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Train traffic has recently been realized as a potential seismic source for imaging and monitoring shallow subsurface structures. High-speed trains usually move on viaducts and generate seismic wavefields through bridge piers. This kind of source is not randomly distributed in space and time and thus indicates the characteristic of self-correlation. We find that correlated sources of one high-speed train running over piers make time-delayed spurious signals in the crosscorrelation functions. The direct-wave interferometry with correlated sources is proposed and demonstrates that stacking of the interferometric responses from high-speed trains with different speeds may reduce the spurious imprint of source self-correlations, but it cannot remediate the problem. The influence of the source wavelet can be eliminated by using cross coherence. Stationary-phase segment selection and stacking can reduce the bias generated by trains moving out of the stationary-phase zone of station pairs. The validity is demonstrated by using synthetic data with inline and out-of-line linear arrays. We apply the proposed scheme to field data with nine high-speed train events. Multimodal Rayleigh waves were retrieved and shallow shear-wave velocities were inverted from multimodal dispersion data in a range of 4–25 Hz. The results bring a novel approach for the imaging and monitoring of the shallow subsurface using high-speed-train-induced vibrations.

Based on the recordings of 42 permanent seismic stations in Northern Jiangsu Basin (NJB) and its surrounding areas, we obtained both the sediment and crustal thickness in the region, using the nearest-neighbor algorithm and H–κ stacking method of receiver functions. We proposed a solution to use the sediment thickness to correct the depth of actual Moho discontinuity beneath the basin areas. Our results indicate that the sediment thickness in the northwest of the NJB is

Seismicity in the Three Gorges Reservoir (TGR) region increased noticeably as the water level of the reservoir rises, since the Three Gorges Dam (TGD) was built in 2003. Here, we determined moment tensors of six earthquakes in the 2014 Zigui earthquake sequence (ZGS) in the TGR region using local and regional broadband seismic waveform data. We also determined the focal depth of the mainshock using its teleseismic waveform data and then relocated the epicenter using its travel times at local stations. High-precision locations of 64 events in the sequence were obtained by combining the double-difference relative location result and the mainshock’s absolute location. The event locations and moment tensor solutions indicate that the ZGS occurred between depth 5 and 9 km on a previously unknown fault striking southwest and dipping ∼80° to the northwest. The event depth distribution and coulomb stress change estimation suggest that the ZGS were not induced directly by the reservoir water but triggered by the stress change from the annual reservoir water level variation. We estimated that the newly found fault has the potential for a magnitude 5.7 earthquake for which ground motion has a 16% probability to exceed the designed maximum intensity level at the TGD, though it would take more than 100 yr to accumulate the needed amount of slip.

SUMMARYGeophysical inverse problems are commonly ill-posed and require proper regularization to gain acceptable solutions. Adopting constraints on the smoothness and/or specified structures of an inverted geophysical model requires the implementation of regularization terms when either structured or unstructured meshes are used. Spatial gradients of the model parameters along axial or arbitrary directions are thus required. However, the calculation of spatial gradients on unstructured meshes is not straightforward since the interfaces between adjacent model cells (i.e. common edges or common faces) are orientated individually and usually are not perpendicular to the axial directions. Depending on the algorithm used, an uneven performance of the regularization is observed on unstructured meshes. To enforce effective and robust regularization terms for geophysical inversions on the unstructured meshes, we propose algorithms for constructing the smoothness and structural similarity operators that take advantage of the inherent merits of the unstructured meshes. Following a detailed introduction of the general inversion formula that we adopted, the smoothness and reference model constraints on triangular and tetrahedral meshes are proposed based on the neighbouring relationships between different model cells within the meshes. Particularly, a quasi-cross-gradient formulation is derived for triangular meshes suitable for the joint inversion of different kinds of geophysical data. Compared to existing algorithms, the new smoothness operator presents an equal or better performance for constraining the model roughness. In addition, the operator exploits the preferred elongation directions of the underground structures by performing varied constraints in different directions. Furthermore, the other new operator could effectively measure structural information of the inverted model even if the algorithms have incorporated sophisticated constraints from other geophysical or geological data. Demonstrated with the applications on synthetic examples, the new algorithms provide advanced regularization techniques for conducting geophysical inversions using unstructured meshes.

