雅鲁藏布江岩浆岩带的形成与板块碰撞、陆壳增生、地壳伸展等地质作用密切相关，揭示其深部展布对理解青藏高原形成演化、岩浆活动和资源效应具有重要意义。我们基于高分辨率航磁异常数据，以深反射地震剖面为深度参考，采用磁力混合Lp范数正则化反演技术，获得雅鲁藏布江岩浆岩带的地壳磁化率三维精细结构，揭示出岩浆岩带的三维架构特征。研究结果表明：（1）南高磁异常带集中分布在雅鲁藏布江缝合带南侧中段，呈现为东西向连续展布的高磁条带，对应日喀则基性-超基性蛇绿岩；北高磁异常带位于雅鲁藏布江缝合带北侧，呈现为横贯缝合带东西端的高磁条带，主要由南冈底斯带隐伏中-基性岩浆岩引起；（2）雅鲁藏布江岩浆岩带呈现“南北分带、东西分段”的特征，南带日喀则蛇绿岩规模小，分布在20 km以浅的上地壳；北带南冈底斯岩浆岩带规模大，分布在30 km以浅的上地壳；且北带因板块俯冲角度差异和南北向裂谷系切割呈东西分段特征；（3）推测雅鲁藏布江岩浆岩带为新特提斯洋北向俯冲的产物，印度板块俯冲前缘已达南冈底斯带之下。

The 2014 Ms 6.5 Ludian earthquake is a typical event in southwest China, resulting in extensive earthquake damage and disasters. Numerous geophysical observations have been conducted around the Ludian earthquake zone to reveal the seismogenic environment, but the underlying seismogenic mechanism remains a subject of debate. The spatiotemporal variation in crustal velocity is closely related to stress state and seismicity. In this article, we present an ambient noise time-lapse tomography algorithm based on the conventional surface-wave direct inversion method and the parallel difference strategy. We apply this algorithm to the ambient noise data recorded by 90 permanent seismic stations around the Ludian earthquake zone from 2011 to 2017, to construct a 3D time-lapse velocity model of the crust in this region. Based on the results, we find significant variations of crustal S-wave velocity (VS) along most active fault zones of the study area. The VS around the hypocenter of the Ludian earthquake decreased during the preseismic stage, reaching its largest variation of −0.015% ± 0.0027% between 2011 and 2014. This decrease in VS is consistent with the time-lapse variations in the gravity and geomagnetic fields around this area, indicating their common responses to physical state variation in the subsurface. The VS gradually increased during the postseismic stage; however, it had not returned to its preseismic levels by 4 August 2017. We deduce that material migration (such as partial melting or aqueous fluids) within the crust strengthened the interactions between rocks and fluids within the active fault zones and their creep behaviors, altering the stress and physical states and then causing variation in the velocity, gravity, and geomagnetic fields. Thus, our time-lapse model of crustal VS could provide significant support for understanding the seismogenic mechanism of the Ludian earthquake. © Seismological Society of America.

Sandwiched by the Siberia and North China cratons, the eastern Central Asian orogenic belt (CAOB) is one of the ideal natural laboratories for studying Phanerozoic continental crust accretion, reworking and metallogenesis in the world. The Solonker suture has been commonly considered as the final Paleo-Asian Ocean closure and is crucial for understanding the tectonic formation and evolution of the eastern CAOB, but its eastern extension remains unknown in Northeast China due to thick cover of the Songliao Basin and a general lack of key geological markers. In addition, the deep metallogenic background of the Mesozoic polymetallic ore deposits in this area is still unclear and is in urgent need of high-resolution structure of crust and upper mantle. In this paper, we assemble seismic data from both permanent and portable seismic stations around the eastern CAOB along with the complete Bouguer gravity anomalies data, and then simultaneously construct the high-resolution 3-D Vs and density models of crust and uppermost mantle around this area by the full-waveform ambient noise and gravity joint inversion. Our Vs and density models reveal approximately symmetric features of velocity and density structures across the eastern CAOB, and suggest that the northern boundary of the Solonker suture extends eastward along the Xilinhot-Tonglu-Baicheng-Daqing line and the southern one is along the Linxi-Changling-Jilin line. Moreover, our Vs model illustrates that the Mesozoic polymetallic ore deposits in the southern Great Xing'an Range are mostly located around the overlay of the upper-crustal high-Vs values and the transition zones of Moho depression and uplift on the west of the NNE-trending Moho gradient belt. The Moho transition zone likely represents the deep crustal boundaries facilitating the ascent of ore-forming magmas and fluids from mantle into upper crust, and the overlay indicates the prospect areas for the future polymetallic mineral exploration.

