用面波方法研究上扬子克拉通壳幔速度结构

本文研究采用单台法和双台法提取了穿越上扬子的基阶面波相速度和群速度频散;通过对提取的面波群速度和相速度频散进行联合反演,得到的1-D SV速度模型显示上扬子块体下地壳S波速度与典型克拉通区域相当,其上地幔顶部80~170 km深处存在高速的岩石圈盖层,较AK135模型要快2%~3%,其岩石圈厚度约为180 km.在上扬子地区,径向各向异性集中分布在300 km以浅的岩石圈与软流圈部分,其中岩石圈部分SH波比SV波波速要快2%~4%,软流圈部分SH波比SV波波速要快3%~5%;Rayleigh波相速度方位各向异性分析结果显示,上扬子块体周期为25~45 s（大致相当于30~70 km深度范围内）的Rayleigh波相速度存在1.8%~2.7%不等的方位各向异性,其快波方向介于147°~174°.我们认为上扬子块体径向各向异性集中分布在岩石圈、软流圈部分,且各向异性随深度变化, 其岩石圈部分各向异性为大陆克拉通化的遗迹,软流圈部分各向异性与现今板块运动相关.     

鄂尔多斯地区上地幔岩石圈三维速度结构面波反演研究

双平面波拟合法是一种新的面波成像方法,反演中考虑地震波场中的非平面波成分,提高反演的分辨率.本文利用双平面波拟合法,反演获得鄂尔多斯地区上地幔岩石圈的速度结构.所用资料为国家数字地震台网69个宽频带地震仪和北京大学34个流动数字地震台观测到的地震波面波资料.首先从面波记录中提取了研究区域20~125 s瑞利波相速度频散曲线,进而得到各个周期瑞利波相速度异常分布图.结果显示,短周期瑞利波相速度异常与地表的构造特征吻合较好,中长周期的瑞利波相速度可以反映出上地幔岩石圈的速度异常分布以及构造特征.由研究区20~125 s的瑞利波相速度分布图可以反演得到地表到地下200 km范围内的三维剪切波速度结构.结果显示,鄂尔多斯块体内部稳定均一,活化或改造的痕迹不明显;鄂尔多斯块体西南缘受到青藏高原的强烈作用,有大量地幔物质流动的痕迹存在;中央转换带下超过200 km深度存在地幔物质上涌,可能与太平洋板块的俯冲和青藏高原板块的挤压有关.     

青藏高原东南缘基于程函方程的面波方位各向异性研究

青藏高原东南缘作为高原物质侧向挤出的前沿地带,是研究岩石圈变形机制、高原物质侧向逃逸和深部动力学等科学问题的关键地区之一.本文利用研究区内540个宽频带流动地震台站记录的远震面波资料,基于程函方程面波层析成像方法获得了青藏高原东南缘周期14~80 s瑞利面波相速度和方位各向异性分布图像.结果显示：14~20 s周期内,面波方位各向异性分布与断裂带的走向和最大主压应力的方向密切相关,可能受到了断裂带和区域构造应力场的共同作用.川滇菱形块体的北部次级块体及丽江—小金河断裂带附近随着面波周期的增加,各向异性快波方向从NS向逐步转变为NE-SW方向,并与断裂带大致平行,而其以南的攀枝花附近表现为高相速度和弱各向异性的特征.我们推测,在川滇菱形块体北部存在明显的下地壳流,流动方向与块体向南的挤出方向基本一致,该地壳流受到攀枝花附近的高速、高强度坚硬块体阻挡,其前缘向西南方向流动.川滇菱形块体中部地区由于坚硬块体的存在,下地壳没有明显的通道流.在红河断裂以西地区,30~60 s周期范围的面波各向异性快波方向和红河断裂大致平行,推测可能与渐新世至中新世早期印支地块向南东方向的挤出密切相关.研究区东北部,四川盆地南缘地壳各向异性以NE-SW和NEE-SWW向为主与SKS快波方向明显不同,推测主要与该地区地壳的早期构造变形有关同时也说明SKS各向异性主要来自上地幔介质;在研究区南部104°E以西的中长周期面波各向异性方向与SKS分裂研究获得的近EW快波方向基本一致,但在104°E以东地区面波各向异性较弱且快波方向与SKS的观测结果存在明显差异,我们推测东部SKS各向异性来源深度至少在150 km以下.     

