Microseismic field affected by local geological heterogeneities and microseismic sounding of the medium

Experiments and numerical model studies have shown that heterogeneities of the Earth’s crust distort the spectrum of the low frequency microseismic field, decreasing spectral amplitudes of a specific frequency f at the Earth’s surface over high velocity heterogeneities and increasing them above low velocity heterogeneities. The frequency f is connected with the depth of a heterogeneity H and the velocity of the fundamental mode of Rayleigh waves V _R (f) through the relation H = 0.5 V _R (f)/f. The low frequency microseismic field is considered as the superposition of trains of Rayleigh fundamental modes with different frequency spectra. The paper proposes an experimentally tested technology enabling the determination of the deep structure of complex geological objects using data on the microseismic background field.     

On the theory of the microseismic sounding method

Based on the theory of surface waves in a media with weak lateral heterogeneities, an explanation is given for the peculiarities that are considered as prospecting indicators in the microseismic sounding method (MSM). According to this theory, during the propagation of surface waves in such media, their local characteristics are the same as in the case of propagation in a horizontally homogeneous medium characterized by the same vertical velocity section as underlying a given point. Since the wave energy flux through a semiinfinite vertical strip is conserved, the amplitude of the wave is redistributed along the vertical. Based on this principle and simple physical considerations, it is shown that above a low-velocity domain, the amplitudes of microseisms should increase at wavelengths approximately exceeding the depth of the anomaly by a factor of three and decrease at small wavelengths. The model calculations show that variation of spectral amplitudes only allows a depth to the anomaly to be estimated rather than its average extent. Practically identical spectra characterize the anomalies in which the product of the vertical extent of the anomaly and the velocity contrast is approximately similar.     

青藏高原东南部地区瑞雷波相速度层析成像

本研究收集了"中国地震科学探测台阵-南北地震带南段"项目325个流动宽频带台站于2011年8月至2012年9月记录的远震垂直向资料,利用双台法测得了3594条独立路径上的瑞雷波相速度频散曲线,反演得到了青藏高原东南部地区周期10~60 s瑞雷波的相速度分布图像.空间分辨尺度图表明,在台站覆盖范围内的绝大部分地区横向分辨率达到50 km.2D相速度分布图显示,青藏高原东南部地区地壳上地幔S波速度结构存在较明显的横向非均匀性.短周期(如10 s)的相速度分布主要受地表沉积层厚度的影响.绝大多数地震发生在周期15 s相速度图上的低速区或高低速的陡变梯度带附近,充分说明该区的强震活动与中上地壳速度结构的变化有直接关系.中等周期(如20~30 s)的相速度分布主要与中下地壳速度结构、地壳厚度密切相关,小江断裂、松潘-甘孜块体呈现最显著的低速,可能暗示这两处的中、下地壳存在低速层.较长周期(如40~60 s)的相速度分布与上地幔顶部热状态和构造活动(如岩浆作用)有关.滇西南地区表现为大范围的显著低速,可能暗示滇西南地区上地幔顶部物质存在部分熔融.不同构造块体下方的频散曲线,具有不同的相速度特征.腾冲火山下方的频散曲线在10~60 s一直为较低的速度,尤其是到40 s以后,相速度随周期的变大增速明显放缓,至60 s比其他任何块体速度都低,暗示腾冲火山区下方的低速至少来自上地幔顶部(约100 km).     

Rayleigh波入射下相邻山体地震响应边界元法模拟

采用间接边界元法（IBEM）,对Rayleigh波入射下两邻近山体的地震响应进行了定量分析。结果表明:与单个山体在Rayleigh波入射下的地震响应相比,两山地形的地震反应规律更为复杂,反应特征受控于入射波频率和山体间距等因素。总体上看:入射Rayleigh波频率较低时,两山间距对双山地形地震反应影响较大。低频波入射,受邻近山体影响,迎波面山体水平位移峰值约为入射波水平位移的6.3倍,放大效应可达单山作用的1.5倍。且山体山脚处竖向位移的频谱振荡更为剧烈,特定频率下山脚的竖向位移反应可达单山放大效应的1.6倍。较高频波入射时,迎波面一侧山体受邻近山体影响较小,且对波表现出明显的屏障效应,背波面山体地震反应强度被削弱。     

