Intracontinental earthquakes usually cause severe casualties and property damage, yet their mechanism is still unclear. The TanLu Fault Zone (TLFZ) has been a typical large intracontinental strike-slip fault in eastern China since the Quaternary, which has hosted many large earthquakes (M >= 6). Generation of large earthquakes is generally closely related to the heterogeneities on and adjacent to the fault plane, which may result from different composition and inclusions of fluids. In this study, we use the receiver function method to obtain detailed distributions of Poisson's ratio and crustal thickness in the central-southern segment of the TLFZ, where the M 8.5 TanCheng earthquake occurred in 1668. We found that the distribution of earthquakes is strongly correlated with the crustal structures, both of which exhibit clear segmentation. The strongly damaged zone (with earthquake intensity >= 10) of the 1668 TanCheng earthquake and present small-to-moderate earthquakes are generally located above a segment that has shallower Moho and particularly higher Poisson's ratio. We propose that the high Poisson's ratio may be caused by intrusive mafic rocks in the ductile lower crust, which plays a role as local stress concentrators and subsequently leads to large intracontinental earthquakes.

The uplift of the Tibetan Plateau in the Cenozoic has greatly affected the tectonics in Asia. But the evolution of the plateau is still unclear. The northeastern Tibetan Plateau is under ongoing expansion and is therefore essential for understanding the tectonic evolution of the plateau. Using data from 675 stations deployed by the ChinArray project, we obtained an anisotropic shear-wave velocity model by Rayleigh-wave phase-velocity Eikonal tomography and further investigated layered anisotropy and deformation patterns in the northeastern Tibetan Plateau. In the upper crust (<20 km depth), the fast directions are parallel with major strike-slip faults. In the lower crust, the fast directions are mostly margin-parallel along the plateau margin (at depths of ∼20 km to Moho), but they are perpendicular to the plateau margin while parallel to the topographic gradient under the high plateau (at depths of ∼40 km to Moho). Our result indicates that there is lower crustal flow under the high plateau due to the differential lateral gravity potential. However, the deep crustal flow is blocked by the surrounding blocks, which results in pure shear along the plateau margin in the northeastern Tibetan Plateau.

The tectonic uplift of the Tibetan Plateau is a focus in the geosciences. Middle-lower crustal flow is a popular model to interpret the geodynamic mechanism on the margin of the Tibetan Plateau. The model predicts different surface and Moho topographies across the plateau boundary due to the different strengths of the surrounding blocks, that is, sharp boundaries on the eastern plateau boundary and gentle variations in the southeastern plateau boundary. Here, we employ receiver function and common conversion point stacking analysis with the seismic waveforms recorded by the dense ChinArray and other local seismic stations to accurately define the Moho topography in southeastern (SE) Tibet. We find that the Moho under the Tibetan Plateau is much deeper than that under the surrounding Yangtze Craton and Indochina block; abrupt Moho changes are found across the southeastern plateau margin, similar to that under the eastern plateau margin. We interpret these sharp Moho variations across the plateau margin to have developed when the Tibetan Plateau was extruded southeastward in the late Miocene. Subsequent gravity collapse resulted in crustal extension and gentle topographic variation, while the sharp Moho slope was preserved.

The Indian plate began to subduct under the Eurasian plate in the late Mesozoic. Since the Eocene, continuous continental collision has resulted in the uplift of the Tibetan Plateau with extensive volcanic activities. Geophysical and geochemical studies have suggested that the Indian plate subducted and that volcanism originated in the deep upper mantle. However, the depth range of the subduction and volcanism and the relationship between them are still unclear. Here, to image the mantle transition zone (MTZ) structures, we deployed new seismic stations and collected as much seismic data as possible from the Tibetan Plateau. We then calculated the receiver functions and analyzed them using the common conversion point stacking method. We found that the MTZ is thickened by similar to 20 km under the Lhasa block, where high-velocity anomalies are obvious in the MTZ. Apparent thinning of the MTZ by similar to 20 km was revealed under the Qiangtang and Songpan-Ganze blocks, confirming a component of depleted mid-oceanic ridge basalt in the magmatic rocks. Our results suggest that the Indian Plate has at least subducted to the MTZ, which has further induced mantle upwelling and volcanism in the Tibetan Plateau.

The eastern margin of the Tibetan plateau is under ongoing expansion. Rise and growth of the Longmenshan are results of the interaction between the Songpan-Ganzi Terrane and the Sichuan Basin. In order to understand the block interactions in the eastern plateau margin, we studied the crustal structures using P wave receiver functions (including H-k staking, CCP stacking, and velocity inversion) with the waveforms recorded by 115 portable stations. The Moho, indicated by clear Ps phases, is generally deeper in the Songpan-Ganzi Terrane than in the Sichuan Basin, while a step is notable under the Longmenshan fault zone. We found a low velocity anomaly in the mid-lower crust below the Songpan-Ganzi Terrane and Longmenshan fault zone, which may imply the accumulated partial melting materials in the deep crust under the eastern plateau margin. Clear P-to-s converted phases are found within the crust of the Sichuan Basin. They look as apparent huge anticline structures, which may be caused by ancient crustal deformation under the stable craton.