基于背景噪声研究青藏高原东北缘瑞利波相速度和方位各向异性

本研究收集了甘肃、青海、宁夏等118个宽频带数字地震台站的连续波形资料,利用噪声互相关,经过计算和筛选,在5~38 s范围内,共得到5773条瑞利波相速度频散曲线.然后采用1°×1°的网格划分,反演获得青藏高原东北缘相速度和方位各向异性分布.结果表明：短周期8~12 s内,鄂尔多斯从低速异常变为高速异常;该周期范围内各向异性结果与区域断裂走向有很好的一致性.18~25 s周期内,祁连地块、松潘-甘孜地块、羌塘地块低速异常范围逐渐变大,随周期增加地壳低速异常与人工探测结果相符;鄂尔多斯表现为速度随周期增加逐渐变大,说明其中下地壳速度相对偏高,不存在低速异常;该周期范围内的各向异性特征表现为,祁连地块和松潘甘孜地块大致呈NW-SE方向,而青藏高原内部快波方向显示了顺时针旋转的形态.在30~35 s范围内面波速度主要受莫霍面深度和莫霍面附近介质速度的影响,与地壳厚度分布有非常好的吻合.综合不同方法获得的各向异性研究结果,支持印度-欧亚板块的碰撞使青藏高原东北缘地壳发生缩短和逐渐隆升的观点,认为整个岩石圈的垂直缩短变形是青藏高原东北缘的主要形成机制.     

利用双平面波干涉面波层析成像方法研究山西断陷盆地及鄂尔多斯地台三维速度结构

本文利用了北京大学在山西地堑的34个台站以及中国地震局台网中心在鄂尔多斯地区46个台站的远震波形数据,运用双平面波干涉的面波层析成像方法,提取瑞利面波相速度频散曲线,开展台阵覆盖区三维速度结构反演,据此分析了研究区地壳和上地幔的结构特征。结果表明,瑞利面波相速度分区特征显著,山西断陷盆地和渭河断陷盆地的相速度整体偏低。鄂尔多斯块体在莫霍面以下有明显高速异常,表明该地块为构造稳定的克拉通块体,鄂尔多斯块体的岩石圈下界面在120～140km的深度左右。与此相反,山西断陷盆地和渭河断陷盆地地下70～120km左右均有低速异常,显示这两个地区构造活动活跃,这也与该区域历史上多次发生强震是相符的。     

南北构造带北段上地幔各向异性特征

对布设在南北构造带北段的中国地震科学探测台阵项目二期674个宽频带流动台站和鄂尔多斯台阵21个宽频带流动台站记录的远震XKS(SKS、SKKS和PKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和"叠加"分析方法求得每一个台站的XKS波的快波偏振方向和快、慢波的时间延迟,并结合该区域出版的122个固定台站的分裂结果,获得了南北构造带北段上地幔各向异性图像.快波方向分布显示青藏高原东北缘、阿拉善块体和鄂尔多斯块体西缘的快波方向主要表现为NW—SE方向,秦岭造山带的快波方向为近E—W方向,鄂尔多斯块体内部的快波方向在北部为近N—S方向,南部表现为近E—W方向.时间延迟分布来看,鄂尔多斯块体的时间延迟不仅明显小于其周缘地区,而且小于其他构造单元,特别是在高原东北缘、阿拉善块体和鄂尔多斯块体的交汇地区的时间延迟很大,反映了构造稳定单元的时间延迟小于构造活跃单元.通过比较快波方向的横波分裂测量值与地表变形场模拟的预测值,并结合研究区地质构造和岩石圈结构特征分析表明,在青藏高原东北缘、阿拉善块体和鄂尔多斯块体西缘各向异性主要由岩石圈变形引起,地表变形与地幔变形一致,地壳耦合于地幔,是一种垂直连贯变形模式;秦岭造山带的各向异性不仅来自于岩石圈,而且其岩石圈板块驱动的软流圈地幔流作用不可忽视;鄂尔多斯块体内部深浅变形不一致,具有弱的各向异性、厚的岩石圈和构造稳定的特征,我们认为其各向异性可能保留了古老克拉通的"化石"各向异性.     