The initiation and evolution of compressional intracontinental orogens are favored by rheologically weak lithosphere underneath; however, how this weakened lithosphere responds to the regional stress regime remains vigorously debated. The Tien Shan mountains in central Asia provide the best example to illustrate the deep deformational responses to intracontinental orogenesis. We present new constraints on the nature of seismic anisotropy in the crust and upper mantle of the central Tien Shan through shear-wave splitting analyses. Our results reveal a sharp change in the orientations of crustal anisotropic fabrics on two sides of the mountains. The convergence-parallel fast orientations in the northern segment are closely related to the lower-crustal simple-shear deformation caused by the underthrusting of the Kazakh Shield, whereas the depth-independent orogen-parallel fast orientations in the southern segment suggest vertically coherent pure-shear thickening of the Tien Shan lithosphere in response to the northward indentation of the Tarim Basin. The thickened lithosphere has partly foundered into the deep mantle, contributing to the accelerated shortening deformation in the late Cenozoic. Our observations demonstrate the complex tectonic processes in the Tien Shan and suggest that the rheological properties of bounding blocks can play a significant role in shaping the lithospheric structures of intracontinental orogens.

The recycling of oceanic lithosphere into the deep mantle at subduction zones is one of the most fundamental geodynamic processes on Earth. During the closure of an ocean, ancient oceanic slabs are thought to be consumed entirely in subduction zones due to their negative buoyancy. Yet, it is recently suggested that small pieces of oceanic slabs could be trapped along paleo-subduction zones. What remains far more enigmatic is whether significant portions of paleo-oceanic lithosphere could eventually avoid the fate of subduction and be accreted to continental lithosphere, thus contributing to continental growth through time. We present seismic evidence for a preserved paleo-oceanic lithosphere beneath the Junggar region in northwestern China. We show that unsubducted oceanic lithosphere in the West Junggar has been preserved beneath the Junggar Basin, becoming a piece of the Eurasian continent. This scenario is likely to have occurred in other continents throughout Earth’s history, providing an additional and commonly underestimated contribution to the growth of continental lithosphere.

Scientists investigating the coupling between the lithosphere, atmosphere, and ionosphere (LAI) require multiple instruments installed on and near the Earth’s surface and aboard satellites orbiting the Earth. However, challenges always occur owing to a lack of observation data from other monitoring instruments and/or the distance between distinct instruments. A novel system for monitoring vibrations and perturbations in the LAI (MVP–LAI) was established in the countryside of Leshan City, Sichuan Province, China, in June 2021. Twelve different types of instruments with 18 devices were set within an area of 50 m × 50 m. The other two were installed approximately 20 km away to avoid interference. These instruments routinely monitor the changes in at least 14 geophysical parameters with short sampling intervals, available for capturing waves propagating from the subsurface to the ionosphere. Once the observations retrieved from the China Seismo-Electromagnetic Satellite and the radio occultation are integrated, the monitoring of vibrations and perturbations can reach ~ 800 km above the Earth’s surface. The system is mainly operated by the China University of Geoscience (Wuhan), China and will officially start functioning in September 2021. All the efforts benefit the investigation of the causal mechanisms of LAI coupling and expose the potential sources of vibrations and perturbations, incorporating observations from other spheres.