南海瑞雷面波群速度层析成像及其地球动力学意义

南海处于欧亚板块、太平洋板块和印度—澳大利亚板块的交会区,是西北太平洋一系列边缘海中最大的边缘海。关于南海的打开以往研究提出了如印度板块与欧亚板块碰撞驱动挤出以及古南海俯冲拖拽等诸多模型。本文力图通过南海海盆及周边各向异性结构来约束南海演化机制。基于同济大学2012和2014年在南海中央海盆进行的两次被动源宽频带海底地震观测试验回收的10台OBS记录仪近1年的地震数据,本文采用三种不同的横波分裂方法,获取了中央海盆针对两次远震的XKS分裂结果以及南海周边20次区域地震提供的S震相分裂结果。SKS分裂结果显示,南海中央海盆下方存在快轴方向为NE-SW向的各向异性,其成因可能与海底扩张时期沿洋脊方向的地幔流以及南海海洋板块俯冲拖拽的地幔流有关。南海及其周边上地幔存在强各向异性,且不同方位观测到的各向异性不同,快轴方向与前人SKS横波分裂结果、GPS和板块运动一致,较好地对应了区域构造运动或者地幔对流模型。各向异性结果与印度—欧亚板块碰撞驱动挤出模型以及古南海俯冲板块拖拽模型预期结果一致,与理想的地幔柱上涌驱动模型不一致。由于海盆各向异性观测特别有限,各向异性结果不能证实亦不能证伪“大西洋型”海底扩张模型、弧后扩张模型和板缘破裂模型,后续还需要更多的观测结果来证实或证伪上述模型。     

南海瑞雷面波群速度层析成像及其地球动力学意义

南海处于欧亚板块、 菲律宾海板块、 太平洋板块和印度-澳大利亚板块的交汇处, 其地质和构造作用十分复杂．通过面波群速度成像, 给出了南海及邻区的三维横波速度分布并分析了其地球动力学意义．南海西部和南部新布设的地震台站使得利用单台法时路径覆盖比过去更好. 特别是在华南地区, 新的台站分布能够弥补该地区地震少且台站少造成的射线密度不够的缺点． 首先运用多重滤波法得到南海周边48个台站周期为14——130 s范围内的基阶瑞雷波频散曲线图; 接着通过子空间反演得到整个区域在不同周期时的群速度分布; 最后通过阻尼最小二乘反演得到不同深度切片上的横波速度分布及不同纵剖面上的横波速度分布． 结果显示: ① 海盆速度较高, 且速度分布很好地勾勒出海盆的轮廓. 浅层较高的横波速度说明海盆都具有洋壳性质, 而深部较高的横波速度则可能对应扩张中心生成洋壳后残留的高速物质． 不同海盆速度上的差异与它们的热流值和年龄大小一致．海盆下的高速异常在60 km以下消失, 且在一定深度范围内由低速区替代． 在低速区下200 km深度, 在南海海盆观测到一条NE-SW走向的高速异常, 可能与古俯冲带有关. ② 环南海出现明显的高速区, 对应俯冲带特征, 且这些高速区速度差异明显且有间断, 说明俯冲带的非均质性和俯冲角度的差异. ③ 在环南海高速区内侧（向南海侧）观测到不连续的低速区． 在浅层, 这些低速区反映了沉积层和地壳的厚度特征. 在地幔, 这些低速区可能对应于古太平洋俯冲带的地幔楔或者也可能反映了南海海盆停止扩张后残留的地幔熔融物质. ④ 南海海盆岩石圈的厚度为60——85 km．      

东北地区背景噪声的Rayleigh和Love波相速度层析成像

本文利用中国数字地震台网位于东北地区的122个宽频地震台站的18个月记录的三分量连续地震噪声数据,采用互相关方法提取了Rayleigh和Love波经验格林函数,并利用时频自动分析技术获取了相应的相速度频散曲线.通过反演频散曲线,获得了Rayleigh和Love波周期为8~35 s的二维相速度分布.结果表明,东北地区相速度的分布存在横向和垂向的不均匀性.短周期的相速度分布同地表地质构造密切相关,松辽盆地及山间沉积盆地呈现低速异常,而大兴安岭、小兴安岭及东部的一些山岭显示高速异常.随着周期的增加,位于中间的松辽盆地变为高低速相间,两侧的造山带呈现低速异常.这种异常的转变,可能是受构造活动或者莫霍面深度的影响.另外,在周期为20~35 s 频段内,Rayleigh和Love波同一周期的相速度在松辽盆地和位于吉林地区的郯庐断裂带表现不一致,表明可能存在径向各向异性.     