鄂尔多斯块体及周缘上地幔各向异性研究

对布设在鄂尔多斯块体及周缘的固定和流动宽频带地震台网共111个台站记录作远震SKS（SKKS）波形资料偏振分析,采用最小切向能量的网格搜索和叠加分析求得每一个台站的SKS（SKKS）快波偏振方向和快、慢波的延迟时间,获得了鄂尔多斯块体及周缘上地幔各向异性图像.在鄂尔多斯块体西缘和北缘,各向异性的快波方向为NW-SE方向,一致性较好;在鄂尔多斯多斯块体南缘,快波方向主要是NWW-SEE和近EW方向;在鄂尔多斯块体东缘,快波方向总体表现为近EW方向,间有NEE-SWW方向和NWW-SEE方向.在鄂尔多斯块体内部,快波方向在北部是近NS方向,而南部则是近EW方向.快、慢波的时间延迟范围是0.48～1.50s,鄂尔多斯块体内部的时间延迟平均值小于其周缘地区.通过分析研究区各向异性特征,认为构造稳定的鄂尔多斯块体内部的各向异性主要来自于古老的华北克拉通保存的＂化石＂各向异性;青藏高原东北缘向NE方向的推挤,造成岩石圈NW-SE方向的拉张伸展,鄂尔多斯块体西缘和北缘下的上地幔物质沿NW-SE方向发生了形变,使得上地幔中橄榄岩的晶格排列方向平行于物质形变的方向;在鄂尔多斯块体南缘,刚性的华北块体和扬子块体碰撞作用,使得各向异性快波方向平行于两个刚性块体的碰撞边界和秦岭造山带的走向.结合该区域绝对板块运动和速度结构研究,认为在秦岭造山带下可能存在一个青藏高原物质东流的地幔流通道;在鄂尔多斯块体东缘的汾河地堑和太行山,相对复杂的各向异性特征可能由于西太平板块俯冲、区域伸展构造、造山运动等共同作用引起的.对于YCI台得到的各向异性参数（快波方向变化范围是45°～106°,时间延迟变化范围是0.6～1.5s）随事件反方位角呈现出π/2周期的变化,符合双层各向异性模型.基于0.125Hz的主频和实测的各向异性参数,我们模拟得到了最佳的双层各向异性模型,下层φlower=132°,δtlower=0.8s,上层φupper=83°,δtupper=0.5s.上层各向异性归功于古老克拉通保留的＂化石＂各向异性,下层各向异性则受到了青藏高原东北缘NE方向推挤导致的岩石圈NW-SE方向的拉张伸展作用.通过该区域各向异性快波方向与全球定位系统（GPS）的观测结果的对比分析,鄂尔多斯块体的周缘壳幔变形符合垂直连贯变形模式,而其内部变形复杂,有待进一步研究.     

青藏高原东南缘基于背景噪声的Rayleigh面波方位各向异性研究

青藏高原东南缘地区是现今地壳形变和地震活动最强烈的地区之一,也是研究青藏高原现今变形机制和构造演化规律的重要区域.本研究使用云南区域地震台网的55个宽频带地震台站连续地震背景噪声数据,采用双台站互相关方法获得Rayleigh(瑞利)面波经验格林函数,提取相速度频散曲线,反演得到云南地区周期5~34s范围内方位各向异性分布图像.反演结果揭示:短周期(5~12s)Rayleigh面波快波优势方向与区域断裂走向有很好的一致性,快波方向随着断裂走向的变化而变化.周期16~26s快波优势方向与反映上地壳特性的5~12s图像总体图像相似,但细节略有不同.其中,滇中块体内易门断裂和滇中块体内东侧的普渡河断裂附近,各向异性快波方向从NS向NW方向旋转;易门断裂以西呈NW向.这反映了青藏高原物质东流和川滇块体受到青藏块体的南东向挤压作用.周期30~34s范围的各向异性,滇缅泰块体和印支块体,快波优势方向为NS和NNW向;而在滇中块体内部,各向异性快波方向呈顺时针旋转变化,可能与青藏高原物质向东逃逸有关.本文还开展了与体波各向异性的对比分析,通过与近震S波分裂、Pms转换波分裂和远震SKS、PKS和SKKS(以后简称为XKS)分裂的对比研究,发现随着周期的增大,得到的快波优势方向与XKS剪切波快波偏振方向趋向一致,与地壳快剪切波偏振方向呈一定夹角.本研究认为,青藏高原东南缘地区壳幔各向异性具有不同的特征和形成机制.     

