南极长城站地壳和上地幔各向异性分析

利用我国第24次和第25次南极科学考察队于2008年2月—2010年3月南极长城站记录到的地震事件数据进行剪切波分裂研究. 选取近震事件对Sg波进行剪切波分裂计算,结果表明快波偏振方向有两个,分别为北东向和近南北向; 慢波延迟时间的范围为1.45—5.17 ms/km,平均值为3.54 ms/km.同时选取长城站记录到的远震数据SKS波震相进行剪切波分裂计算,得出上地幔快波偏振方向优势取向为北东向, 慢波延迟时间平均值为1.60 s. 剪切波分裂结果显示长城站地区地壳和上地幔具有明显的各向异性, 并显示长城站地区地壳与上地幔快波偏振方向几乎平行,表明壳幔变形的一致关系.另外,地壳和上地幔各向异性的快波偏振方向不仅与长城站附近的海沟方向平行,同时也与绝对板块的运动方向平行.该结果进一步说明了绝对板块的运动是构成上地幔各向异性的主要原因.     

采用剪切波分裂方向约束的板块绝对运动模型研究

板块绝对运动（即岩石圈板块相对于深部地幔的运动）导致软流圈深度存在较强的剪切作用,为软流圈呈现较强的地震波速各向异性提供了一种物理解释.相应地,软流圈地震各向异性的实测数据为反演板块绝对运动提供了一种定量的约束.本文利用前人发表的由474个剪切波分裂数据组成的全球软流圈地震各向异性方向数据集,结合板块相对运动模型MORVEL,通过加权最小二乘法反演板块绝对运动.计算结果表明,由实测数据约束的板块绝对运动模型不能将观测数据拟合到原始数据测量误差要求的统计水平上,反映出以地震各向异性方向指示板块绝对运动方向存在原始数据测量误差之外的系统性误差.该误差或可归因于板块绝对运动控制软流圈地震各向异性的物理机制复杂性以及小尺度地幔流动的区域复杂性.因此,地震各向异性方向只能从统计平均的意义上约束板块绝对运动方向,两者间的差别除了测量误差外还应包含复杂物理成因的模型误差.在原始数据测量误差的基础上增加20°的模型误差并且剔除一个离群数据后,本文得到了最优拟合剪切波分裂数据集的SKS473模型,并与由热点数据反演得到的板块绝对运动模型进行对比.由于约束板块绝对运动的地震各向异性数据和热点数据都存在误差较大、地理分布不均的局限性,结合两类数据的联合反演或可成为未来建立更高精度板块绝对运动模型的有效途径.

     

基于横波分裂的青藏高原多圈层各向异性研究进展

有关青藏高原横波分裂的各向异性研究已经开展了近30年,在理论方法和实际应用方面取得了重要进展,并获取了大量的横波分裂测量结果,为认识青藏高原壳幔各向异性变形特征和动力学机制提供了重要的依据.本文首先介绍了地震各向异性的来源与应用,随后回顾了横波分裂分析方法的发展,简述了各种横波分裂方法的原理,最后通过总结近30年来青藏高原上地壳、整个地壳和上地幔横波分裂各向异性研究成果,系统分析了青藏高原壳幔各向异性变形特征.基于各横波分裂结果的对比分析来看,XKS波分裂测量结果最为稳定,近震直达S波分裂测量结果次之,而Pms波分裂测量结果相对离散,往往相同区域内不同的研究结果差异较大,主要原因可能是相比XKS波和近震直达S波,Pms波的信噪比较低,次要原因可能是各研究在方法和处理分析等方面的差异.     

利用DONET海底观测网研究日本南海海域俯冲带地震波各向异性

基于DONET海底观测网的直达S波地震记录,采用波形旋转互相关方法和最小特征值最小化方法求得了日本南海海域俯冲带横波分裂快轴方向和分裂时间,获得了该俯冲带地震波的各向异性结果.结果显示:该俯冲带地震波的各向异性快轴方向基本平行于南海海槽走向,分裂时间为0.1—0.96 s.这表明:日本南海海域俯冲带各向异性来源于太平洋...     

