利用Rayleigh波相速度和群速度联合反演青藏高原东北缘S波速度结构

利用青藏高原东北缘地区固定和流动地震台网2007年8月到2012年1月期间记录的远震波形,运用小波变换频时分析方法分别测定了1216和653条周期从15到140 s的台站间基阶Rayleigh相速度和群速度频散曲线.通过对上述频散进行反演,重构了青藏高原东北缘分辨率高达0.5°×0.5°的2-D相速度和群速度分布图.然后通过对所提取到的每个格网点Rayleigh波相速度和群速度频散进行联合反演,得到了研究区下方一维S波速度结构.最后通过线性插值,得到了青藏高原东北缘下方地壳上地幔三维S波结构.结果表明,印度板块向北俯冲已经达到班公—怒江缝合带附近;在柴达木盆地北部祁连山下面我们发现了亚洲板块,且其没有表现出明显的向南俯冲的迹象;在两大板块中间,我们观测到延伸到250 km深度的低速异常,该低速异常可能是地幔物质底辟上涌现象造成的.     

Azimuthal anisotropy of Rayleigh waves beneath the Tibetan Plateau and adjacent areas

The crustal and upper mantle azimuthal anisotropy of the Tibetan Plateau and adjacent areas was studied by Rayleigh wave tomography. We collected sufficient broadband digital seismograms trav-ersing the Tibetan Plateau and adjacent areas from available stations, including especially some data from the temporary stations newly deployed in Yunnan, eastern Tibet, and western Sichuan. They made an adequate path coverage in most regions to achieve a reasonable resolution for the inversion. The model resolution tests show that the anisotropic features of scope greater than 400 km and strength greater than 2% are reliable. The azimuthal anisotropy pattern inside the Tibetan Plateau was similar to the characteristic of tectonic partition. The crustal anisotropy strength is greater than 2% in most re-gions of East Tibet, and the anisotropy shows clockwise rotation surrounding the eastern Himalayan syntaxis. Vertically, the anisotropy direction indicates a coherent pattern within the upper crust, lower crust, and lithosphere mantle of the Tibetan Plateau, which also is consistent with GPS velocity field and SKS fast polarization directions. The result supports that the crust-mantle deformation beneath the Tibetan Plateau is vertically coherent. The anisotropy strength of crust and lithospheric upper mantle in Yunnan outside the Tibetan Plateau is lower than 2%, so SKS splitting from core-mantle boundary to station should largely be attributed to the anisotropy of asthenosphere.     

Upper mantle structure of the Apulian plate from Rayleigh waves

Phase velocities of Rayleigh waves for the Adriatic Sea area are obtained in the period range 25–190 sec along the path (l'Aquila-Trieste) AQU-TRI and 20–167 sec along the path (Trieste-Bari) TRI-BAI.The phase velocities are systematically higher than the known values for the surrounding regions. The data inversion indicates the presence of a lithosphere typical of stable continental areas with clear high-velocity lid (V _s 4.6 km/sec) overlying a well developed low velocity zone (V _s 4.2 km/sec).P. F. Geodinamica C.N.R., Roma Pubbl. N. 189.     

The Pn wave velocities and the relief of Moho in the Tibetan Plateau

Eleven PASSCAL broadband digital seismic stations were employed in the Tibetan Plateau by the Sino-US team from September, 1991 to June, 1992. Seven of them were distributed along the Qinghai-Tibet highway, others in Maqin and Yushu of Qinghai Province, Linzhi and Xigatze of Tibet. The data included 31 local earthquakes recorded by these stations from July, 1991 to January, 1992. Considering the characters of digital data, we identified the seismic phases carefully in several methods using SAC softwares (Seismic Analysis Code) in SUN workstation. We compared the seismic phases after converting the seismograms of the single stations to the seismic profiles; analyzed the first arrivals of P waves in the incident planes by rotating 3 component seismic records; identified the seismic phases from the particle motion pictures. The Pn apparent velocities were calculated in the distance range of 230–1200 km from Linzhi earthquakes, western Changtang earthquakes, Xitieshan and Gonghe earthquakes and the earthquakes in India. The results show that the Pn velocities change slightly in the Tibetan Plateau (8.0–8.1 km/s). These values near the velocities at the uppermost mantle of the normal continents. The Moho undulation in the Tibetan Plateau was also studied by using Pn data by the time-term method. The primary results indicate that the Moho beneath the Tibetan Plateau is flat, and its depths are 67–70 km. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, Supp., 593–600, 1992.     