利用背景噪声资料研究海原断裂带及邻区Rayleigh波相速度和方位各向异性

海原断裂带是位于青藏高原东北缘的一条NWW走向大型边界断裂带,为了研究该区域地壳速度结构与变形特征,使用中国地震局地震预测研究所布设的跨海原断裂带流动地震台阵(SACHY-Array)和研究区域内固定地震台站共61个台站(40个流动台站和21个固定台站)的垂向连续波形数据。采用背景噪声互相关方法,提取了面波相速度频散曲线,反演得到周期范围为5～30 s、分辨率1°×1°的Rayleigh波相速度和方位各向异性图像。结果表明,短周期5～10 s内,河西走廊过渡带东部、鄂尔多斯地块西北部以及银川地堑南部均呈现低速异常体,祁连造山褶皱带东段表现为相对高速体;海原断裂带西南侧快波偏振方向为NWW、 NW向,鄂尔多斯西缘及邻区快波偏振方向主要为近NS向,各向异性方向与区域断裂走向基本一致。15～30 s周期内,河西走廊过渡带东部及银川地堑南部的低速异常逐渐减弱且范围不断减小,15 s周期左右,河西走廊过渡带东部的低速体在烟筒山断裂带下方有错断的趋向,陇中盆地中央、鄂尔多斯西南缘均存在高速异常体;方位各向异性方向与短周期基本一致,不过各向异性强度较弱。本文认为,海原断裂带是高低速过渡带,位于断裂带...     

秦岭及周边地区瑞雷波方位各向异性

除了使用前人提取的面波频散曲线外, 还从秦岭及周边地区布设的59个宽频流动地震台的数据和中国地震局及各省局台网的数据中筛选出的地震事件波形和台站间噪声互相关格林函数中提取瑞雷波群速度频散曲线. 利用二维面波层析成像反演获得了瑞雷波周期为10—50 s的各向同性群速度及方位各向异性分布. 结果显示: 周期为10 s的各向同性群速度和方位各向异性分布与各构造单元存在明显的对应关系; 周期为10—50 s的面波在四川盆地和鄂尔多斯盆地内的快波方向多为近NS向. 与前人研究结果不同的是, 本文得到的秦岭、 大巴山构造带周期为10—50 s的面波快波方向均与山脉走向近似平行, 且与SKS波分析得到的快波方向一致. 这表明秦岭和大巴山之下整个岩石圈的快波方向都与山脉走向平行, 预示着秦岭和大巴山整个地壳, 甚至岩石圈发生了类似的变形. 由于四川盆地和鄂尔多斯盆地面波快波方向与SKS波结果差别较大, 推测青藏高原隆升扩展对这两个盆地的地壳基本无影响, 但对其岩石圈上地幔却产生了重大影响.      

Love-Rayleigh wave incompatibility and possible deep upper mantle anisotropy in the Iberian peninsula

Love and Rayleigh wave phase velocities are analyzed with the goal of retrieving information about the anisotropic structure of the Iberian lithosphere. The cross-correlation method is used to measure the interstation phase velocities between diverse stations of the ILIHA network at periods between 20 and 120 s. Despite the 2-D structure of the network, the Love wave data are too few to enable an analysis of phase velocity azimuthal variations. Azimuthal averages of Love and Rayleigh wave phase velocities are calculated and inverted both in terms of isotropic and anisotropic structures. Realistic isotropic models explain the Rayleigh wave and short-period Love wave phase velocities. Therefore no significant anisotropy needs to be introduced in the crust and down to 100 km depth in the upper mantle to explain our data. A discrepancy is observed only at long periods, where the data are less reliable. Love wave data at periods between 80 and 120 s remain 0.15 km/s faster than predicted by isotropic models explaining the long-period Rayleigh wave data. Possibilities of biases in the measurements due to interferences with higher modes are examined but seem unlikely. A transversely isotropic model with 8% of S-wave velocity anisotropy in the upper mantle at depths larger than 100 km can explain the whole set of data. In terms of a classical model of mantle anisotropy, this corresponds to 100% of the crystals perfectly oriented in the horizontal plane in a pyrolitic mantle. This is a rather extreme model, which predicts at time delay between 0 and 2 seconds for split SKS.     