东南亚下方核幔边界D″层地震各向异性

本文利用中国国家数字地震台网(CDSN)和国际数字地震台网(FDSN)的宽频带远震资料,采用S和ScS震相剪切波差异分裂方法对东南亚下方核幔边界D″层地震各向异性进行研究.共获得了来自7个深震(400km)的24对高质量的S和ScS剪切波分裂参数,其中S震相的分裂参数与前人上地幔各向异性研究结果十分吻合,表明S震相观测结果反映了观测台站下方的上地幔各向异性.通过对上地幔各向异性校正后的波形数据进行进一步的分裂参数分析,最后得到了24个可靠的ScS剩余各向异性分裂参数.结果显示,在经度107°~112°范围内分裂参数均为空值(Null);而在该经度范围东西两侧区域,主要表现为明显分裂特征.这种分裂参数的分布指示了核幔边界D″层的各向异性特征的区域变化,我们推测这种变化可能与停滞在核幔边界的古特提斯和/或古太平洋俯冲板块相关.     

中国大陆及邻区SKS波分裂研究

SKS波分裂测量是研究大陆地幔的形变特征、探索大陆动力学和演化过程的重要工具. 本文对中国大陆及邻区地震台站的SKS波分裂现象进行了研究. 选用中国数字化宽频带地震台网(CB台网)和美国IRIS数据中心提供的三分量宽频带数字化地震资料，使用SC(Silver and Chan,1991)方法，得到了中国大陆及周边地区80多个台站下方上地幔各向异性参数，即快波偏振方向φ和快慢波到时差δt. 快波偏振方位在相同地块有一定的优势排列方向，大多数台站快波偏振方向都能与过去或现今大规模的构造运动得到很好的符合. 整个研究区域所得到的分裂延迟时间在0.4～2.4s之间，平均为1.2s. 根据SKS波测量得到的分裂参数，分析了该研究区域各向异性介质的特性，从而探索与岩石弹性各向异性相关的地球内部动力学过程.     

中国东北地区北部上地幔各向异性及其动力学意义

中国东北地区广泛发育新生代板内火山,晚中生代以来岩石圈遭受过多期拉张作用.作为中国唯一的深震孕育区,中国东北地区受到太平洋板块的西向俯冲,使得其成为研究岩石圈变形、板块俯冲和板内火山成因及其相互作用关系的天然实验室.通过分析架设在中国东北地区北部的147个流动和固定台站的SKS波形数据,共计得到了377对各向异性参数和251个无效分裂结果.结果表明,中国东北地区东西两侧具有不同的各向异性分布:西部地区各向异性方向变化范围为N143-199°E,平均N169°E,与晚中生代岩石圈伸展方向一致;其各向异性延迟时间平均值约为0.8s,说明来自地幔的各向异性比较微弱,主要由残留在岩石圈中的古老变形所引起.同时,在松辽盆地和佳木斯地块部分区域,观测到延迟时间较小的各向异性(~0.4s),可能是由于岩石圈的拆沉和热地幔物质的上涌侵蚀了保留在岩石圈的古老形变所致.在研究区东部,NNW-SSE朝向的各向异性被观测到,并伴随较大的延迟时间(大于1.0s),可能与太平洋板块撕裂回撤而产生的地幔流动有关.此外,近W-E方向的各向异性只在佳木斯地块被观测到,而太平洋板块在地幔过渡带中的俯冲可能是其产生的主要成因.     

华东地区SKS波分裂研究

本文搜集整理了华东地区6个区域地震台网的宽频带数字地震台站SKS波记录资料, 使用最小切向能量的网络搜索法, 得到了华东地区157个台站下方上地幔各向异性参数。 测算结果表明, 华东地区各向异性快波偏振方向主体为NW—SE向、 南部近E—W向, 逐渐呈旋转趋势, 与绝对板块运动方向一致; 其各向异性主要来自于上地幔, 地壳与沉积层对各向异性影响较小; 研究区内地下浅部与深部物质的运动模式基本一致, 壳幔变形存在垂直连贯变形的特征。     

中国东北诺敏河火山岩石圈变形:来自剪切波分裂的证据

中国东北新生代板内火山广泛发育,其中诺敏河火山由于上地幔结构研究的匮乏,火山成因尚不明确.利用布设在诺敏河火山周围的40个流动台站所记录到的远震剪切波数据,测量得到82对各向异性参数和219个无效分裂结果.结果表明,研究区快慢波延迟时间变化范围为0.4~1.4 s,平均0.78±0.21 s;各向异性快波方向范围为N77°W-N18°E,绝大多数快波方向集中在N6.9°W±9.87°,平行于中生代晚期岩石圈伸展变形方向,推测由残留在岩石圈中的化石各向异性所引起.同时,在火山中心及周边部分台站,只观测到无效分裂而没有观测到有效分裂结果,可能是由于残存在岩石圈内的古老形变被上涌的热地幔物质所侵蚀.

