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Moho depth of the Qilian orogen revealed by wide-angle reflection and refraction profiles
Wu Guowei, Xiong Xiaosong, Gao Rui, Chen Xuanhua, Li Yingkang, Wang Guan, Ren Haidong
 doi: 10.19975/j.dqyxx.2021-067
[Abstract](11) [FullText HTML](8) PDF 3155KB(4)
The Qilian Orogenic Belt is located on the northeastern edge of the Qinghai-Tibet Plateau, 1500 km from the front of the Himalayan collision zone to the south, manifesting as an orogenic belt with a NW orientation between the Alxa Block to the north and the Qaidam Block to the south (approximately 1000 km long from east to west and 200~400 km wide from north to south). It is cut by the NE-trending Altyn left strike-slip fault to the west and surrounded by the northeastern Tibetan Plateau, Qilian Mountain, and Hexi Corridor to the north. To the southeast, it is connected to the west Qinling orogenic belt from the early Mesozoic, and the eastern margin is adjacent to the Ordos Block. It has played an important role in the collisional process between the Indian and Asian continental plates, and the orogenic process of the Qinghai-Tibet Plateau since the Cenozoic. Exploration of the deep structure of the crust is a key means to study the uplift and northward expansion processes of the Qinghai-Tibet Plateau and understand the intracontinental deformation from the collision between India and Eurasia. This study selects the wide-angle reflection and refraction profiles conducted in this area since the 1980s and summarizes the Moho depth. The results show that the Moho becomes shallower from west to east as a whole, and the deepest Moho in the west section can be explained by two-way subduction and underplating. The deepest Moho of the Qilian orogenic belt is located near the Hala Lake in the North Qilian orogenic belt. The low-velocity anomalies with high conductivity in the middle crust reflect the large detachment zone produced by the decoupling deformation of the upper and lower crust in this location, which accommodates the compressional stress from the south direction. The cumulative stress in the east section with the shallowest Moho is released by the left-lateral strike-slip fault of the Haiyuan fault and thrust faulting developed in the crust.
Recent progress on wave gradiometry method
Cao Feihuang, Liang Chuntao
 doi: 10.19975/j.dqyxx.2022-054
[Abstract](3) [FullText HTML](2) PDF 10707KB(0)
Wave gradiometry method is a data processing technique based on dense seismic arrays and is suitable for a variety of seismic signals/phases such as P-waves, S-waves, Rayleigh waves, Love waves, and ambient noise. Since both temporal and spatial differences within the wave field are fully considered by this method, more seismic wave propagation parameters and medium physical properties can be obtained; these include stress, rotation, seismic velocity, azimuth, geometrical spreading, radiation pattern, azimuthal anisotropy, and Q value. Since its introduction in 2007, wave gradiometry has been widely used to study strong ground motion in river valleys, shallow lunar crust, fault systems and the inversions of the velocity and anisotropy models of shallow Earth, crust, or mantle. Based on different signal processing techniques, wave gradiometry method has been developed into different branches such as the wave gradiometry analyses based on Fourier transform, wavelet transform and Hilbert transform. These methods have branched into further research based on different reference coordinate systems, network types, or seismic phases/signal sources. In this paper, the methodology, recent progress, developmental trend, and method comparisons of wave gradiometry are described in detail.
Codefmap APP: A seismic deformation simulation application based on Android system
Ni Ruisheng, Xu Wenbin
 doi: 10.19975/j.dqyxx.2022-037
[Abstract](731) [FullText HTML](82) PDF 5063KB(78)
If coseismic deformation of the earthquake region can be obtained in a short time, it is important for timely evaluate the disaster and cooperate with the development of earthquake relief work. Space geodesy technology has the advantages of high monitoring accuracy and high spatial resolution, and has been widely used in seismic deformation monitoring related fields. The coseismic deformation obtained by processing space geodetic data can intuitively show the surface deformation caused by the earthquake and provide a reference for judging the disaster situation. However due to the lag of data acquisition, it is often impossible to provide coseismic deformation map in a short time after the earthquake. In this paper, using the near real-time focal mechanism solutions of the USGS NEIC, seismic elastic dislocation model and seismic empirical formula, a seismic deformation simulation application based on Android smartphone (namely Codefmap APP) is developed based on Java and Python language. The program has the functions of earthquake catalog query at any time in the world, actively obtaining USGS NEIC source parameters, displaying epicenter position and automatically calculating coseismic deformation. For important events, the deformation simulation results can be given within one day, especially for blind earthquakes. It can provide a reference basis for determining potentially dangerous areas and early earthquake relief to a certain extent.