Depth constraints on azimuthal anisotropy in the Great Basin from Rayleigh-wave phase velocity maps

We present fundamental-mode Rayleigh-wave azimuthally anisotropic phase velocity maps obtained for the Great Basin region at periods between 16 s and 102 s. These maps offer the first depth constraints on the origin of the semi-circular shear-wave splitting pattern observed in central Nevada, around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached. Our phase velocity study helps constrain the three-dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western United States. The dispersion measurements were made using data from the USArray Transportable Array. At periods of 16 s and 18 s, which mostly sample the crust, we find a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semi-circular pattern of fast seismic directions. Away from central Nevada the fast directions are ～ N–S in the eastern Great Basin, NW–SE in the Walker Lane region, and they transition from E–W to N–S in the northwestern Great Basin. Our short-period phase velocity maps, combined with recent crustal receiver function results, are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. At longer periods (28–102 s), which sample the uppermost mantle, isotropic phase velocities are significantly reduced across the study region, and fast directions are more uniform with an ～ E–W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere–asthenosphere boundary at depths of ～ 60 km. We interpret the fast seismic directions observed at longer periods in terms of present-day asthenospheric flow-driven deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere. Our results also provide context to regional SKS splitting observations. We find that our short-period phase velocity anisotropy can only explain ～ 30% of the SKS splitting times, despite similar patterns in fast directions. This implies that the origin of the regional shear-wave splitting signal is complex and must also have a significant sublithospheric component.     

用Rayleigh面波方位各向异性研究中国大陆岩石圈形变特征

中国大陆地质构造历史非常复杂,岩石圈长期积累的形变较大,而利用地震面波传播的各向异性是研究岩石圈形变特征的强有力手段. 本文利用双台窄带通滤波-互相关方法与基于图像分析的相速度频散曲线提取技术,提取Rayleigh面波相速度频散资料,进而反演中国大陆及邻区20～120 s周期Rayleigh面波相速度方位各向异性空间分布图像. 检测板测试结果显示：中国大陆大部分区域的方位各向异性横向分辨率在5°左右. 各向异性研究结果表明：中国大陆地壳上地幔方位各向异性特征存在显著的空间差异,反映出形变特征的空间差异;104°E以东地区地壳上地幔各向异性弱于西部地区,表明其构造变形总体弱于西部地区. 青藏地块及其东缘地区地壳与上地幔顶部变形最为强烈. 但东部的局部地区如华南地块与珠江口地区、鄂尔多斯盆地西南缘以及秦岭-大别造山带,较强的各向异性显示这些区域在不同时期也经历了强变形. 青藏地块内中短周期快波方向自西向东顺时针旋转变化可能指示板块碰撞与挤压过程中软弱物质的流变方向. 青藏地块西部中下地壳和上地幔形变模式相似,可能处于壳幔耦合状态;而中东部及东缘地区地壳上地幔形变模式存在明显差异,壳幔似乎不具备垂直连贯的形变特征. 位于青藏地块北部的塔里木盆地、柴达木盆地以及祁连褶皱带同样经历了强变形. 包括四川盆地在内的上扬子地块快波方向的变化显示中地壳与下地壳上地幔形变模式不同,而形变特征一致的下地壳与上地幔应为强耦合. 大约以103°E为界,龙门山断裂带可分为南西段和北东段,南西段处于低速区,而北东段位于高速区,且方位各向异性强度明显大于南西段;2008年5月12日汶川M_S8.0级地震沿断裂带的单侧破裂模式除与北东段的高应力积累有关外,还可能与北东段地下介质物性存在密切关系,高速坚硬岩体的发育有利于应变能的积累与集中释放.     