SUMMARY The separation of regional-residual anomalies plays an important role in the processing of potential field anomalies for obtaining better understandings of the nature of the underground sources. Many methods have been developed to achieve the separation of anomalies that are of distinct wavelengths. On the other hand, fewer studies have addressed the separation of local anomalies from the observed potential field anomalies. In this paper, we introduce a new process for separating localized anomalies from the observations under the Cartesian and spherical coordinates. The separation is achieved using the equivalent source technique and an iterative inversion process which is to refine and finalize the separated local anomalies. Additionally, we introduce an inversion method for determining the equivalent sources that are of varying dimensions, as well as a quantitative measurement to assess the accuracy of the separation process. Verified with synthetic examples, the proposed method could extract arbitrary shaped local anomalies from the rest with low error levels. Subsequently, we apply the method to the construction of a 3-D model of the mantle uplift beneath the Von Kármán crater (VKC) on the Moon. The VKC is the landing site of the Chinese lunar exploration mission Chang'e 4, which lies in the northwestern portion of the South-Pole Aitken (SPA) basin on the far side of the Moon. Multiple generations of mare basalts are identified within the VKC, which indicates a complex geological history of the basin. Insights into the evolutionary history of this region can be obtained by investigating the deep crustal structure of the VKC using topographic and gravity data. Processed with the proposed method, the 3-D structure we obtain provides evidence for separated mantle uplifting events triggered by the two impact events that created the VKC and the Von Kármán M crater, respectively.

SUMMARY Rotation of coil-type magnetometers caused by seismic waves can change the magnetic flux of the earth's magnetic field through the coils and thereby give rise to rotation-induced magnetic fields (RIMFs), which have contribution to so-called coseismic magnetic disturbances during earthquake events. In this study, we present a method to numerically calculate the rotational motion and RIMFs excited by a seismic source in a horizontally layered model. Like computing synthetic seismograms, this method allows calculating the full response of magnetic disturbances during an earthquake event. Numerical examples are conducted to investigate characters of the magnetic fields under excitation of different kinds of sources. The results show that the P-SV-mode waves mainly contribute to radial and vertical magnetic disturbances, while the SH-mode waves dominate tangential magnetic disturbances. We also apply this method to the 2017 Jiuzhaigou earthquake. The synthetic magnetic signals explain well the observed data, suggesting that the coseismic magnetic fields observed during this earthquake are mainly caused by the rotation of the magnetometers.

A new efficient random search algorithm is introduced for solving inversion problems in geophysical studies. The proposed algorithm is inherently a stochastic optimization method which is built on the concept of gradient descending and Levy flights. Therefore, the algorithm is referred to as the Levy gradient descent (L-GD). In which, the Levy flights is a special class of random walk which consists of many short steps along with a few large steps. Such movements are observed in a varying range of fields, including animals’ foraging patterns, fluid dynamics, transport of light and so on. Meanwhile, the Levy flights typically shows much higher speed in searching for sparsely located targets compared to the well-known Brownian walks, which make them preferable to drive random search algorithms. As shown in the paper, besides optimal solutions of the inverse problems, the L-GD algorithm could also produce estimations on the error distributions of the resultant model parameters. Following a detailed introduction of the methodology, parameter settings of the algorithm are discussed in length through statistical experiments. Subsequently, the proposed algorithm is evaluated using numeric tests and shows attracting properties of the global convergence and significant higher searching efficiency compared to commonly adopted stochastic optimization techniques in geophysical inversions. Moreover, the L-GD algorithm is applied to the inversions of gravity and seismic travel time data and has achieved the same accuracy as gradient-based optimization methods. Meanwhile, though error estimations generated by L-GD algorithm are essentially qualitative, they could still provide valuable information to help evaluating the resultant model parameters, which is of great importance for practical geophysical inversions.

The dominant driving forces for the east-west extension of the Tibetan Plateau since the mid-late Miocene remain vigorously debated. Proposed hypotheses encounter difficulties in reconciling the geological observations of more developed north-trending rifts in southern Tibet as well as the discrepant extension magnitudes among them. With seismic recordings collected from our recently deployed and existing seismic arrays, we locate a mid-crustal simple shear zone characterized by convergence parallel anisotropy beneath the southern plateau, which is likely caused by the underthrusting of the Indian Plate. Furthermore, a zone of reduced S-wave velocity is also resolved between the two rifts with highest extension rate, indicative of the convective removal of the lower Indian mantle lithosphere. Taken together, our results suggest that the enhanced extension occurring in southern Tibet are controlled by both the shear tractions induced by the advancing Indian Plate and the increased buoyancy due to asthenospheric upwelling.