P-wave crustal velocity structure in western Sichuan and eastern Tibetan region

A deep seismic sounding profile located in the western Sichuan and eastern Tibetan region extends from Batang (Zhubalong) to Zizhong, Sichuan. It passes through the Songpan-Garzê Fold System and the Longmenshan Tectonic Zone, and ends in the Yangtze Craton. Based on the travel times of phases on the profile, incorporating information on the relevant amplitudes, we determined 2-D P-wave crustal velocity structure along the profile, analyzed the principle differences between the crustal and upper mantle structure in the Western Sichuan Plateau and Sichuan Basin, discussed the deep feature of the major faults on the profile, the tectonic relation between the Yangtze Craton and the Tibetan Plateau and the deep structural environment where strong earthquakes occurred.     

青藏高原东南缘瑞利波群速度分布特征及其构造意义探讨

青藏高原东南缘是研究印度—欧亚板块碰撞过程、块体间相互作用和壳幔变形机制的重要地区.本文利用川滇地区流动地震台阵和固定地震台网共557个台站的连续波形数据,基于改进的背景噪声处理流程和分析方法得到了6023条瑞利波群速度频散曲线,反演获得了6~48 s的瑞利波群速度分布图像.结果显示在四川盆地内部短周期群速度分布较好地揭示了盆地内沉积层厚度的横向变化.在30~48 s周期,四川盆地西部群速度存在南低、北高的特征,推测是南部中下地壳和上地幔温度较高引起的.温度的增高降低了地壳的力学强度,在青藏高原东向挤压作用下盆地西南部地壳更易发生变形,并导致脆性上地壳在新生代产生地壳缩短和褶皱、断裂等地质活动.攀枝花及其周边地区从地壳浅部至上地幔深度的高速异常体,可能与基性和超基性岩的侵入有关.该高速体具有较大的介质强度,在一定程度上阻碍了青藏高原物质东南向的运移,这可能是造成丽江—小金河断裂两侧巨大高程差异的重要因素.自24 s开始,南盘江盆地表现为明显的高速异常,与华南块体西南部其他区域的深部结构存在明显差异.反演的S波速度结构揭示,自中上地壳至上地幔,南盘江盆地的速度一直高于北侧其他区域.结合此地区的地壳运动模式,推测介质S波速度较高、力学强度较大的南盘江盆地对青藏高原物质的东南向逃逸具有一定的阻挡作用.     

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…     

Rayleigh wave phase velocity tomography and strong earthquake activity on the southeastern front of the Tibetan Plateau

To investigate the relationship between velocity structure and earthquake activity on the southeastern front of the Tibetan Plateau, we make use of continuous observations of seismic ambient noise data obtained at 55 broadband stations from the regional Yunnan Seismic Network. These data are used to compute Rayleigh wave Green's Functions by cross-correlating between two stations, extracting phase velocity dispersion curves, and finally inverting to image Rayleigh wave phase velocity with periods between 5 and 34 s by ambient noise tomography. The results show significant lateral variations in crustal and uppermost mantle structures in the studied region. Phase velocity anomalies at short periods(5–12 s) are closely related to regional tectonic features such as sediment thickness and the depth of the crystalline basement. The Sichuan-Yunnan rhombic block, enclosed by the Honghe, Xiaojiang and Jianchuan faults, emerges as a large range of low-velocity anomalies at periods of 16–26 s, that inverts to high-velocity anomalies at periods of 30–34 s. The phase velocity variation in the vicinity of the Sichuan-Yunnan rhombic block suggests that the low-velocity anomaly area in the middle-lower crust may correspond to lower crustal channelized flow of the Tibetan Plateau. The spatial distribution of strong earthquakes since 1970 reveals that the Yunnan region is inhomogeneous and shows prominent characteristics of block motion. However, earthquakes mostly occur in the upper crust, with the exception of the middle-Yunnan block where earthquakes occur at the interface zone between high and low velocity as well as in the low-velocity zones, with magnitudes being generally less than 7. There are few earthquakes of magnitude 5 at the depths of 15–30 km, where gather earthquakes of magnitude 7 or higher ones which mainly occur in the interface zone between high and low velocities with others extending to the high-velocity abnormal zone.     