A method for inversion of layered shear wavespeed azimuthal anisotropy from Rayleigh wave dispersion using the Neighborhood Algorithm

Seismic anisotropy provides important constraints on deformation patterns of Earth's material. Rayleigh wave dispersion data with azimuthal anisotropy can be used to invert for depth-dependent shear wavespeed azimuthal anisotropy, therefore reflecting depth-varying deformation patterns in the crust and upper mantle. In this study, we propose a two-step method that uses the Neighborhood Algorithm(NA) for the point-wise inversion of depth-dependent shear wavespeeds and azimuthal anisotropy from Rayleigh wave azimuthally anisotropic dispersion data. The first step employs the NA to estimate depthdependent VSV(or the elastic parameter L) as well as their uncertainties from the isotropic part Rayleigh wave dispersion data. In the second step, we first adopt a difference scheme to compute approximate Rayleigh-wave phase velocity sensitivity kernels to azimuthally anisotropic parameters with respect to the velocity model obtained in the first step. Then we perform the NA to estimate the azimuthally anisotropic parameters Gc/L and Gs/L at depths separately from the corresponding cosine and sine terms of the azimuthally anisotropic dispersion data. Finally, we compute the depth-dependent magnitude and fast polarization azimuth of shear wavespeed azimuthal anisotropy. The use of the global search NA and Bayesian analysis allows for more reliable estimates of depth-dependent shear wavespeeds and azimuthal anisotropy as well as their uncertainties.We illustrate the inversion method using the azimuthally anisotropic dispersion data in SE Tibet, where we find apparent changes of fast axes of shear wavespeed azimuthal anisotropy between the crust and uppermost mantle.     

利用远震研究四川盆地及其周缘瑞利面波相速度和方位各向异性

基于四川盆地及周边的245个宽频带台站2010年9月—2014年9月期间的远震记录,提取双台路径瑞利面波相速度频散资料,反演得到四川盆地20～120s的高分辨率瑞利面波相速度及各向异性空间分布.在丰富区域地球物理基础数据的同时,结合已有研究成果对地壳上地幔变形耦合进行探讨,结果表明短周期(20～30s)的相速度分布与四川盆地的地质构造特征相吻合,作为川滇地块、松潘—甘孜地块和四川盆地之间的边界——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;松潘—甘孜地块,特别是川滇地块中下地壳普遍表现为明显的低速异常,表明中下地壳相对软弱;而四川盆地的中下地壳整体呈现相对高速,表明四川盆地具有相对坚硬的中下地壳.研究区域东南角接近北扬子地块与南扬子地块的缝合部位,呈现高速异常.四川盆地南部和东南邻区不同周期均具有较强的各向异性,且快波方向较为一致,反映这些地区不同深度变形耦合较好.四川盆地西部、北部及东北部邻区,不同周期的各向异性快波方向变化较大,不同深度变形耦合较差.这些特征与绕喜马拉雅东构造结的物质流动被扬子地块的高速地壳阻挡的宏观认识基本一致.     

青藏高原东南缘地震各向异性及其深部构造意义

青藏东南缘是青藏高原物质东流的通道,为了更全面了解复杂的岩石圈结构和强烈的变形特征,本文介绍了青藏东南缘岩石圈各向异性的形态,综合其他研究者得到的该区域壳幔各向异性结果,增加了部分新的资料,更新了青藏东南缘岩石圈方位各向异性图像,探讨了区域深部构造意义.
基于近场小震、远震和背景噪声资料计算结果,青藏东南缘地震各向异性展现出独特的区域空间分布和垂向层次性分布形态,展现了3个主要特征.（1）青藏东南缘上地壳各向异性与地表变形测量结果相符,快剪切波偏振方向（即快波方向）呈现与地表运动特征一致的发散性,与主压应力方向一致,但受到地质构造的影响.（2）青藏东南缘下地壳方位各向异性展现了更好的方向一致性,但方位各向异性程度相对较弱,在红河断裂带西北端部和小江断裂带下方有两个下地壳低速区,其方位各向异性程度与上地壳相当.（3）青藏东南缘岩石圈方位各向异性,呈现南、北分区特征,南北分界线大致在26°20'N,快波方向在北部近似为NS方向,在南部近似为EW方向.
本文推测：（1）在26°20'N北侧的上地幔有较厚的高速体,高速体南侧边缘呈现出近EW走向的直立墙形构造,其南侧软弱的上地幔物质在EW方向上流动,导致了岩石圈方位各向异性特征在空间发生突然的变化,快波方向由北部的NS变为南部的EW方向;（2）小江断裂带是现今的华南地块的地壳西边界,但岩石圈尺度的方位各向异性展现出的趋势性表明,华南地块的上地幔物质越过了小江断裂带到达其西侧,揭示了华南地块与青藏地块接触碰撞造成的岩石圈物质变形和上地幔软流圈物质运移的深部图像.地震各向异性能揭示区域深部构造与介质变形的信息,不同观测资料的综合分析有助于获得更清晰的各向异性三维图像.     