川西地区台阵环境噪声瑞利波相速度层析成像

2006年中国地震局地质研究所地震动力学国家重点实验室在川西地区(26°N~32°N,100°E~105°E)布设了由297台宽频带数字地震仪组成的流动观测台阵.利用该密集台阵29°N以北156个台站2007年1~12月份的地震环境噪声记录和互相关技术,我们得到了所有台站对的面波经验格林函数和瑞利波相速度频散曲线,并进一步反演得到了观测台阵下方2~35 s周期的瑞利波相速度分布图像．本文结果表明,观测台阵覆盖的川滇地块、松潘-甘孜地块和四川盆地的地壳速度结构存在显著差异,具体表现为：(1)短周期（2~8 s）相速度分布与地表构造特征相吻合,作为川滇地块、松潘-甘孜地块和四川盆地之间的边界断裂,龙门山断裂带和鲜水河断裂带对上述三个地块上地壳的速度结构具有明显的控制作用,四川盆地前陆低速特征表明相应区域存在较厚的（约10 km）沉积盖层;(2)中周期（12~18 s）相速度分布表明,川滇地块和松潘-甘孜地块中上地壳速度结构存在明显的不均匀横向变化,并形成了尺度不同且高、低速相间的分块结构,而四川盆地中地壳整体上已经表现出相对高速;(3)长周期（25~35 s）相速度分布表明,松潘-甘孜地块,特别是川滇地块中下地壳表现为广泛的明显低速异常,意味着它们的中下地壳相对软弱,而四川盆地的中下地壳呈现整体性的相对高速,意味着四川盆地具有相对坚硬的中下地壳,并且以汶川地震的震中为界,龙门山断裂带的地壳结构显示了北段为高速异常,南段为低速异常的分段特征．     

Laboratory measurements of spatial fluctuation and attenuation of elastic waves by scattering due to random heterogeneities

To study the effects of strong scattering on elastic waves, spatial fluctuation and scattering attenuation ofP waves were examined by laboratory experiments for 2-D models of random media approximately characterized by a triangular correlation function in the range of 2<ka<33, wherek is the wave number anda is the correlation distance of the heterogeneities, i.e., the heterogeneity size. The results obtained are as follows: (1) Forka>10, both the intensity and the correlation distance of the amplitude fluctuation are approximate for any phase of theP-wave train. The correlation distance nearly agrees with the heterogeneity size. These fluctuation properties are quite consistent with the theoretical prediction by the forward-scattering approximation. (2) For 3<ka<6, the fluctuation intensity becomes stronger in later phases of theP-wave train. This shows that scattering is approximately isotropic, and therefore, the scattered energy increases with time within theP-wave train. The correlation distance of the amplitude fluctuation disagrees with the heterogeneity size, and it shows a frequency-dependent property decreasing from 7a to 4a with the increase ofka from 3 to 6. These properties for 3<ka<6 have not yet been predicted theoretically. (3) Forka<3, though the fluctuation is considerably smaller compared with that ofka>10 and 3<ka<6, the fluctuation property is considered similar to that of 3<ka<6. (4) The observed scattering attenuation,Q ^–1, increases withka forka<3, has a peak aroundka=35, and then decreases withka. (5) When _min = 15° and = 0.075, the theoreticalQ ^–1 curve, predicted by the approximate theory of Wu, roughly matches the observedQ ^–1 values, where _min is the minimum scattering angle measured from the propagation direction of theP waves and is the rms of fractional velocity fluctuation. This suggests that the energy scattered in the range of >15° is lost from theP waves, while the energy scattered in the range of <15° is retained; and that the approximate theory overestimates by about three times the value of the model media used owing to the neglect of multiple scattering. (6) When the size of velocity heterogeneities responsible for forward scattering at 3<ka<6 is estimated from the _min value of 15° on the basis of Wu's theory, it nearly agrees with the correlation distance for the initial phase of theP-wave train.     