The southeastern (SE) Tibetan plateau is uplifting accompanying strong seismicity, but the relationship between the crustal structure and tectonic process in this region is still unclear. Seismic anisotropy provides important clues for the deformation mechanism. Here we applied the harmonic decomposition approach and further conducted a Markov-Chain-Monte-Carlo (MCMC) inversion to analyze the crustal anisotropy from receiver functions using waveforms recorded by dense stations in the SE Tibetan plateau. More spatial details on the crustal anisotropy and interface are determined consequently. While the anisotropies in the upper crust are relatively weak (~2%), the anisotropies in the mid-lower crust are mostly stronger than 4%. The fast polarization directions (FPDs) in the mid-lower crust show a dominant circular pattern, in good coherence with the topographic contours, indicating that the crustal deformations in the SE Tibetan plateau may be mainly controlled by the gravity potential. Because of the significant Moho steps along the plateau margin, the extruded deep crustal materials are blocked by the strong crust around so that the minerals are aligned to be sub-parallel to the plateau margins.

青藏高原东北缘是青藏高原隆升的前缘地带,其构造变形特征一直是研究的热点.前人在该区域进行过面波成像研究,但不同结果存在较明显的差异,可能与不同成像方法的分辨率有关,此次研究分别使用基于射线的方法和基于程函方程的方法进行了瑞利波相速度成像.笔者对比了20 s、30 s和60 s的成像结果,两种方法的结果表现出相似的速度特征.总体的速度特征与研究区域的主要构造单元分布相吻合,青藏高原东北缘表现出低速异常,鄂尔多斯块体表现出高速异常.在银川河套地堑,基于程函方程的成像在20 s和30 s得到更明显的低速异常,低速异常体的分布与地堑的轮廓吻合更好,说明基于程函方程的方法对数据利用更充分.

SUMMARYFocal mechanism solutions (FMSs) reflect the stress field underground directly. They provide essential clue for crustal deformations and therefore improve our understanding of tectonic uplift and expansion of the Tibetan Plateau. In this study, we applied generalized Cut and Paste and P-wave first-motion methods to determine 334 FMSs (2.0 ≤ Mw ≤ 6.4) with the data recorded by a new temporary network deployed in the NE Tibetan Plateau by ChinArray project. We then used 1015 FMSs (including 681 published FMSs) to calculate the regional stress field with a damped linear inversion. The results suggest dominant thrust and strike-slip faulting environments in the NE Tibetan Plateau. From the Qilian thrust belt to the Qinling orogen, the maximum horizontal stress orientations (${S_\mathrm{ H}}$) rotate clockwise from NNE to NE, and further to EW, showing a fan-shaped pattern. The derived minimum horizontal stress orientations (${S_\mathrm{ h}}$) are parallel to the aligned fabrics in the mantle lithosphere indicated by shear wave splitting measurements, suggesting vertically coherent deformation in the NE Tibetan Plateau. Beneath the SW Qinling adjacent to the plateau, however, the stress orientations in the shallow and deep crust are different, whereas the deep crustal stress field indicates possible ductile crustal flow or shear.

The SE Tibetan Plateau is located between the eastern Himalayan syntaxis and the stable Yangtze craton; its tectonic evolution is important for understanding the plateau expansion. However, interactions between the plateau and its adjacent subduction zone and craton are still unclear. In this study, we determine updated high‐resolution P and S wave tomographic models of the crust and upper mantle beneath the SE Tibetan Plateau. We confirmed the existence of a high‐velocity layer in the upper mantle down to 200‐km depth beneath the Sichuan basin, the core of the Yangtze craton, representing the stable cratonic root. Another high‐velocity body is located beneath Burma, reflecting the subducting Indian slab in the upper mantle and possibly the remnant Burman slab in the mantle transition zone. A strong low‐velocity column exists above the high‐velocity body in the transition zone and reaches the Moho under the Tengchong volcano. In addition, we find high‐velocity fragments in the upper mantle beneath the Yangtze craton and northern Indochina, which may reflect delaminated lithosphere beneath the SE Tibetan Plateau. The lithospheric delamination may be closely related to mantle flow extruded from the high plateau and possible thermal upwelling from the lower mantle. Our results provide new insight into the deep interactions between the Tibetan plateau and its surrounding blocks near the SE plateau margin.