High precision coseismic deformation monitoring method based on time-series InSAR analysis
Wu Xiongxiao, Feng Guangcai, He Lijia, Lu Hao
 doi: 10.19975/j.dqyxx.2022-023
[Abstract](71) [FullText HTML](34) PDF 7176KB(22)
Interferometry Synthetic Aperture Radar (InSAR) technology has become an important tool for monitoring surface deformation with its all-day, all-weather ground monitoring and high spatial resolution, and has been widely applied to seismic deformation monitoring. Currently, the most commonly used technique for coseismic deformation monitoring is differential InSAR (D-InSAR). However, the traditional D-InSAR is susceptible to spatial and temporal uncorrelation in areas such as waters and densely vegetated areas, resulting in serious contamination of the coseismic deformation field. In addition, the seismic deformation field sometimes contains obvious atmospheric delay that can affect source parameter inversions. Therefore, improving the quality of coseismic deformation is of great significance for future seismic deformation monitoring and parameter inversion. The multi-temporal InSAR (MT-InSAR) technique, which is widely used in inter-seismic and post-seismic deformation monitoring, is able to suppress the effects of spatiotemporal decorrelation and atmospheric noise. In this paper, we propose a high-precision coseismic deformation monitoring method based on time-series InSAR analysis to obtain high-precision coseismic deformation results. The accuracy of coseismic deformation field is mainly improved by selecting appropriate interferograms and selecting stable points. With the support of sufficient Sentinel-1A/B satellite SAR data, numerous interferograms were generated using a large number of pre- and post-earthquake images. Interferograms that are less affected by errors are selected for study according to certain criteria to reduce the impact caused by atmospheric delay errors. At the same time, setting threshold to select stable point target to improve the accuracy of deformation field. Taking the 2018 Hualian MW6.4 earthquake in Taiwan China as an example, the data processing flow of high-precision coseismic deformation monitoring method is introduced in detail. Compared with the results of the traditional D-InSAR method, the proposed method can reduce the noise error and improve the signal-to-noise ratio of coseismic deformation. The high-precision coseismic deformation monitoring method is applied to obtain the seismic deformation of 14 different magnitudes and locations. The deformation results show that the method can improve the accuracy of deformation field by selecting stable points, and it is generally applicable.
Multiple surface wave tomography methods and their applications to the Tibetan Plateau
Li Lun, Cai Chen, Fu Yuanyuan, Fang Hongjian
 doi: 10.19975/j.dqyxx.2022-019
[Abstract](234) [FullText HTML](86) PDF 5842KB(90)
Surface wave tomography is a widely used geophysical method to measuring seismic velocity and anisotropy in the crust and upper mantle. This paper briefly reviewed the history of surface wave tomography, and summarized the principles and advantages of multiple surface wave tomography methods (i.e., two-station method, two-plane-wave method, Eikonal and Helmholtz surface wave tomography, ambient noise tomography, and direct surface wave tomography). The theory and application of the two-station method are straightforward, but this method requires the earthquake and seismic stations to be located in the same great circle. This restriction results in a low lateral resolution phase velocity map for regions where stations are sparse and deployment times short. The two-plane-wave method can overcome the effect of scattering and multipathing of seismic wave propagation on phase velocity dispersions, but this method requires high-quality surface wave waveforms and is usually suitable for regional seismic networks. The Eikonal and Helmholtz surface wave tomography can directly compute the phase velocities and azimuthal anisotropy straightforwardly without any processes of forward modeling and inversion, but this method is limitedly suitable for high-density and orderly seismic arrays. In comparison to the Eikonal surface wave tomography method, the Helmholtz surface wave tomography approach analyzes both the phase and amplitude of the waveform and can produce better results. Ambient noise tomography can obtain high-resolution crustal structure without seismic events. However, this method is difficult to obtain long-period phase velocities, resulting in a lack of constraints