     

Shear wave anisotropy in D＂ region beneath the western Pacific

Using seismic shear phases from 47 Tonga-Fiji and its adjacent region events recorded by the CENC and IRIS, and from 26 northeast Asia and north Pacific events recorded by IRIS, we studied the shear wave anisotropy in D" region beneath the western Pacific utilizing the ScS-S differential travel time method and obtained the splitting time values between the radial and transverse components of each ScS wave corresponding to each core-mantle boundary (CMB) reflection point. We found that most shear waves involved horizontally polarized shear wave components traveling faster than vertically polarized shear wave components through the D" region. The splitting time values of ScS wave range from ?0.91 s to 3.21 s with an average value of 1.1 s. The strength of anisotropy varies from ?0.45% to 1.56% with an average value of 0.52%. The observations and analyses show that in the D" region beneath the western Pacific the lateral flow is expected to be dominant and the vertical transverse isotropy may be the main anisotropic structure. This structure feature may be explained by the shape preferred orientation of the CMB chemical reaction products or partial melt and the lattice preferred orientation of the lower mantle materials caused by the lateral flow at lowermost mantle.     

西太平洋下D"区的剪切波各向异性

利用来自汤加—斐济及周边地区发生的47次地震并由中国地震台网中心和IRIS台站记录到的数据,与来自东亚北部及北太平洋地区发生的26次地震并由IRIS台站记录到的数据, 使用ScS-S相对走时分析方法研究了西太平洋下D区的剪切波各向异性, 得到了核幔边界反射点上ScS波径向分量和横向分量的分裂时间, 并计算了D区的地震波各向异性强度. 发现ScS波通过西太平洋下D区时普遍呈现出径向分量走时大于横向分量走时的模式. ScS波的分裂时间值为-0.91~3.21s, 平均值为1.1 s; 各向异性强度值为-0.45%~1.56%, 平均值为0.52%. 观测和分析结果表明, 西太平洋下D区水平流应占支配地位,各向异性结构主要是具有垂直对称轴的横向各向同性介质. 形成这种结构的可能原因是地幔底部水平流导致的核幔边界反应产物或部分熔融物质的形状优选方向, 或下地幔物质的晶格优选方向.      

塔里木盆地南部上地幔各向异性研究

塔里木南缘位于塔里木块体同青藏高原碰撞的前缘,是认识青藏高原同周围块体相互作用的重要位置.横波分裂方法可以获得岩石圈及软流圈地幔的介质各向异性特征,进而探讨岩石圈变形和地幔流动.本文利用横波分裂方法对中国科学院地质与地球物理研究所、北京大学和南方科技大学联合布设在塔里木盆地南部的8个宽频带流动地震台站记录的SKS和SKKS震相进行分析,获得了塔里木盆地南部上地幔各向异性参数.分析结果显示,研究区快波偏振方向总体比较一致呈近东西向,但存在盆地边缘到盆地内部变化,而慢波延迟时间分布在0.3~1.5 s,差异较大.综合前人对青藏高原北缘和阿尔金断裂周边的横波分裂研究结果,研究区内大部分地区地幔流动方向和绝对板块运动方向保持一致,推测青藏高原北缘同塔里木盆地接触带,青藏高原北缘软流圈南东东向流动对塔里木块体下方地幔流动造成扰动,随着深入到塔里木块体内部,干扰越来越弱;同时青藏高原北部软流圈物质可能对塔里木盆地岩石圈产生影响,随着地幔软流圈物质持续向北运动逐渐冷却,对盆地内部岩石圈的影响程度减弱.