The downward continuation of gravity field can provide valuable information for 3-D gravity-field modelling, shallow-layer geological interpretation, source depth estimation and so on. However, downward continuation is ill-posed, and traditional approaches often suffer from computational instability, poor noise resistance and limited continuation depth, making it a longstanding challenge in gravity data processing. We present a new approach for fast, stable and large-depth downward continuation of gravity anomalies by using frequency-domain 3-D imaging. First, we utilize the frequency-domain 3-D imaging approach to invert the gravity anomalies at the original observational plane to quickly obtain the equivalent density model in the subsurface. Then, we apply the optimized strategy of frequency-domain 3-D forward calculation on the equivalent density model to rapidly obtain high-precision gravity anomalies at the downward-continuation plane. The synthetic data tests prove the effectiveness of our approach, and demonstrate that our approach enables fast, stable, robust noise resistance and large-depth downward continuation of large-scale gravity anomalies data, and has superior performance compared to the traditional regularized filtering approach and spatial-domain equivalent-source approach. The real data test of the free-air gravity anomalies data in the central South China Sea also verifies the fast, stable and reliable downward continuations of large depths by our approach. The 3-D gravity-field model built by our approach will provide significant support for the tectonic studies and resource exploration in this area.

The high-magnetic anomaly belt in the northern slope of the South China Sea has long been associated with a Mesozoic volcanic arc. However, its crustal architecture remains unclear, limiting the understanding of its geological implications. We assembled high-resolution reduced-to-the-pole aeromagnetic anomaly data around the northern South China Sea and then applied the 3-D magnetic inversion method based on mixed Lp norm regularization, with the constraint of multiple deep seismic reflection profiling data, to obtain high-resolution 3-D crustal susceptibility model. Our model confirms that the high-magnetic anomaly belt is caused by deep-seated magnetic bodies, which, supported by borehole and seismic data, are identified as a Mesozoic volcanic arc. We found that the Mesozoic volcanic arc described by high susceptibility bodies extends northeastward from the Dongsha Rise to Taiwan Island and primarily distributes within the depth range of 20~28 km. Our findings provide important constraints for understanding the pre-Cenozoic tectonic evolution of the northern South China Sea. © 2025 by the authors.

Density interface inversion is an important approach for quantitative interpretation of gravity data, used to reveal the morphology of structural interfaces such as the basement, Moho boundary, and the lithosphereasthenosphere boundary (LAB). However, common density interface forward modeling and inversion algorithms in Cartesian coordinate systems are unsuitable for large regional scales, while conventional algorithms in spherical coordinate systems are not suitable for ground observation plane and rarely account for the density variations underground. In this article, we develop algorithms for gravity forward modeling and iterative inversion of variable-density interface in the spherical coordinate system. The gravity forward modeling formula of variable-density tesseroid unit in a spherical coordinate system is derived based on the GaussLegendre quadrature (GLQ) integration algorithm, and the improved adaptive subdivision strategy is utilized to make the forward modeling algorithm suitable for ground observation plane with high precision. Then, the iterative inversion algorithm for variable-density interface in a spherical coordinate system is proposed based on our gravity forward modeling algorithm and Bott's iteration algorithm. We validate the effectiveness and reliability of the proposed algorithms by using a spherical shell theoretical model and a variable-density interface model. Finally, we apply the proposed algorithms to the gravity data on ground observation plane to derive the depth distribution of the LAB beneath Chinese mainland. The inversion result indicates that the lithospheric thickness generally exhibits a trend of being thicker in the west and thinner in the east, with 110 degrees E as the boundary. The thickest region exceeds 250 km, located in the central Tibetan Plateau, while the thinnest area is approximately 60 km, found from the eastern Northeast China to the eastern South China.