青藏高原东南部作为板块碰撞的前缘地带一直是地球科学研究的热点,为了揭示碰撞前缘地带地壳结构特征,作者利用布设在中国青藏高原东南部的38个宽频带流动台站记录的2487条远震P波接收函数,采用接收函数CCP叠加(共转换点叠加)和H-κ叠加两种方法获得了研究区域详细的地壳厚度图像和泊松比值.研究结果显示:两种方法获得的地壳厚度特征具有较好的一致性;青藏高原东南部地壳厚度存在明显的东西差异和南北差异;喜马拉雅构造区内莫霍面深度变化较大,介于65～80 km之间;拉萨地体内莫霍面深度介于72～80 km之间;雅鲁藏布缝合带两侧地壳厚度突变,缝合带北侧和南侧地壳厚度相差约8 km.研究区域平均泊松比值较小,为0.24,和大多数造山带泊松比偏低的特征类似.研究区域中下地壳广泛存在强转换界面,该界面可能对应中下地壳高速层的上界面,埋深40～70 km,表明壳内发生深熔或部分熔融作用,导致壳内发生重力分异,在中下地壳形成了高速薄层.

地球物理学是一门运用基本物理理论和数学分析手段解决地学问题的交叉应用学科,因此专业课程的设置侧重于理论和应用的紧密结合,实验教学亦成为地球物理专业课程重要的配套教学内容.南京大学地学院在地球物理基础课程教学中,逐步建立起由岩石物性测量、地球物理场观测和地球物理场资料处理三大模块构成的模块化教学实验体系.教学实验设计密切结合课程教学逻辑和框架结构,通过各模块的设计目标、模块之间的关联性,帮助学生建立地学问题和物理过程的清晰关联,加深对基础理论的理解,提升课堂教学效果.

The subduction of the Indian Plate beneath SE Tibet and its related volcanism in Tengchong are important geologic processes that accompany the evolution of the Tibetan Plateau. However, it is still not clear whether the subduction and volcanism are confined to the upper mantle or if they extend deep into the mantle transition zone (MTZ). Here, we imaged MTZ structures by using receiver function methods with the waveforms recorded by more than 300 temporary stations in SE Tibet. The results show significant depressions of both the 410-km and 660-km discontinuities and a thickened MTZ (260–280 km) beneath SE Tibet. The depression of the 660-km discontinuity (by 10–30 km) and the thickened MTZ correlate well with high P-wave velocity anomalies in the MTZ, indicating the presence of a subducted Indian slab within the MTZ. Significant depression of the 410-km discontinuity (by 10–20 km) beneath the Tengchong volcano indicates that the volcano originates from the MTZ and is closely related to the subducted Indian slab. Our results confirm the deep subduction of the Indian plate and the deep origin of the Tengchong volcano. However, it remains unknown whether a slab gap exists and contributes to the Tengchong volcano.

The northeastern Tibetan plateau margin is the current expansion border, where growth of the plateau is ongoing. We analyze shear-wave splitting at ChinArray stations in the NE Tibetan Plateau and its margin with the stable North Chine Craton. The measurements provide important information on the seismic anisotropy and deformations patterns in the crust and upper mantle, which can be used to constrain the expansion mechanism of the plateau. Along the margin and within the craton, the dominant NW–SE fast polarization direction (FPD) is NW–SE, subparallel to the boundary between the plateau and the North China Craton. The shear-wave splitting measurements on the NE Tibetan Plateau itself generally reflect two-layer anisotropy. The lower-layer anisotropy (with NW–SE FPDs) is consistent in the whole region and FPDs are the same as those in the North China Craton. The upper-layer FPDs are parallel to crustal motion rather than surface structures within the high plateau. The two-layer anisotropy implies the presence of deformed Tibetan lithosphere above the underthrusting North China Craton. The NE Tibetan shows similar deformation patterns at the surface (inferred from GPS) and within the mantle (inferred from shear-wave splitting), but significant crustal anisotropy (parallel to crustal motion) requires mid-lower crustal channel flow or detachment to drive further tectonic uplift of the plateau.

We applied the gCAP algorithm to determine 239 focal mechanism solutions \(\left( {3.0 \le M_{\text{W}} \le 6.0} \right)\) with records of dense ChinArray stations deployed in Yunnan, and then inverted 686 focal mechanisms (including 447 previous results) for the regional crustal stress field with a damped linear inversion. The results indicate dominantly strike-slip environment in Yunnan as both the maximum (σ 1) and minimum (σ 3) principal stress axes are sub-horizontal. We further calculated the horizontal stress orientations (i.e., maximum and minimum horizontal compressive stress axes: S H and S h, respectively) accordingly and found an abrupt change near ~26°N. To the north, S H aligns NW-SE to nearly E-W while S h aligns nearly N-S. In contrast, to the south, both S H and S h rotate laterally and show dominantly fan-shaped patterns. The minimum horizontal stress (i.e., maximum strain axis) S h rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in west Yunnan, and further to NE-SW in the South China block in the east. The crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions. Therefore, the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.

