The ongoing India-Asia collision principally regulates Cenozoic tectonic deformation of the Asian interior, and causes reactivation of one most active and spectacular intraplate orogen, the Tian Shan, as a visible manifestation of the long-distance orogenic effects. Most previous studies focused on crustal-scale strain transfer and the role of the rigid Tarim in crustal thickening in its surrounding areas, which makes deep process-related physical mechanism of the Tian Shan rejuvenation due to Indian indentation still poorly understood. Here, we conduct systematic numerical modeling to explore the effects of the thermo-rheological properties of the orogen-featured blocks and the convergence rate on intraplate deformation propagation and mountain building. According to our models, it is the collision of Tibetan lithospheric mantle with the rigid Tarim basin beneath northern Tibetan crust, rather than the progressively northward crustal thickening, that expedites long-distance stress transfer through the Tarim basin to the remote Tian Shan area and triggers reactivation of the ancestral orogen. The model results, together with the well-established geological and geophysical constraints, not only well account for the ~30-million-year time lag of uplift of the Tian Shan from the onset of the India-Asia collision, but also reconcile the first-order crustal and lithospheric structures of the Tibetan plateau and Tian Shan.

Since ~10 Ma, the northeastern margin of the Qinghai Tibet Plateau has been growing and expanding northeastward, and has formed a series of fault zones, namely Haiyuan, Xiangshan-Tianjingshan, Yantongshan, and Niushoushan, respectively, which becomes the leading edge of the northward growth and expansion of the Tibet Plateau. The historical earthquakes are also frequent in this area. On the basis of previous geological investigation, tectonic geomorphology, geochronology, seismic tomography, high-precision seismic reflection profile and other data, three-dimensional finite element models of the spatio-temporal evolution of the arc-shaped structural belts in the northeastern margin of the Tibet Plateau are established, considering the complexity of geometry, material, boundary conditions and bottom decollement layer. Compared with a two-dimensional model, the three-dimensional finite element model can better reflect the temporal and spatial distribution of fold-and-thrust belts in the northeastern margin of the Tibet Plateau. The three-dimensional model can well simulate the length variation, spatial interval, mechanism variation and asymmetric distribution of Haiyuan fault, Xiangshan-Tianjingshan fault and Yantongshan fault. The 1920 Ms 8.5 Haiyuan earthquake and the 1709 Ms 7.5 Zhongwei earthquake occurred in the first (Haiyuan fault) and the second (Xiangshan-Tianjingshan fault) arc-shaped plastic strain concentration zones, respectively. Most of the earthquakes with M ≥ 6 are located in the plastic strain concentration zones. There is a significant correspondence between epicenter distribution and fault zones, which are both related to the calculated arc-shaped plastic strain concentration zones. The simulation results are in good agreement with the geological investigation, structural geomorphology and geophysical data, and can also explain the dynamic causes of historical earthquakes in the study area.

The Tibetan Plateau resulted from diachronous collisions since the Paleozoic and the ongoing India-Eurasia collision constructed the plateau to its current configuration. Previous studies have indicated an eastward-extrusion of the plateau since the Miocene, which is evidenced by one of the scenarios along the sinistral Eastern Kunlun fault zone. However, it remains ambiguous regarding the easternmost termination of the Eastern Kunlun Fault zone. In this study, with employment of data from short-period dense array of seismometers that were deployed in the Ruo’ergai Basin of easternmost Tibet, we will carry out a receiver function study to first document the fine crustal structure and then locate the eastward extent of the Eastern Kunlun fault zone. Numerical models will be constructed to understand deep processes of extrusion and how stress accumulated. The results will contribute significantly to understanding the deep mechanism for extrusion processes within the plateau margin.

The Cenozoic tectonic deformation history of the Qilian orogenic belt in the northeastern margin of the Qinghai-Tibet Plateau is complex, with many possible tectonic patterns. The unresolved issue is whether the Asian lithospheric mantle subducted beneath the Tibetan Plateau. In this study, two-dimensional elastic-plastic models are established, with Drucker-Prager plastic yield criterion. The length of the model is 360 km, crossing the Qilian Mountains. Southwest of the model is the edge of the Qaidam Basin, and northeast is the north of the Hexi Corridor, including the South, Central and North Qilian Mountains. The model is 100 km deep, which is divided into upper crust layer, lower crust layer and lithospheric mantle layer. Moho depth changes from 50 km to 65 km along the profile. The subduction passage of the Asian lithosphere is considered as a decollement layer, which adopts frictional contact. According to a reference model, the dynamic processes of crustal thickening, fault development and surface uplift of the Qilian orogenic belt after Cenozoic activation are reproduced. The simulated fault development sequence is from South Qilian Mountain, Central Qilian Mountain, North Qilian Mountain, and to south part of Hexi corridor, sequentially. The fault in South Qilian Mountain dips north, and that in North Qilian Mountain dips south, showing obvious bilateral structural style. Based on a series of testing models, we indicate that the position of the subduction front of the Asian lithospheric mantle and the cutting depth of the Haiyuan fault play an important role in the sequence and style of fault system evolution. Decollement has an obvious influence in the tectonic shortening, fault evolution and surface uplift of the study area.