Group velocity distribution of Rayleigh waves and crustal and upper mantle velocity structure of the Chinese mainland and its vicinity

Introduction A lot of results have been achieved on the study of crust and upper mantle structure in Chinese mainland by the surface wave dispersion. The seismometer can be classified into three types based on the development from the analog to digital instrument. FENG, et al (1981) measured the dispersion curves of group and phase velocity along 28 paths, with 19 earthquakes recorded by 15 base analog seismic stations, subdivided the Chinese mainland into 5 blocks and obtained the average c…     

扬子与华夏块体缝合带区域的Rayleigh及Love波相速度层析成像

扬子与华夏块体碰撞拼合构成华南大陆, 其间存在多条大型断裂带, 两个块体缝合于哪些断裂尚存争议, 这限制了对华南构造演化的认识.本研究使用布设于扬子和华夏块体之间的184个流动密集台站以及华南地区273个固定台站的三分量连续波形数据(2014年10月至2016年12月), 采用背景噪声层析成像方法获得研究区域内Rayleigh和Love波周期为4~25 s的二维相速度.短周期的相速度与地壳中上部的盆地和褶皱等地质构造单元吻合; 长周期的相速度在东部地区整体表现为高速异常, 可能是受较薄的地壳厚度影响, 西南部地区Rayleigh波呈现高速异常, Love波则呈现显著低速异常, 这种差异可能反映该区域地下介质存在径向各向异性.成像结果显示, 扬子与华夏块体的Rayleigh和Love波相速度存在较大的差异, 结合华南地区已有的地质与地球物理学研究成果, 本研究认为扬子与华夏块体可能在地壳浅层缝合于绍兴—江山—萍乡—永州—河池一线, 深部缝合带则可能由石台—九江—长沙延伸至永州—河池线.

     

Propagation of Rayleigh waves in soils

Summary Propagation of Rayleigh type waves in soils is considered in this paper. It is a well known fact that soils do not behave like an ordinary isotropic elastic medium where the ratio of Young's modulus to the modulus of rigidity is much less than that in sandy soils. Considering the velocity of Rayleigh type wave as less than that of distortional wave (which is an observed fact) a probable value of this ratio is determined, and also assuming the value of this ratio based on some experimental data, the velocity of wave propagation in the medium is deduced.     

Feasibility of waveform inversion of Rayleigh waves for shallow shear-wave velocity using a genetic algorithm

Conventional surface wave inversion for shallow shear (S)-wave velocity relies on the generation of dispersion curves of Rayleigh waves. This constrains the method to only laterally homogeneous (or very smooth laterally heterogeneous) earth models. Waveform inversion directly fits waveforms on seismograms, hence, does not have such a limitation. Waveforms of Rayleigh waves are highly related to S-wave velocities. By inverting the waveforms of Rayleigh waves on a near-surface seismogram, shallow S-wave velocities can be estimated for earth models with strong lateral heterogeneity. We employ genetic algorithm (GA) to perform waveform inversion of Rayleigh waves for S-wave velocities. The forward problem is solved by finite-difference modeling in the time domain. The model space is updated by generating offspring models using GA. Final solutions can be found through an iterative waveform-fitting scheme. Inversions based on synthetic records show that the S-wave velocities can be recovered successfully with errors no more than 10% for several typical near-surface earth models. For layered earth models, the proposed method can generate one-dimensional S-wave velocity profiles without the knowledge of initial models. For earth models containing lateral heterogeneity in which case conventional dispersion-curve-based inversion methods are challenging, it is feasible to produce high-resolution S-wave velocity sections by GA waveform inversion with appropriate priori information. The synthetic tests indicate that the GA waveform inversion of Rayleigh waves has the great potential for shallow S-wave velocity imaging with the existence of strong lateral heterogeneity.     

Analysis of Rayleigh waves in saturated porous elastic media by finite element method

A numerical procedure for the analysis of Rayleigh waves in saturated porous elastic media is proposed by use of the finite element method. The layer stiffness matrix, the layer mass matrix and the layer damping matrix in a layered system are presented for the discretized form of the solid-fluid equilibrium equation proposed by Biot. In order to consider the influence of the permeability coefficient on the behavior of Rayleigh waves, attention is focused on the following states: ‘drained’ state, ‘undrained’ state and the states between two extremes of ‘drained’ and ‘undrained’ states. It is found from computed results that the permeability coefficient exerts a significant effect on dispersion curves and displacement distributions of Rayleigh waves in saturated porous media.     