SUMMARY The Tuareg Shield was assembled by oceanic closures and horizontal movements along mega-shear zones between approximately 20 terranes during the Pan-African Orogeny (750–550 Ma). Although there is an ongoing debate about its origin, the exhumation of the Tuareg Shield is assumed to be related to Cenozoic intraplate volcanism. The Gour Oumelalen is a key region of the Tuareg Shield and is located in the northeastern part of the Egéré-Aleksod terrane, corresponding to the eastern boundary of the Archean–Palaeoproterozoic microcontinent LATEA (Central Hoggar). The eastern boundary of the study area corresponds to a Neoproterozoic suture zone separating two old microcontinents, LATEA and the Orosirian Stripe. We deployed two magnetotelluric (MT) profiles consisting of 33 broad-band MT stations and combined these with aeromagnetic data, aiming to define the crustal structure in detail. The resistivity cross-sections obtained from the 3-D inversion of full impedance tensor and tipper data from stations along the profiles, confirm the main Precambrian faults, some of which are covered by Quaternary sediments and hence, have not yet been deciphered. The cross-sections also highlight the Cretaceous–Quaternary sedimentary basins represented by low resistivities. The upper crust is typically cratonic with a high electrical resistivity. On the contrary, the lower crust shows a drastic drop in resistivity (

Geophysical, petrological and geochemical studies have shown that the lithospheric mantle is highly heterogeneous in its physical properties and chemical compositions. They provide critical information on the evolution of the lithosphere-asthenosphere boundary (LAB). Considerable efforts by geophysical approaches have demonstrated that LAB is characterized by profound changes in physical properties such as seismic velocities and electrical resistivity at an average depth of ~70 km in the oceanic areas. In the continental regions, however, the variations in the physical properties at LABs are not that significant and their depths vary greatly with tectonic environments (e.g., orogens vs. cratons). Mid-lithospheric discontinuities (MLDs; ~60 to 120 km depths) have been equally well documented by various techniques, including deep seismic reflections and refractions, receiver functions analyses of teleseismic Ps and Sp phases, teleseismic and ambient-noise based surface-waves tomography. Except in some regional cases, MLDs are usually featured by reduced velocities (i.e., negative velocity gradients). It appears that MLDs are statistically correlated with the low-resistivity layers, as imaged for the European continent. Many hypotheses have been proposed to explain the origin of MLDs, including the accumulation of volatile-bearing assemblages due to metasomatism of magmas/fluids and/or metamorphism; the change of deformation patterns of rocks in the lithospheric mantle and finally a “frozen” LAB. Here we review the related advances and try to reconcile available data and models. We first provide a brief introduction about the discovery of MLDs, by summarizing their velocity, radial and azimuthal anisotropies as well as their internal structures across Australian, North American and Euro-Asian continents. Then we discuss the possible origin of MLDs, by considering mantle heterogeneities in mineralogy and petrology and in particular the potential role of volatile-rich minerals such as phlogopite, amphibole and carbonates. Finally, we conclude that MLDs may be frozen LABs. It sheds light on the potential importance of MLDs in Earth’s evolution and especially the global tectonic and the changes over time. We present a working hypothesis using a dynamically-evolved LAB model. MLDs began as oceanic LABs, with their depths increasing gradually from the mid-ocean ridges to subduction zones due to change in temperature and age. During continental accretion, the “inherited” oceanic LAB merged with the continental lithosphere and became deepened along with the cooling of the new continental lithosphere. In brief, the old LAB was “frozen” into the continental lithosphere and produced MLDs. Several possible mechanisms supporting this hypothesis are proposed, and the conditions that are tenable from recent results of geodynamic modeling studies in Africa are presented. The advantages of this hypothesis are highlighted. Its potential in explanation of scientific puzzles of solid earth is evaluated.