The Yulin River, flowing across the Dongbatu Shan in the northern Tibetan Plateau, consists of multiple geomorphological features, including alluvial fans, fluvial terraces, knickpoints, and paleo-channels. These provide a natural laboratory for exploring the late Quaternary fluvial landform evolution in response to both climatic change and tectonic activities in the region of the Altyn Tagh Fault. In this research, we investigated the distribution, chronology sequences and sediment characteristics of the fluvial landforms along the Yulin River based on high-precision (0.5 m-resolution) Worldview satellite images and field observations. Optically Stimulated Luminescence, Carbon fourteen, and ¹⁰Be terrestrial cosmogenic nuclides methods were used to establish the chronological sequences. Geometric features of the fluvial landform surfaces were measured by Real-time kinematic global positioning system. Our results suggested that the Yulin River had incised into the alluvial Fan2 since nearly 160 ka and then transformed into fluvial incision system forming 10 major terraces in three different stream sections during the late Pleistocene. Terrace abandonment ages indicated that river incision mainly happened during periods of climate transition, suggesting that terrace evolution was partly controlled by climate changes. Moreover, terrace exposure ages showed an upstream younging trend along the river, demonstrating a probable response to knickpoint retreat. The abandonment ages of the paleo-channels showed that Dongbatu Shan had gone through a rapid uplift during the Late Middle Pleistocene. Combined with terrace deformation and ages, fault uplift rates were constrained, which were lower than the river incision rates during the Late Quaternary, indicating that crustal shortening was mainly absorbed by the growth of the regional anticline. Collectively, our results suggested that the evolution of fluvial landforms along the Yulin River were a result of the correlation of regional tectonic movements and climate fluctuation, which is significant to understand the NW-outward splay of the Altyn Tagh Fault.

The Sanmenxia basin, located in the southeast margin of Shanxi rift and filled with Cretaceous-Paleogene fluvial and lacustrine sediments, is a faulted basin bounded by a series of normal strike-slip faults. It provides a valuable opportunity to investigate the early tectonic deformation in Weihe graben. In this study, we report an integrated rock magnetism and AMS analysis of two sections from Sanmenxia basin spanning the interval from Late Cretaceous to Paleogene. The results demonstrate that the magnetite and hematite are asserted to be the main magnetic carriers of remanence, and the paramagnetic minerals and subordinate hematite are major contributors to the AMS in both two sections of Sanmenxia basin. Besides, the inverse magnetic fabrics on Haoyang River section (HYRS) may result from the contribution of goethite and single domain (SD) magnetite in deposits. Together with the relatively low corrected anisotropy values, the tightly grouped minimum principal axes almost perpendicular to the bedding plane and the well-defined magnetic lineation generally parallel to the bedding trend indicate the primary sedimentary fabrics in Sanmenxia basin are overprinted by the initial deformation. The magnetic lineation on two sections is NW-SE and NNW-SSE oriented, denoting the NW-SE stretching during late Cretaceous-Eocene and NW-SE compression in Oligocene-Miocene. This deformation process may be related to the India-Eurasia convergence and/or the subduction of western Pacific plate during late Cretaceous-Miocene.

Mid-ocean ridges and mantle plumes are two principal regions of mantle upwelling and magma generation on the Earth. However, these two systems are not always isolated but communicated in some cases. Mantle plumes would deliver hot and compositionally distinct material toward nearby mid-ocean ridges, imposing large geophysical and geochemical anomalies along the ridge axis. Yet, many basic aspects of plume-ridge interaction are still enigmatic, especially why this interaction system are more abundant near slow-spreading ridges than fast-spreading ridges. Here, we use a 2D thermomechanical numerical modeling method to simulate the process of mantle plumes-ridge interaction, exploring the mechanism behind this behavior. Three major physical parameters, (i) the spreading rate of ridge, (ii) the plume radius, and (iii) the plume excess temperature, are investigated, and their influences on interaction distance between the plumes and spreading ridge are systematically documented. Our numerical experiments suggest three different geodynamic regimes: (a) ridge sucks plume, (b) plume dragged away by the ridge, and (c) plume underplating beneath the ocean lithosphere. Using systematic numerical models of plume-ridge interaction, we show that for fast-spreading ridge, increasing the ridge spreading rate may inhibit rather than enhance ridge suction, as a result of increased shear stress between the ridge and underplating plume materials.

