structural features of the south-eastern part of Iberia, such as the existence of lateral and vertical heterogeneity in the whole study area, or the location of the Moho discontinuity at 30 km of depth (with an average S-velocity of uppermost mantle of 4.7 km/s), have been revealed. Other important structural features revealed by this analysis have been that the uppermost of Iberian massif shows higher velocity values than the uppermost of the Alpine domain, indicating that the massif is old and tectonically stable. The average velocity of the crust in Betic cordillera is of 3.5 km/s, while in the Iberian massif is 3.7 km/s. All these features are in agreement with the geology and other previous geophysical studies.     

地层特性随机场插值方法应用研究

地质勘探中钻孔的数量常是有限的,钻孔以外未知的地质特性需要进行插值或推断。实践中一般假定两钻孔之间的地质特性为线性分布,用直线连接。线性连接方法简单,但精度难以保证。因此,有必要进一步研究简便且精度高的插值方法。对常用的空间插值数学方法作一简要评价,认为Kriging法更适用于地质特性随机场的插值。并尝试将Kriging法应用于南水北调穿黄工程砂土地基标准贯入击数及液化范围的空间插值,采用合适的变差函数以反映砂层的空间相关特性,由插值结果得到地基砂层标贯击数及液化范围的沿线分布,从而可以更好地了解地层特征的变化情况。     

Shear wave velocity anisotropy of salt- and polymer-amended kaolinite

Shear wave velocity (V_s) anisotropy of kaolinite mixed with sodium chloride (NaCl) and organic polymer (polyethylene oxide, xanthan gum, and chitosan) solutions was investigated using a custom-made floating wall consolidometer-type bender element testing system. The addition of salt and polymers influenced the microfabric anisoopy of platy kaolinite particles, thus resulted in the increment or decrement in the V_s anisotropy. The V_s of kaolinite in all three orthogonal directions increased as the NaCl concentration increased; however, the V_s anisotropy decreased. PEO and chitosan increased the V_s of kaolinite, while xanthan gum exhibited counter-effects. V_s anisotropy (V_s?hh/V_s?vh and V_s?hv/V_s?vh) of polymer amended kaolinite was found to decrease. In addition, both salt- and polymer-modified kaolinite did not show V_s cross-anisotropy.

     

Vibrational characteristics of soil deposits with variable wave velocity

The paper studies certain dynamic characteristics of soil deposits with wave velocities increasing with depth according to (1 + μz)^m, where m = 1, 2/3, 1/2, 1/4 and 0. Analytical solutions are presented for the fundamental periods, mode shapes and amplification functions due to vertically propagating shear waves and the effects of type (m) and rate (μ) of heterogeneity are systematically investigated. The analytical–numerical techniques are used to study the attenuation with depth in a deposit of the vertical and horizontal displacements due to travelling of Rayleigh waves. Differences attributed to different heterogeneities are discussed in connection with the machine isolation problem and the steady-state vibration technique for soil exploration. Finally, the dependence on soil heterogeneity of the time–distance response curves, obtained during seismic refraction surveys, is graphically illustrated.     

含AVO 效应的转换波速度分析方法

当转换波地震记录存在振幅值随炮检距变化,尤其是极性发生反转时,传统转换波速度分析计算结果不准确。为此,按照PP 波AB 相似系数速度分析方法,解决PP 波AVO 效应的思路,对存在 AVO 现象的转换波资料进行AB 相似系数方法的拓展,推导出用于解决转换波含有AVO 现象的AB 相似系数方法,为提高转换波速度分析精度提供了理论支撑。