We present a magnetotelluric study of the Lei-Hu-Ling volcanic system in the Haikou Volcanic Zone, Hainan, China. Data acquired from 111 broadband magnetotelluric sites was used to construct a 3D resistivity model of the magmatic system. The 3D inversion shows that a well-developed shallow low-resistivity zone in the southwest of the Changliu-Xiangou Fault corresponds to the Fushan depression, and a large conductor presents in the depths of 4–10 km underlying the Lei-Hu-Ling volcanic system. The deep conductor displays a N–S lineation oblique to the Changliu-Xiangou Fault, indicating that the fault does not control the deeper magmas path. We prefer to attribute the deep conductor to accumulation of high-salinity fluids in the storage zone with porosity of 1% to 1.6%, which was associated with the transportation of magmatic fluids and volatiles supplied by a partial melting zone at greater depths.

Uplifting mechanisms for the Tien Shan, an active intracontinental orogenic belt, have been under debate for decades, a key issue being how the convergence has been accommodated at depth. Here we investigate the Moho structure across the central Tien Shan by common-conversion-point imaging and H-k-c stacking of receiver functions from a dense array. The observed Moho exhibits distinct characteristics among sub-blocks. Southward-dipping diffuse Moho is imaged in the Southern Tien Shan (STS), in contrast with the relatively flat and sharp Moho beneath the Tarim Basin. This feature along with the large Moho offset beneath the South-Boundary Fault suggests that the shortening and thickening of Tien Shan crust rather than the underthrusting of the Tarim Basin are responsible for the uplift of the STS. In the Northern Tien Shan, however, the imaged Moho doublet provides direct evidence for the underthrusting of the Kazakh Shield accommodating the convergence there.

The role of the Tarim Block in the reconstruction of the Neoproterozoic supercontinent Rodinia remains contentious. Here we report a suite of high–Mg gabbroic dykes from the Yingfeng area in northwestern Quanji Massif, which is considered as a fragment of the Tarim Block in NW China. Magmatic zircons from these dykes yield to have a weighted mean 206Pb/238U age of 822.2 ± 5.3 Ma, recording the timing of their emplacement. The gabbros have high MgO (9.91–13.09 wt%), Mg numbers (69.89–75.73) and CaO (8.41–13.55 wt%), medium FeOt (8.50–9.67 wt%) and TiO2 (0.67–0.93 wt%), variable Al2O3 (13.04–16.07 wt%), and high Cr (346.14–675.25 ppm), but relatively low Ni (138.72–212.94 ppm), suggestive of derivation from a primary magma. The rocks display chondrite-normalized LREE patterns with weak fractionation but flat HREE patterns relative to those of the N-MORB. Their primitive mantle normalized trace elemental patterns show positive Rb, Ba and U but negative Th, Nb, Ti and Zr anomalies, carrying characteristics of both mid-ocean ridge basalts and arc basalts. The eHf(t) values of the zircons from these rocks vary from +4.7 to +13.5 with depleted mantle model ages (TDM) of 1.23–0.85 Ga, and the youngest value nearly approaching that for the coeval depleted mantle, suggesting significant addition of juvenile materials. Our data suggest that the strongly depleted basaltic magma was probably sourced from a depleted mantle source that had undergone metasomatism by subduction–related components in a back-arc setting. Accordingly we postulate that a subduction–related tectonic regime possibly prevailed at ∼0.8 Ga along the southeastern margin of the Tarim Block. Combining with available information from the northern Tarim Block, we propose an opposite verging double–sided subduction model for coeval subduction of the oceanic crust beneath both the southern and northern margins of the Tarim Block during early Neoproterozoic.