     

湖北地区上地幔各向异性及其动力学意义

SKS波分裂测量是研究大陆地幔的形变特征、探索大陆动力学和演化过程的重要工具.本文选用湖北数字地震台网(HB台网)和中国数字化宽频带地震台网(CB台网)提供的三分量宽频带数字化地震资料,使用旋转相关法和切向能量最小法,计算得到了湖北地区23个地震台站下方上地幔各向异性参数.结果表明,快波偏振方向较为复杂:大别造山带内,呈平行于大别造山带构造走向的NWW向,且与绝对板块运动方向基本一致,反映印支—燕山期扬子板块与华北板块南北向挤压碰撞造山的过程,而地幔流动则是造成这种南北挤压的力源.扬子板块内NWW向虽与绝对板块运动趋势一致,但比较离散,受地幔对流影响不明显,而NE向各向异性主要与燕山后期之后应力场的转变有关.分析认为,该地区的上地幔各向异性为地幔流动和固结在岩石圈中的"化石各向异性"共同作用的结果.根据上地幔各向异性方向与主要构造带的走向平行,可以作为地壳变形与上地幔连贯变形的证据.整个研究区域得到的分裂延迟时间在0.5~1.35 s之间,厚度在58~155 km之间.     

Insight into craton evolution: Constraints from shear wave splitting in the North China Craton

The multi-episodic tectonic activities from the Precambrian to Cenozoic, including nucleus formation, cratonic amalgamation, and rejuvenation, make the North China Craton (NCC) an ideal natural laboratory for studying craton evolution. Spatial change in the upper deformation records is an important aspect for understanding cratonic formation and rejuvenation. In this study, we performed seismic shear wave splitting analysis using SKS phases from 50 portable stations. Two different methodologies, shear wave splitting measurement and amplitude analysis of transverse/radial components, produced mutually consistent splitting results. These results showed that the seismic anisotropy beneath the Ordos Block can be divided into three subgroups reflecting the tectonic control. Combining these results with those from previous splitting studies in the eastern NCC, we suggest that the Proterozoic amalgamation generated the seismic anisotropy in the boundary zone between the Ordos Block and the Trans-North China Orogen, while the anisotropy in the eastern Trans-North China Orogen and eastern NCC were possibly associated with the lithospheric rejuvenation during the Late Mesozoic to Cenozoic.     

Large-scale coherent anisotropy of upper mantle beneath the Italian peninsula comparing quasi-Love waves and SKS splitting

We document quantitatively observations of quasi-Love waves obtained at permanent (Italian National Seismic Network) and temporary seismic stations deployed in Italy between 2003 and 2006 (Retreat, CAT/SCAN projects). We analyzed large earthquakes with source parameters that favor quasi-Love wave generation within this time-span, including the Sumatra–Andaman earthquake of 12/26/04. The presence or the absence of the quasi-Love phase is compared to the smoothed anisotropic pattern defined by the numerous SKS splitting measurements obtained in peninsular Italy, and to the Italian upper mantle structure as defined by seismic tomography. The large-scale anisotropic features, responsible for shear-wave splitting and documented also by Pn and surface-wave anisotropy, generally display the correct geometry to explain the scattered quasi-Love waves. Quasi-Love observations do not demand a tilted-axis anisotropic geometry. We argue instead for anisotropy with laterally-variable horizontal symmetry axis in the upper mantle below the Italian peninsula.     

裂隙参数对衰减各向异性影响的数值模拟

Propagation through stress-aligned fluid-filled cracks and other inclusions have been claimed to be the cause of azimuthal anisotropy observed in the crust and upper mantle.This paper examines the behavior of seismic waves attenuation caused by the internal structure of rock mass,and in particular,the internal geometry of the distribution of fluid-filled openings Systematic research on the effect of crack parameters,such as crack density,crack aspect ratio(the ratio of crack thickness to crack diameter),pore fluid properties(particularly pore fluid velocity),VP/VS ratio of the matrix material and seismic wave frequency on attenuation anisotropy has been conducted based on Hudson’s crack theory.The result shows that the crack density,aspect ratio,material filler,seismic wave frequency,and P-wave and shear wave velocity in the background of rock mass,and especially frequency has great effect on attenuation curves.Numerical research can help us know the effect of crack parameters and is a good supplement for laboratory modeling.However,attenuation is less well understood because of the great sensitivity of attenuation to details of the internal geometry.Some small changes in the characteristics of pore fluid viscosity,pore fluids containing gas and liquid phases and pore fluids containing clay can each alter attenuation coefficients by orders of magnitude.Some parameters controlling attenuation are therefore necessary to make reasonable estimations,and anisotropic attenuation is worth studying further.     