青藏高原东部Love波偏振研究

利用布设在云南西部的50个流动观测台站记录的三分量数字面波资料,采用频率域奇异值分解法,分析了中短周期Love波在青藏高原东部传播中的偏振状况. 结果显示Love波的传播呈现明显的有规律的偏离大圆路径的异常,地壳-上地幔速度结构显著的横向不均匀性可能是造成这些偏振异常的主要原因. 同时利用以前研究中得到的剪切波速度模型计算了Love波到达台站的理论偏离情况,大多数理论计算与实际观测结果的偏离方向基本一致,但理论偏离角通常小于观测值. 这样的结果在一定程度上验证了所采用的青藏高原速度结构模型是基本正确的,同时也说明了模型对研究区域复杂的速度结构的反映还不够精细.     

青藏高原东北缘祁连山西段与东段岩石圈结构差异研究

作为青藏高原东北缘的祁连山,其东段与西段的地貌以及新生代地表变形样式差异显著.在新生代青藏高原向北东扩展这一共同构造背景之下,引起祁连山东段和西段这些差异的深部原因尚不清晰.本文基于一条宽频密集台站观测剖面,利用P波速度结构约束下接收函数和面波频散的联合反演方法给出祁连山东段的壳内及岩石圈地幔结构.结合前人在青藏高原东北缘的研究成果,对比祁连山东段和西段的岩石圈结构,探讨东、西段壳内变形方式的差异,从而揭示引起祁连山东西段这些差异背后的可能原因.东西段岩石圈结构的差异主要有：1) 壳内低速层差异.在20~40 km的深度处,祁连西段(测线BB′)中地壳存在壳内低速物质(3.2~3.4 km·s^-1),而祁连东段(测线AA′)的中地壳并不存在低速层; 2) 岩石圈地幔差异.祁连西段的岩石圈地幔低速异常表现为分块的特征,而祁连东段的岩石圈地幔表现为比较统一的高速体.通过对比研究发现祁连西段壳内强度相对软弱,地壳的缩短变形程度更大,表现为高海拔的山脉以及更厚的地壳,而祁连东段壳内变形及缩短程度相对较弱,表现为地势相对较平坦的临夏盆地和陇中盆地以及较薄的地壳厚度.东、西段壳内强度以及变形方式的差异可能与深部岩石圈地幔的结构相关.

     

The upper mantle structure of the Tibetan Plateau and its implication for the continent-continent collision

The present continent had been assembled by the accretion of a series of terrains after their mutual colli-sions. Thus, the continental collision process plays an important role in the tectonics, the deformation and the movement in the continent. The Indo-Eurasian colli-sion had not only produced the grand geological structures and landscape, such as the Himalayan Mountain and the Tibetan Plateau, but also played a decisive role in the tectonic deformation and seismic-ity of Eastern Asia si…     

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).     

Apparent phase velocities and fundamental-mode phase velocities of Rayleigh waves

Rayleigh wave dispersion data usually appear in the form of multimodal spectra for a layered model structure. The number of dispersion modal curves depends on the number of layers in the model. The measured dispersion velocities from the frequency–wavenumber (F–K) space, however, may not represent the true phase velocities of the fundamental-mode dispersion curve, but apparent phase velocities. The present study discusses how multimode curves are generated in the multichannel analysis of surface waves (MASW) method and the cause of the apparent velocity produced by the F–K method. Results from a field trial demonstrate that apparent phase velocities fail to reveal thin layers with low velocities. A better agreement of the inverted model with the geotechnical record is obtained by using the data points extracted from the fundamental-mode curve of the MASW spectral image.     