3-D shear wave velocity and density models are important for understanding the structures, material composition, tectonic deformations and dynamical mechanisms of the Earth's crust. Such models are usually based on surface wave tomography and gravity inversion, which provide high resolution in the vertical and horizontal directions, respectively. The joint inversion of surface wave and gravity methods can promote the mutual constraints and complementary advantages of seismic and gravity information, improving the imaging resolution and reducing the uncertainty in the individual methods. However, the traditional joint inversion methods directly construct only the shear wave velocity model, excluding density models. We present a joint inversion method of surface wave and gravity data that simultaneously constructs both crustal shear wave velocity and density models. Unlike the previous studies, we reconfigure the seismic kernels of surface wave tomography to preserve the seismic kernel of density. Moreover, the gravity kernel of density is combined with the seismic kernels to establish an objective function of simultaneous joint inversion. Consequently, the imaging resolution of density structure is improved. Our method is validated on the northeastern Tibetan plateau. The inversion results show that the Dingxi, Jiuzhaigou and Jishishan earthquakes occurred in the high-low-anomaly transition zones of shear wave velocity or density, implying that they were induced by accumulation of strain energy in the upper crust of the northeastern Tibetan plateau when the surrounding harder blocks extruded during the tectonic deformation process.

The mechanism responsible for the lateral expansion and uplift of the eastern Tibetan Plateau remains a topic of ongoing debate, partly due to discrepancies in the results of seismic velocity and anisotropy. In local earthquake tomography, hypocentral uncertainties can cause significant errors in the tomographic model. However, this issue has received limited attention in previous studies. In this work, we employ the weighted least-squares (WLS) method to solve the tomographic inversion problem. A power exponent coefficient, which is called weighting level, is introduced into the weighting matrix to control the relative contribution of the data with different hypocentral errors to the final tomographic result. Our data set contains high-quality Pg, Pn and Sg arrival times of local earthquakes recorded by the dense Chinese seismic network in eastern Tibet during 2008–2022. We comprehensively analyse the inversion results derived from the WLS inversions with different weighting levels to evaluate the robustness of isotropic velocity anomalies and azimuthal anisotropy. The most robust feature of our results is a striking low-velocity (low-Vp) zone surrounded by high-velocity (high-Vp) anomalies and fault parallel fast-velocity directions (FVDs) of azimuthal anisotropy in the lower crust beneath the western side of the Longmenshan fault zone. Taking into account many previous results of the region, we deem that the low-Vp zone reflects hot and wet upwelling flow from the deep asthenosphere, which ascends to the lower crust along the fault zone. At the NE margin of the Tibetan Plateau, significant low-Vp anomalies exist in the lower crust and the FVDs are consistent with the motion direction of the Tibetan block revealed by GPS (Global Positioning System) observations. We think that lower crustal flow exists beneath NE Tibet, which controls the plateau expansion toward the northeast. A low-Vp anomaly appears at 30 km depth beneath the Sichuan Basin. However, as the weighting level increases, the amplitude of this low-Vp anomaly decreases by more than 6 per cent, suggesting that this low-Vp anomaly has a lower accuracy than the other features. © The Author(s) 2024. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Passive seismic interferometry techniques enable the retrieval of virtual shot gathers at receiver locations. However, it is difficult to distinguish body waves from surface waves in seismic ambient noise data, which leads to failed dispersion measurements, artificial deep reflections, and misleading geologic interpretations. We develop a new method that uses the waveforms and their corresponding f-k k spectra to identify body waves and surface waves. Our technique functions as a data classification procedure before wavefield retrieval and subsequent imaging. A field data example is conducted to test the feasibility of this technique. The field data results demonstrate that our technique can reconstruct satisfactory high signal-to-noise-ratio wavefields for reliable imaging and interpretation purposes. The method can also serve as a validation tool for identifying surface wave-related artificial events in the obtained virtual source reflection image. The marker boundaries of the Songliao Basin in Northeast China are interpreted in the obtained profile and are in agreement with the well-logging data. In addition, two normal faults are detected, which correspond to the extensional rift environment during the early Cretaceous.