on the mantle and lithosphere structure. Direct surface wave tomography can obtain the shear-wave velocity and azimuthal anisotropy from dispersive curves without the inversion process of phase velocity maps. We conducted detailed comparisons of the phase velocity maps at the short-intermediate periods (20~40 s) previously obtained from these surface wave tomography methods in the central-northern, northeastern and southern Tibetan Plateau. The comparisons demonstrated that the Rayleigh and Love wave phase velocity maps from different tomography methods show highly consistency in velocity variation patterns at same periods. The results show that the plateau's interior by characterized with low-velocities in phase velocity maps at intermediate and long periods, whereas relatively high velocities dominate adjacent regions, such as the Qaidam Basin and Sichuan Basin. These features indicate that the Tibetan Plateau's mid-lower crust and upper mantle tend to be weaker and easily deform during the northward movement of the Indian plateau and the barrier of the strong blocks (e.g., the Qaidam Basin and the Alashan Platform). The short periods phase velocity maps (i.e., 20~40 s) reveal that the mid-lower crustal flow of the Tibetan Plateau escapes southward surrounding the weakening zones (i.e., the Red River Fault and the Xianshui River Fault) due to the barrier of the strong Chuandian tectonic block. The Qilian Orogen also shows low velocities in the phase velocity maps at the short-intermediate periods, which is likely due to thermal anomaly with localized mantle upwelling. It is noteworthy that the Rayleigh-wave and Love-wave phase velocities at different period ranges (e.g., 4~200 s) can be obtained via incorporating earthquake surface wave tomography and ambient noise tomography. Those phase velocities can be used as input to inverting three-dimensional shear wave velocities and radial anisotropy models simultaneously. This paper envisions that integrating higher-mode earthquake surface wave tomography, adjoint tomography, and joint inversion could obtain higher resolution and more reliable crustal and upper mantle structures.
Development status of deep seismic reflection profile detection technology
Wang Guangwen, Lu Zhanwu, Li Wenhui, Wang Haiyan, Cheng Yongzhi, Chen Si, Cai Wei
 doi: 10.19975/j.dqyxx.2021-055
[Abstract](160) [FullText HTML](74) PDF 3484KB(60)
The deep seismic reflection profile detection technology developed from petroleum seismic exploration. By using dynamite source, long spreads and multi-coverage, this detection technique can receive reflection signals from the crust or upper mantle. Fine time profiles within the crustal scale obtained through denoising, static, superposition and migration processes are the basis for studying the characteristics of deep structures. The deep seismic reflection profiling technique is an important means to explore the process of tectonic evolution and plays an irreplaceable role in other geophysical methods. Since it was first proposed by the United States in the last century, this exploration technique after decades of development. Relying on a series of deep exploration plans, deep reflection exploration technique has obtained many important deep reflection profiles and solved many geological problems, including the evolution process of orogenic, basin structural model, deep structural characteristics of ore concentration area and so on. Deep reflection exploration technique is nowadays a widely accepted research tool by geologists and geophysicists. At present, deep seismic reflection detection technology has developed into a systematic, mature and reliable deep structure detection method. It is often used as a pioneer in the study of deep fine structure in critical area. We summarized examples of deep seismic reflection profiles in recent years. A series of new progresses and applications of deep seismic reflection detection technology are summarized in terms of acquisition technology, data processing and comprehensive interpretation, including high-precision vibroseis acquisition technology, line drawing technology, full-waveform inversion technology, integrated interpretation, etc. The application of these new technologies not only effectively improves the imaging quality of deep seismic reflection profiles but also solves the problems of complex terrain and inconvenient acquisition in deep seismic reflection. The deep seismic reflection detection technology play an increasingly significant role in solving geological problems in critical area.