青藏高原东北缘及邻区Rayleigh和Love波背景噪声层析成像

基于青藏高原东北缘及邻区流动密集地震台阵——喜马拉雅二期2013年12月至2015年8月期间的三分量连续波形数据,采用背景噪声成像方法获得了Rayleigh波周期为6~30 s和Love波6~25 s的二维相速度.6~12 s Rayleigh和Love波相速度在鄂尔多斯盆地及银川—河套地堑呈现明显的低速异常,而在西秦岭造山带和中亚造山带则显示高速异常.16~25 s的相速度同时受中下地壳及上地幔顶部速度结构和地壳厚度影响.此周期范围内,位于青藏高原的祁连地块和松潘甘孜地块北部呈现大范围相速度低速异常,青藏高原周边的鄂尔多斯和西秦岭造山带表现为高速异常.青藏高原与周边块体相速度的横向不均匀性,可能反映了构造活动或者地壳厚度的差异.此外,中亚造山带在周期16~20 s时,Rayleigh波相速度高低相间,但Love波大范围高速异常,两者差异可能反映了径向各向异性的影响.

     

A review on the lithospheric structures in the Tibetan Plateau and constraints for dynamics

In the last decade, several international joint projects were conducted in the Tibetan Plateau by Chinese, American and French geophysicists and geologists. In the present review, the results from vertical reflections, wide-angle reflections and broadband digital seismic recordings are reviewed and compared. Constraints for the dynamics of continent-continent collision from the lithospheric structures, seismicity, focal mechanism and anisotropy are discussed.The velocities ofPn,Sn, , were accurately determined by using their travel times from local events. They evidenced that the uppermost mantle underneath the Tibetan Plateau was similar to that of the ordinary continental mantle.The reflection profile from INDEPTH-I furnishes convincing evidence that the Indian crust penetrates into the Tibetan lower crust. The results from teleseismic waveform inversion reveal that the Moho discontinuity dips northwards, and an offset of Moho occurs near Bangong suture.The fact that materials within the Tibetan Plateau escape laterally has been proposed by several authors. Recent data and studies provide further convincing evidence that eastward mass transfer does occur, and their paths and natures are investigated.Some authors suggested that the large strike slip faults (Kun Lun, Xianshuihe) in the eastern plateau may be related to the lateral extrusion. However, most of the strike slips are left-lateral, and extrusion could not occur without right-lateral strike slips. Recent observations of the focal mechanisms and geological structure indicate that the earthquakes in the Yanshiping-Changdu belt are left-lateral strike slip. It is the southeast zone of the left-lateral slip faults in the eastern Tibetan plateau. Geological and seismological evidence show that the Bencuo-Jiali belt is the only large right-lateral fault in the eastern plateau. It was proposed that the present eastward extrusion occurs between the Yangshiping-Changdu left-lateral strike slip and the Bencuo-Jiali right-lateral strike slip. The other left-lateral strike slips north of the Yangshiping-Changdu belt are considered to be the fossils of the ancient flow paths.     

利用剪切波分裂研究青藏高原东北缘及东南缘上地幔各向异性

利用从IRIS上下载的青藏高原东北缘238个台站以及中国地震科学探测台阵喜马拉雅一期350个台站记录到的远震波形数据,通过采用剪切波分裂方法,获取各个台站下方各向异性分裂参数——快波偏振方向（φ）和分裂时差（δt）,从而得到青藏高原东北缘、东南缘上地幔的各向异性特征。研究结果表明,祁连块体、阿拉善块体和鄂尔多斯块体北部,快波方向为NNW-SSE,明显不同于GPS测量得到的近NE-SW的地表位移场方向,延迟时间平均~0.85 s;羌塘块体以及松潘-甘孜褶皱带的西部,快波方向呈现沿顺时针方向旋转的趋势,并且与GPS测量得到的地表位移场方向一致,延迟时间平均为~1.24 s;松潘-甘孜褶皱带东部、秦岭造山带与鄂尔多斯块体南部的交界处,快波方向呈现无序分布,与GPS方向表现出不一致的分布,延迟时间平均~1.08 s;川滇块体北部,快波方向近似N-S方向,与GPS测量得到的地表位移场方向相同,平均延迟时间为~0.925 s;位于北纬26°以南的川滇块体南部,快波方向近E-W方向分布,明显不同于GPS地表位移场方向,平均延迟时间~1.065 s。综合分析推测,羌塘块体、松潘-甘孜褶皱带的西部以及川滇块体北部,地表形变和深部之间的变形是相互耦合的;祁连块体、阿拉善块体和鄂尔多斯块体北部,松潘-甘孜褶皱带东部、秦岭造山带与鄂尔多斯块体南部的交界处以及川滇块体南部,壳幔之间可能存在解耦现象。     