D″ anisotropy inverted from shear wave splitting intensity

The D″ layer, located at the bottom of the mantle, is an active thermochemical boundary layer. The upwelling of mantle plumes, as well as possible plate subduction in the D″ layer, could lead to large-scale material transformation and mineral deformation, which could result in significant seismic anisotropy. However, owing to limited observations and immense computational cost, the anisotropic structures and geodynamic mechanisms in the D″ layer remain poorly understood. In this study, we proposed a new inversion method for the seismic anisotropy in the D″ layer quantitatively with shear wave splitting intensities. We first proved the linearity of the splitting intensities under the ray-theory assumption. The synthetic tests showed that, with horizontal axes of symmetry and ray incidences lower than 30º in the D″ layer (typical SKS phase), the anisotropy is well resolved. We applied the method to the measured dataset in Africa and Western Europe, and obtained strong D″ anisotropy in the margins of the large low shear-wave velocity provinces and subducting slabs. The new method makes it possible to obtain D″ anisotropy, which provides essential constraints on the geodynamical processes at the base of the mantle.     

Anisotropy and Flow in Pacific Subduction Zone Back-arcs

—We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the Tonga, southern Kuril, Japan, and Izu-Bonin subduction zones using shear-wave splitting in S phases from local earthquakes and in teleseismic core phases such as SKS. The observed splitting in all four subduction zones is consistent with a model in which the lower transition zone (520–660 km) and lower mantle are isotropic, and in which significant anisotropy occurs in the back-arc upper mantle. The upper transition zone (410–520 km) beneath the southern Kurils appears to contain weak anisotropy. The observed fast directions indicate that the geometry of back-arc strain in the upper mantle varies systematically across the western Pacific rim. Beneath Izu-Bonin and Tonga, fast directions are aligned with the azimuth of subducting Pacific plate motion and are parallel or sub-parallel to overriding plate extension. However, fast directions beneath the Japan Sea, western Honshu, and Sakhalin Island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing. Models of back-arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in Tonga and Izu-Bonin, but further three-dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern Kurils and Japan. The fast directions in the southern Kurils and Japan may require strain in the back-arc mantle that is driven by regional or global patterns of mantle flow.     

云南地区上地幔各向异性研究

对云南23个数字地震台11次地震的SKS记录，采用理论切向分量与实测切向分量拟合的方法，确定了快S波的偏振方向和快、慢波之间的时间延迟．结果表明，除鹤庆台外，在各台都观测到了S波分裂现象；云南地区的快方向总体特征是北北东向，时间延迟变化范围为0.5～2.0ｓ．在地质构造复杂地区断层对分析的影响很大．分析表明，作为青藏高原与华南块体之间的过渡带，云南地区的S波快方向反映了印度板块向欧亚板块俯冲是该地区地球动力学的基本背景，而由于青藏高原隆起造成的康滇菱形块体的南东-南南东向运动是造成复杂构造、应力环境的重要因素．快方向与上地幔运动的方向存在差异，说明在云南地区低速层或者软流层的运动与地壳块体的运动之间存在着复杂的耦合作用，构造驱动力如同向北东方向张开的手掌．从时间延迟出发，推断各向异性层的厚度为60～225ｋｍ．其变化范围与低速层埋深的变化范围（104～260ｋｍ）相当，认为各向异性层顶面可能在地壳底部，也可能在低速层，且在不同地点是不相同的，这与云南及周边地区莫霍面变化剧烈有因果关系．进一步推断出上地幔的各向异性主要存在于岩石圈而不是整个上地幔．      

西北太平洋俯冲带大地幔楔及地幔过渡带各向异性证据和特征

地震层析成像研究清晰给出了地球深部俯冲板片的大尺度形态,但与俯冲过程相关的地幔流动特征仍不明确.在俯冲地幔楔系统中,前人观测到了与海沟平行和垂直的快波偏振方向.本文研究了西北太平洋俯冲板片在地幔过渡带中停滞形成的"大地幔楔"中的各向异性特征.对具有长期稳定观测数据的MDJ台站SKS震相和区域深源地震的直达S波震相进行了...