目前解释青藏高原东缘的生长与扩张有诸多动力学模型，如：刚性块体挤出模型、连续变形和中下地壳流模型。由于受到岩石层结构模型分辨率的限制，青藏高原演化和变形的动力学过程仍不清楚。我们利用最新布设在青藏高原东南缘的地震台阵，通过接收函数和瑞利波联合反演得到了该区高分辨率三维岩石层横波速度模型，更好地揭示了壳内低速带(LVZ)分布特征。我们的速度模型显示研究区壳内存在两个低速通道，这两个低速通道边界与该区主要走滑断裂相对应，且沿着东喜马拉雅构造结顺时针分布，这与该区地壳物质顺时针运动模式比较一致。此外，我们观测到该区域主要大地震分布在这两个低速通道边界区域。据此，我们提出塑性流动和剪切变形在青藏高原的隆升和变形过程中都起了重要作用。

了解华南各岩石圈块体壳幔结构和各向异性方面的差异是揭示华南深部构造演化的基础.本文利用布设于华南的两条宽频地震测线观测数据,采用多种地震学方法对华南的地壳上地幔结构和各向异性进行了研究.接收函数结果表明,华南地区地壳厚度和岩石圈厚度都较薄,地壳厚度自东南沿海向西北内陆增厚,扬子克拉通的泊松比(波速比)低于华夏块体,表明扬子克拉通地壳较华夏块体更偏长英质.约北纬29°以北的扬子克拉通地幔转换带厚度明显增厚,可能是由地幔转换带底部停滞的冷的古太平洋板片或中生代克拉通碰撞残留造成的.层析成像结果显示华南上地幔具有很强的横向差异性,上地幔中的强烈低速异常体可能对应了晚中生代发生广泛岩浆作用时的岩浆房和岩浆通道.台湾下方的上地幔存在南北横向差异明显的高速异常,分别对应台湾南部向东俯冲的欧亚板块及台湾北部向北俯冲的菲律宾海板块.俯冲的欧亚板块在台湾南部是连续的,而在台湾中北部,由于与菲律宾海板块的相互作用,俯冲的欧亚板块被折断.剪切波分裂结果显示,以江绍断裂为界,华夏块体与扬子克拉通的岩石圈地幔各向异性存在明显的横向变化,表明两者的构造演化过程有显著差异.

Competing geodynamic models, such as rigid-block extrusion, continuous deformation, and the mid-lower crustal flow, have been proposed to describe the growth and expansion of eastern Tibet. However, the dynamic processes responsible for plateau evolution and deformation remain poorly understood partly due to resolution limitations of previous models of lithospheric structure. On the basis of joint inversion of Rayleigh wave dispersion and receiver functions using data from a newly deployed seismic array, we have obtained a high-resolution 3D image that reveals the distribution of low-velocity zones (LVZs) with unprecedented clarity. The prominent feature of our model is two low-velocity channels that bound major strike-slip faults in SE Tibet and wrap around the Eastern Himalaya Syntaxis, consistent with the clockwise movement of crustal material in this region. Most large earthquakes in this region occurred in the boundaries of the LVZs. We propose that ductile flow within these channels, in addition to shear motion along strike-slip faults, played a significant role in accommodating intensive lithospheric deformation during the eastward expansion of Tibet in the Cenozoic.

We measured shear-wave splitting of teleseismic XKS phases (i.e., SKS, SKKS and PKS) recorded by more than 300 temporary ChinArray stations in Yunnan of SE Tibet. The first-order pattern of XKS splitting measurements shows that the fast polarization directions (φ) change (at ∼26–27°N) from dominant N–S in the north to E–W in the south. While splitting observations around the eastern Himalayan syntax well reflect anisotropy in the lithosphere under left-lateral shear deformation, the dominant E–W φ to the south of ∼26°N is consistent with the maximum extension in the crust and suggest vertically coherent pure-shear deformation throughout the lithosphere in Yunnan. However, the thin lithosphere (<80 km) could account for only part (<0.7 s) of the observed splitting delay times (δt, 0.9–1.5 s). Anisotropy in the asthenosphere is necessary to explain the NW–SE and nearly E–W φ in these regions. The NE–SW φ can be explained by the counter flow caused by the subduction and subsequent retreat of the Burma slab. The E–W φ is consistent with anisotropy due to the absolute plate motion in SE Tibet and the eastward asthenospheric flow from Tibet to eastern China accompanying the tectonic evolution of the plateau. Our results provide new information on different deformation fields in different layers under SE Tibet, which improves our understanding on the complex geodynamics related to the tectonic uplift and southeastward expansion of Tibetan material under the plateau.