The design earthquake (DE) of nuclear power plant (NPP) sites in South Korea is determined by taking into account the maximum ground motions expected at sites through a geological and seismological characteristics evaluation within a radius of 320 km from sites and a detailed geological survey within a radius of 8 km from sites. It is evaluated in a deterministic way according to U.S. Federal Regulations 10 CFR Appendix A to Part 100, a foreign regulation subject to the Nuclear Safety and Security Commission (NSSC) Notice No. 2017-15.On September 12, 2016 in South Korea, the Gyeongju earthquake occurred in an underground fault that was not revealed on the surface, making it difficult to identify the characteristics of the causative fault of the Gyeongju earthquake by ordinary surface geological surveys alone. Meanwhile, significant earthquakes in South Korea, such as the Odaesan earthquake in 2007 and the Pohang earthquake in 2017, tend to occur in such underground faults. The NPP sites in South Korea have accumulated experiences in evaluating DE, taking into account capable faults identified on the surface. Based on this, it is also necessary to prepare draft technical standards for DE evaluation of NPP sites by the causative faults of earthquakes that do not involve surface faulting in the future. It is expected that the technical standards prepared will allow the stable calculation of earthquake ground motion by the underground causative fault with a certain quality or higher and reconfirm the adequacy of DE of NPP sites. 

Atmospheric electrostatic field in the near-surface layer is usually downward under fair weather condition. While the electrostatic field may become upward a few days before an earthquake. During seismogenic process, the microfracture activity will lead to the formation of many cracks in the Earth’s crust near the epicenter, from which underground radon escape to the near ground air. Then radon will ionize the air with radiation causing the atmospheric electrostatic field in the near surface shows upward signals which can be recorded by atmospheric electric field meter on the surface of the Earth. In this paper, the abnormal signal is extracted from the data of the atmospheric electric field meter of several stations, and the correlation between the signal and the earthquake is analysed. As weather condition is the main cause of disturbance of atmospheric electrostatic field, we build a neural network model to automatically recognize whether the readings of atmospheric electrostatic field are disturbed by weather, and then it would be easier to judge whether the reverse signal is related to earthquakes. 

Tan and Helmberger (2007) developed a pure path upper mantle shear velocity model, PAC06, along the corridor from Tonga-Fiji to southern California. Although the sampled oceanic lithospheric age ranges from ~10 Ma to 125Ma, the 1D PAC06 model explains the S wave multiples up to S5 well. The similarity between PAC06 and PA5, sampling old oceanic lithosphere (100-125 Ma), indicates a more uniform oceanic lithosphere structure along the whole path from Fiji-Tonga to southern California. However, PAC06 is still debatable because of the possible path average effect. To further test the applicability of PAC06, we study an older corridor from Tonga-Fiji to Alaska, sampling oceanic lithosphere from ~55Ma to 125Ma, to investigate possible age dependence of the oceanic lithosphere structure. We selected 31 events, with different depths occurring 2016-2021 in Tonga-Fiji, recorded by USArrays in Alaska, which include rich multibounce S phases. Verified by modeling travel times and waveforms of G phase and S multiples turning at different depths simultaneously, we demonstrate that the PAC06 can well describe the lithospheric structure of the corridor we studied. The similar lithospheric structures of two corridors with different ranges of age suggest little variation in the lithospheric mantle velocity structure beneath the Pacific with very different lithospheric ages. Furthermore, we find a low-velocity anomaly in the mantle transition zone and top of the lower mantle associated with delayed SS and SSS, which may be related to the Hawaii plume.

The Mid-Atlantic Ridge (MAR), as the largest and most completely developed slow spreading ridge, crosses the whole Atlantic Ocean from north to south. In this study, we use ETOPO1 topographic data, EIGEN-6C4 satellite gravity field model and CRUST1.0 model to calculate the gravity anomaly which can reflect heterogeneous distribution of upper mantle density in the South Atlantic Ocean. Then we construct the upper mantle density structure by 3D gravity inversion method. Our study mainly aims at investigating the formation of oceanic rise and deep geophysical features of hotspots in the South Atlantic Ocean. The high density anomaly inserting into low density anomaly beneath east of South Georgia and The South Sandwich Islands (SGSSI) reflects the subduction zone between American Plate and Antarctic Plate. However, this subduction is relatively weak beneath the northwest of SGSSI. The Rio Grande Rise and Walvis Ridge at both sides of MAR respectively present obvious low density anomalies, indicating they are probably influenced by mantle plume. We suggest the interaction between Tristan Da Cunha Hotspot and MAR during the opening of the Atlantic Ocean dominates the formation of the Rio Grande Rise and Walvis Ridge. About the hotspots in the South Atlantic Ocean, the low density anomaly band beneath the northern seamounts chain started at the southern end of Walvis Ridge indicates the track of Tristan Da Cunha Hotspot. Moreover, not all the hotspots present deep low density anomaly feature. In our result, the low density anomaly in the whole upper mantle beneath the Bouvet Hotspot indicates the probable existence of deep mantle plume. The Tristan Da Cunha Hotspot and Shona Hotspot have relatively weaker deep low density anomaly characteristic. This research is supported by the China Ocean Mineral Resources Research and Development Association Thirteen Five-Year Major Program under contract No DY135-S1-1-06.