Anisotropic tomography of the Atlantic Ocean

We present the first regional three-dimensional model of the Atlantic Ocean with anisotropy. The model, derived from Rayleigh and Love wave phase velocity measurements, is defined from the Moho down to 300 km depth with a lateral resolution of about 500 km and is presented in terms of average isotropic S-wave velocity, azimuthal anisotropy and transverse isotropy.The cratons beneath North America, Brazil and Africa are clearly associated with fast S-wave velocity anomalies. The mid-Atlantic ridge (MAR) is a shallow structure in the north Atlantic corresponding to a negative velocity anomaly down to about 150 km depth. In contrast, the ridge negative signature is visible in the south Atlantic down to the deepest depth inverted, that is 300 km depth. This difference is probably related to the presence of hot-spots along or close to the ridge axis in the south Atlantic and may indicate a different mechanism for the ridge between the north and south Atlantic. Negative velocity anomalies are clearly associated with hot-spots from the surface down to at least 300 km depth, they are much broader than the supposed size of the hot-spots and seem to be connected along a north-south direction.Down to 100 km depth, a fast S-wave velocity anomaly is extenting from Africa into the Atlantic Ocean within the zone defined as the Africa superswell area. This result indicates that the hot material rising from below does not reach the surface in this area but may be pushing the lithosphere upward.In most parts of the Atlantic, the azimuthal anisotropy directions remain stable with increasing depth. Close to the ridge, the fast S-wave velocity direction is roughly parallel to the sea floor spreading direction. The hot-spot anisotropy signature is striking beneath Bermuda, Cape Verde and Fernando Noronha islands where the fast S-wave velocity direction seems to diverge radially from the hot-spots.The Atlantic average radial anisotropy is similar to that of the PREM model, that is positive down to about 220 km, but with slightly smaller amplitude and null deeper. Cratons have a lower than average radial anisotropy. As for the velocities, there is a difference between north and south Atlantic. Most hot-spots and the south-Atlantic ridge are associated with positive radial anisotropy perturbation whereas the north-Atlantic ridge corresponds to negative radial anisotropy perturbation.     

Seismic structure of Iceland from Rayleigh wave inversions and geodynamic implications

We have constrained the shear-wave structure of crust and upper mantle beneath Iceland by analyzing fundamental mode Rayleigh waves recorded at the ICEMELT and HOTSPOT seismic stations in Iceland. The crust varies in thickness from 20 to 28 km in western and northern Iceland and from 26 to 34 km in eastern Iceland. The thickest crust of 34–40 km lies in central Iceland, roughly 100 km west to the current location of the Iceland hotspot. The crust at the hotspot is ∼32 km thick and is underlain by low shear-wave velocities of 4.0–4.1 km/s in the uppermost mantle, indicating that the Moho at the hotspot is probably a weak discontinuity. This low velocity anomaly beneath the hotspot could be associated with partial melting and hot temperature. The lithosphere in Iceland is confined above 60 km and a low velocity zone (LVZ) is imaged at depths of 60 to 120 km. Shear wave velocity in the LVZ is up to 10% lower than a global reference model, indicating the influence of the Mid-Atlantic Ridge and the hotspot in Iceland. The lowest velocities in the LVZ are found beneath the rift zones, suggesting that plume material is channeled along the Mid-Atlantic Ridge. At depths of 100 to 200 km, low velocity anomalies appear at the Tjornes fracture zone to the north of Iceland and beneath the western volcanic zone in southwestern Iceland. Interestingly, a relatively fast anomaly is imaged beneath the hotspot with its center at ∼135 km depth, which could be due to radial anisotropy associated with the strong upwelling within the plume stem or an Mg-enriched mantle residual caused by the extensive extraction of melts.