The marine controlled-source electromagnetic (CSEM) method is of increasing importance as a powerful geophysical tool in hydrocarbon exploration. Related to this, three-dimensional (3-D) forward modeling provides an effective method for marine CSEM survey planning and sensitivity analysis (Frechet derivative). To further evaluate the feasibility of this measurement system as well as its resolution capabilities, we carried out a 3-D sensitivity distribution experiment. The sensitivity calculation used in this work is based on reciprocity theorem and directly affects the accuracy and efficiency of the inversion computation process. This is also a key problem in Gauss-Newton type inversion algorithms. Thus, we analyze the sensitivity distribution of the electric field for both a homogeneous half-space and resistors embedded in conductive sediments. To illustrate the potential of this approach in a 3-D survey, we then adopt a series of simple numerical models to simulate hydrocarbon reservoirs in real situations across a typical suite of configurations. We show that the sensitivity simulation can enhance our understanding of sensitivity characteristics. Indeed, via comparisons and the analysis of sensitivity distributions, we demonstrate the feasibility of this approach for the detection of hydrocarbon reservoirs, and provide additional information for planning and designing marine CSEM surveys.

Recent geological, geochemical and geophysical data have inclined to support the presence of a remnant Paleozoic oceanic lithosphere beneath the Western Junggar, southwestern Chinese Altaids. However, regional high-resolution geophysical data have been rarely deployed to image its geometry, making it difficult to trace its evolution and final geodynamic setting. Presently, two magnetotelluric (MT) profiles are deployed across the northwest margin of the Junggar basin and the southern Darbut belt to image the electrical structure of the crust and lithospheric mantle. High-quality data at 102 sites and the quasi-2D indications of phase tensor skew angles and impedance phase ellipses for relatively short periods (up to 500 s) allow us to invert the two profile data by a 2-D scheme. The resistivity cross-section of a NW-SE striking LINE2 sheds light on a fossil intraoceanic subduction system, and reveals the Miaoergou intrusions as a bowl-like pluton, indicating that the multi-phase intrusions primarily formed in a post-collisional setting. The resistivity cross-section of striking NE-SW LINE1 reveals a possible oceanic slab with relatively lower resistivity underlying the low-resistivity sedimentary strata and high-resistivity melange. Given that the profile of LINE1 cuts the out-rise zone of a subducted slab developed during the late Paleozoic, the 2-D resistivity model may thus represent the zone that have experienced heterogeneous deformation, reflecting subduction with barrier variation parallel to the ancient trench. Moreover, as shown in previous results, the new MT data also illustrate that the Darbut Fault is a thin-skinned structure, which has been erased at depths during the subsequent magmatism.

The first tomographic images of P wave azimuthal and radial anisotropies in the crust and upper mantle beneath the Alps are determined by joint inversions of arrival time data of local earthquakes and teleseismic events. Our results show the south dipping European plate with a high-velocity (high-V) anomaly beneath the western central Alps and the north dipping Adriatic plate with a high-V anomaly beneath the Eastern Alps, indicating that the subduction polarity changes along the strike of the Alps. The P wave azimuthal anisotropy is characterized by mountain chain-parallel fast-velocity directions (FVDs) in the western central Alps and NE-SW FVDs in the Eastern Alps, which may be caused by mantle flow induced by the slab subductions. Our results reveal a negative radial anisotropy (i.e., Vph Vpv) in the low-velocity mantle wedge, which may reflect the subvertical plate subduction and its induced mantle flow. The results of anisotropic tomography provide important new information on the complex mantle structure and dynamics of the Alps and adjacent regions.

Knowledge of heat flow and associated variations of temperature with depth is crucial for understanding how the Earth functions. Here, we demonstrate possible heat shielding effects that result from the occurrence of mafic intrusions/layers (granulitic rocks) within a dominantly granitic middle crust and/or ultramafic intrusions/layers (peridotitic rocks) within a dominantly granulitic lower crust; heat shielding is a familiar phenomenon in heat engineering and thermal metamaterials. Simple one-dimensional calculations suggest that heat shielding due to the intercalation of granitic, granulitic and peridotitic rocks will increase Moho temperatures substantially. This study may lead to a rethinking of numerous proposed lower crustal processes.