Development and application of uppermost mantle Pn tomography
He Yuhui, Guan Yurui, Kong Hua, Lü Yan
 doi: 10.19975/j.dqyxx.2022-052
[Abstract](23) [FullText HTML](16) PDF 3636KB(8)
The ray path of Pn waves is concentrated in the limited depth range of the uppermost mantle, which has unique advantages in ray transverse coverage density. Therefore, the Pn phase is the dominant phase for studying the velocity and anisotropic structure of the uppermost mantle. The lateral variation of Pn wave velocity reflects the temperature and composition difference of the uppermost mantle, and Pn anisotropy can reflect the movement of the mantle material and deformation characteristics. The high accuracy of Pn wave velocity and anisotropy structure in the uppermost mantle can provide key information about the lithospheric structure, plate movement, and deep thermal material migration process. With development and improvement, Pn tomography has become a mature technology to study the structure of the uppermost mantle and has been applied to obtain structural information such as crustal thickness, upper mantle velocity, and anisotropy on a global scale. This method characterizes the lateral heterogeneity of the global upper mantle structure and provides further understanding of the Earth's internal structure and plate subduction, continental collision deformation, volcanic activities, and other dynamic processes. With increasing global seismic stations and observation data, a large amount of high-quality Pn data provide favorable conditions for the study of the structure of the uppermost mantle. This paper reviews the development of the Pn tomography method and its applications in the world.
Recent advances in distributed acoustic sensing applications for seismic imaging
Zhang Lina, Xie Jun, Chi Benxin, Liu Hongping, Bao Feng
 doi: 10.19975/j.dqyxx.2022-049
[Abstract](45) [FullText HTML](13) PDF 3071KB(22)
Distributed fiber-optic seismic sensing is a next-generation seismic acquisition technology. It records dynamic strain based on the phase change of back-scattered Rayleigh light caused by incident seismic waves. This method generates a dense record of the seismic wavefield using optic fibers which are durable in extreme environments and have low maintenance costs. Consequently, it has been widely used for surveys of urban areas, the ocean bottom, boreholes, and glaciers. In this study, we focus on its applications in seismic imaging and the monitoring of seismic velocity change. Previous studies have employed both passive and active sources to explore structures ranging from near-surface depths to the Moho. Although impressive achievements have been made, the research community still faces challenges such as data quality, signal fidelity, storage, processing, and information mining of large data volumes. Therefore, active and passive high-resolution 4D imaging, instrument transfer function calculation, and effective big data mining should be conducted as part of future studies to achieve high-density and high-precision seismic imaging with this fiber-optic sensing technology.
Review of progress in passive seismic reflection exploration
Ruan Xiaomin, Chen Mingchun, Liu Zhendong, Wang Zhihui, Chen Miao, Zhang Xingang
 doi: 10.19975/j.dqyxx.2022-046
[Abstract](22) [FullText HTML](23) PDF 10729KB(9)
Seismic exploration is a key method for the study of underground structures. Active seismic surveys can acquire high signal-to-noise ratio reflection data; however, the operation of active seismic surveys is complicated, and the exploration cost is high. Passive seismic exploration is another type of seismic exploration, which requires no active human build source but utilizes the natural noise recorded by seismograph stations or geophones. As a low-cost and environmentally friendly method, passive seismic reflection exploration can be used to create higher resolution seismic profiles than the surface wave method and plays an increasingly important role in underground mineral exploration, dynamic monitoring of carbon storage, and urban underground structure detection. However, passive source reflection imaging technology faces several challenges. For example, because ambient noise is mainly controlled by surface wave energy, the reflected body wave signal can be weak and difficult to extract. Actual underground sources are limited in number and are unevenly distributed, which leads to artifacts in the virtual shot gathers reconstructed by seismic interference. Constraints of massive data computation and storage for long-term observation by a large number of geophones. With the rapid development of portable node geophones and high-performance computing, passive seismic reflection exploration has achieved considerable progress in both method research and practical applications in recent years. This paper briefly reviews the history of seismic interferometry and the construction of virtual shot gathers using various seismic interferometry methods and then introduces in detail how the reflection signals from ambient noise records dominated by surface waves are identified and extracted. We discuss the identification and extraction of weak body wave reflection signals based on various characteristics of surface and body waves, such as differences in signal-to-noise ratio, velocity, and azimuth angle. We then focus on the processing of passive source reflection data, including the beginning of raw data preprocessing, virtual shot gather static correction, coherent noise suppression, multiple suppression, velocity analysis, and direct migration imaging. We also introduce examples of passive seismic reflection applications, including CO2 storage site monitoring, metal mining, and coal mine underground structure research. Finally, we give an outlook for research prospects in passive seismic reflection exploration.