Generation of earth stratified models compatible with both ellipticity and phase velocity observations of Rayleigh waves

In the eigenvalue eigenvector decomposition used in inversion techniques, we can sometimes obtain eigenvalues close to zero. Related to these eigenvalues close to zero, a set of stratified earth models is generated. These are compatible with the theoretical observations according to a given model. The method is applied to ellipticities and phase velocities of Rayleigh waves, and supply proof of the advantage of working with both sets of data together.     

大别-苏鲁及其邻近地区基于背景噪声的瑞雷波和勒夫波相速度层析成像

采用与作者2014年发表的“大别-苏鲁及其邻近地区基于背景噪声的勒夫波群速度成像”文章相同的资料,用频时分析提取5 000余条瑞雷波和4 000余条勒夫波相速度频散曲线,反演得到了8—32 s的瑞雷波和勒夫波相速度分布图像．结果显示,瑞雷波与勒夫波相速度分布具有很好的一致性．8 s的相速度分布与地表构造特征相吻合,造山带与隆起区均表现为高速,盆地因其规模不同而显示不同程度的低速．随着周期的增大,大别 苏鲁的高速带由强变弱,但始终存在．16—24 s的高速可能主要受到中地壳高速的控制,而32 s的高速则可能与上地幔顶部的高速有关．比较大别造山带与苏鲁造山带的平均频散曲线,发现大别造山带和苏鲁造山带的勒夫波频散曲线均高于AK135模型计算的理论频散曲线,而瑞雷波则没有这一现象. 这可能意味着两个地区有比较强烈的径向各向异性．      

瑞雷波斜入射下层状半空间中沉积谷地周围的三维散射研究

针对层状半空间中沉积谷地对斜入射瑞雷波的三维散射问题, 采用直接刚度法计算自由场波场, 以层状半空间中移动斜线均布荷载动力格林影响函数求解三维散射波场, 建立了求解该问题的间接边界元方法. 通过与已有结果的比较, 验证了该方法的正确性, 并以均匀半空间以及弹性基岩上单一土层场地中沉积谷地为例进行了计算分析. 研究结果表明： 层状半空间与均匀半空间中沉积谷地对瑞雷波的散射存在显著差别; 层状半空间中瑞雷波的振动模态对沉积附近位移幅值有着重要影响; 土层刚度和厚度等参数也对沉积附近位移幅值大小及空间分布有着显著的影响.     

Dispersion of Rayleigh waves along the Prague-Warsaw profile

Summary Phase velocities of Rayleigh waves propagating along the Prague-Warsaw profile have been determined. A layered model of the Earth's crust and upper mantle, satisfying the observed dispersion of Rayleigh waves, has been derived. In constructing this model, the results of deep seismic soundings along international profile VII were also taken into account.     

华北地区瑞雷面波相速度层析成像

利用华北科学探测台阵190个宽频带台站和8个甚宽带台站2006年10月至2009年5月记录的远震垂直向资料,用小波变换频时分析方法测定了1587条独立路径的基阶瑞雷波相速度频散曲线,并使用Ditmar & Yanovskaya 方法反演得到111°~120°E,37°~42°N区域内、周期10~60 s的高分辨率相速度分布图像.本文的研究结果表明,华北地区的地壳上地幔横波速度结构存在横向非均匀性.短周期(如10 s)的相速度分布与地表地质构造具有明显的相关性,随着周期的增大(如15 s),地形的控制作用相对减弱.中周期(如30 s)的相速度分布差异暗示华北克拉通中、东部地区下地壳具有不同的物质组成,图中相速度梯度带与太行山重力梯度带位置十分相近,说明该重力梯度带两侧的地壳速度结构差异较大.与短周期相比,研究区内长周期(如60 s)相速度分布的横向非均匀性明显减弱,表明研究区上地幔顶部速度结构差异较地壳的小.与全球典型大陆克拉通相比,华北克拉通10~60 s的相速度频散明显偏低,其频散特征与全球典型裂谷地区相类似,这暗示华北克拉通已经遭受了明显的侵蚀与破坏.