Crustal structure of northeastern margin of the Tibetan Plateau by receiver function inversion

Using seismic data of about one year recorded by 18 broadband stations of ASCENT project,we obtained 2547 receiver functions in the northeastern Tibetan Plateau.The Moho depths under 14 stations were calculated by applying the H-κ domain search algorithm.The Moho depths under the stations with lower signal-noise ratio(SNR) were estimated by the time delay of the PS conversion.Results show that the Moho depth varies in a range of ~40–60 km.The Moho near the Haiyuan fault is vague,and its depth is larger than those on its two sides.In the Qinling-Qilian Block,the Moho becomes shallower gradually from west to east.To the east of 105°E,the average depth of the Moho is 45 km,whereas the west is 50 km or even deeper.Combining our results with surface wave research,we suggest a boundary between the Qinling and the Qilian Mountains at around 105°E.S wave velocities beneath 15 stations have been obtained through a linear inversion by using Crust2.0 as an initial model,and the crustal thickness that was derived by H-κ domain search algorithm was also taken into account.The results are very similar to the results of previous active source studies.The resulting figure indicates that low velocity layers developed in the middle and lower crust beneath the transition zone of the Tibet Block and western Qinling,which may be related to regional faults and deep earth dynamics.The velocity of the middle and lower crust increases from the Songpan Block to the northeastern margin of Tibetan Plateau.Based on the velocity of the crust,the distribution of the low velocity zone and the composition of the curst(Poisson's ratio),we infer that the crust thickening results from the crust shortening along the direction of compression.     

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.     

青藏高原东北缘至鄂尔多斯地块壳幔电性结构及构造变形研究

为了获取青藏高原东北缘至鄂尔多斯地块的壳幔电性结构,研究祁连造山带、鄂尔多斯地块及六盘山构造带的构造变形,布设一条甘肃陇西至陕西黄陵的近东西向大地电磁测深剖面,获取了91个大地电磁测深点的响应.经过对全剖面观测资料的数据处理、分析及二维反演,获得了剖面壳幔电性结构模型.研究结果表明：剖面横向可划分为三个区块,分别对应祁连造山带、六盘山构造带与鄂尔多斯地块;祁连造山带东段可能残存沟弧盆体系的构造格架,青藏高原北东向生长可能是在这一先存格架上的叠加与改造;六盘山构造带壳幔结构复杂,以中地壳拆离断层为界,上地壳发育拆离断层系统而下地壳挤压缩短增厚;鄂尔多斯地块成层性较好,地块总体较为稳定,但局部经历了与地幔上涌相关的物质与结构再造.     

青藏高原东南缘地壳结构与变形机制研究进展

新生代青藏高原的隆升改变了整个亚洲的构造格局,对气候、环境均产生了重要的影响,但高原的隆升扩展机制众说纷纭.青藏高原东南缘作为扩展前缘,其构造演化对了解整个高原的扩展机制具有重要的意义.本文总结了近年来对青藏高原东南缘地壳结构研究的最新进展,特别是2011年中国地震科学探测台阵计划开展以来,利用密集地震台阵取得的新成果,探讨了青藏高原东南缘地壳的结构与变形机制.这些研究发现青藏高原的地壳由高原向外围减薄,但在高原边界断裂附近存在地壳厚度突变带;下地壳中存在两个独立的低速异常,一个位于松潘—甘孜块体下方,被高原的边界断裂所围限,另一个位于小江断裂带下方,呈NE-SW向展布.我们认为青藏高原东南缘下地壳物质被边界(丽江—小金河)断裂所围限,并没有继续向边缘流出,但是地壳挤出产生的应力作用继续向东南方向传递,造成了小江断裂带附近的地壳变形.