Through the approximate solution (i.e., finite Fourier series) of Laplace′s equation in the rectangular coordinate system, the rectangular harmonic analysis (RHA) is widely used in regional and local potential field modeling. In this study, the RHA method and its application effect of combined gravity and gravity gradient tensor (GGT) data sets are discussed. Firstly, the calculation methods of harmonic coefficients and spectrum are provided. Secondly, the validity of the field modelling and data processing is analyzed by synthetic tests. Then, the method is applied to the real data sets over Vinton dome in Louisiana, U.S.A. Finally, three pieces of conclusions are drawn as following:(1) If only the sparse ground gravity data is utilized for the recovery, the anomaly field in the data gaps may be wrongly reconstructed; If only the GGT data is adopted, the gravity and GGT anomaly fields can be correctly recovered except the amplitude of the gravity anomaly field; If jointing the gravity and GGT data sets, both gravity and GGT anomaly fields with high resolution can be accurately recovered.(2) Integrating both gravity and GGT data sets, different components are physically coupled and constrained each other, that is, the RHA is weakly influenced by noises, gross errors and even the systematic errors of few components, and thus to avoid the loss of the useful information the regularization is completely not necessary.(3) The RHA can effectively fuse the irregularly distributed multi-component observation data sets from different surveys, that is, can maximize the usage of the effective information of each data, and thus can improve the resolution and accuracy of recovering as well as the reliability of data processing (e.g., interpolation, spatial continuations, denoising, anomaly separation, component transformation, spectrum analysis).

The structure of the western edge of the Pacific large low shear velocity province (LLSVP) has not been accurately resolved because of the limited station coverage. Here, we provide a detailed image of the western edge of the Pacific LLSVP by using multiple phases (S, ScS, SKS and SKKS) with events in the southwestern of the Pacific Rim recorded by China National Seismic Network, F-Net and Global Seismographic Network. We find the western edge of the Pacific is dominated by strongly delayed ScS and high ScS/S amplitude ratios, which implys focusing effects generating from the low velocity zone at the core-mantle boundary. We outline the low velocity zone using ScS-S and SKKS-SKS travel time residuals. After 2D and 3D waveform modelling, we determine the height of the low velocity zone is ~60 km, and the shear-velocity perturbation (δVs) is no more than -4%. Additionally, we find a separate small-scale nearly circular low velocity zone located ~1000 km away from the Pacific LLSVP, which is about -6% in δVs and 120 km in height. This unique low velocity zone may represent a plume originated from the core-mantle boundary.

Isostatic gravity anomalies have significant meaning in studying lithospheric structure. Normally, Airy model Vening Meinesz (VM) model are used to compute isostatic anomalies with a constant value of lithospheric strength. However, the actual lithospheric strength varies greatly in the global area. Some scholars have considered the variations in lithospheric strength for calculating isostatic anomalies. However, such work will minimize the isostatic anomaly variance. For the above reasons, we propose a new method:(1) Based on Vening Meinesz model, different Moho flexure models are computed by adopting different “degree of regionality” ranging from 20-212 km with a step size of 1 km. (2) Then the maximum absolute value of the correlation between different VM Moho flexure models and CRUST1.0 Moho model is computed in a fan-spherical cap window; both the corresponding “degree of regionality” and the correlation value are stored temporarily. (3) Rotating the fan-spherical cap window N times with 2π/N degrees each time and repeating step (2) to obtain N values of “degree of regionality” and the correlation at the point, the final “degree of regionality” and correlation value are assigned to the point by averaging the sum of these N values. (4) Sliding the window, steps of (2) and (3) are repeated to obtain global VM “degree of regionality” and correlation results. (5) According to the results of step (4), when computing the Moho flexure of any point, we take the “degree of regionality” of the point into the computation to get the global Moho flexure. (6)Calculating global isostatic gravity anomalies. The proposed method takes into account the variation of global lithospheric strength and can overcome the problem of isostatic anomaly minimizing. The results reflect the global lithospheric strength, flexural mechanism and isostatic state.