Current status and scientific progress of the Zhangheng-1 satellite mission
Zeren Zhima, Liu Dapeng, Sun Xiaoying, Yang Yanyan, Zhao Shufan, Yan Rui, Zhang Zhenxia, Huang He, Yang Dehe, Wang Jie, Chu Wei, Wang Qiao, Xu Song, Hu Yunpeng, Lin Jian, Tan Qiao, Huang Jianping, Lu Hengxin, Guo Feng, Zhou Na, Li Wenjing, Shen Xuhui
 doi: 10.19975/j.dqyxx.2022-043
[Abstract](16) [FullText HTML](8) PDF 4427KB(7)
The Zhangheng-1 electromagnetic satellite is the space-based observation platform of China's stereoscopic earthquake observation system. Its scientific objective is to obtain the global geomagnetic field, electromagnetic field (waves), ionospheric plasma parameters, and high-energy particles for monitoring the dynamic variation of the ionosphere and the seismo-ionospheric disturbances over China and its surrounding areas to compensate for the deficiency of the ground-based observation system. The project explores new ways of earthquake monitoring and prediction by using space science. The first test probe of the Zhangheng-1 electromagnetic satellite series was successfully launched in February 2018, and has been stably operating in orbit for more than four years. The second is an operational probe that will be launched in early 2023. The in-flight commissioning test and cross-calibration work show that the Zhangheng-1 electromagnetic satellite can provide good data quality to support geophysics and space physics studies, and has obtained valuable scientific results in recent years. The global geomagnetic reference model built by Zhenghang-1 data is the first global geomagnetic field model built using only Chinese satellite data, allowing Chinese scientists to take an important role in the computation of the global geomagnetic reference model (IGRF) for the first time in over a century. The ionospheric electron density 3D model based on Zhangheng-1 satellite data can present the ionospheric structure characteristics. In natural hazards monitoring, Zhangheng-1 has shown good response ability to disturbances related to earthquakes, volcano eruptions, and geomagnetic storms. In terms of the lithosphere-atmosphere-ionosphere coupling mechanism study, the full-wave calculation method can provide the electromagnetic field changes between the lithosphere-ionosphere waveguide and the ionospheric wave propagation feature. The results from the full-wave model prove the capability of the Zhangheng-1 satellite's electromagnetic payloads to detect low-frequency electromagnetic emissions induced from the earthquake epicenter. The simulation and observation studies suggest that the Zhangheng-1 satellite can reflect seismic activities, very low frequency (VLF) transmitter, magnetic anomalies in the lithosphere, and lightning activities in the atmosphere. These recent scientific results show that the Zhangheng-1 electromagnetic satellite is consistent with other similar types of electromagnetic satellites worldwide, indicating its great potential in scientific application.
Recently research advances on the polar cap patches
Wang Yong, Zhang Qinghe, Xing Zanyang, Ma Yuzhang, Zhang Duan
 doi: 10.19975/j.dqyxx.2022-050
[Abstract](33) [FullText HTML](18) PDF 6672KB(4)
The polar cap patch is a common ionospheric structure. It often appears in the F region of the ionosphere over the polar caps; these patches are usually characterized by electron densities that can be even twice greater than that of the surrounding area. Untangling the formation and evolution of the polar cap patch can unveil the transportation of energy and momentum through magnetosphere-ionosphere-thermosphere couplings. However, owing to their wide horizontal reach (~100~1000 km), these high electron density structures greatly obstruct radio wave propagation over the polar caps, particularly at their edges, seriously impeding efforts tied to communication, navigation, and positioning. Therefore, the study of polar cap patches is not only of significance within space physics but also of great value within the context of space weather monitoring and prediction. In this paper, recent progress in polar cap patch research is summarily reviewed. The focus of our review includes the possible mechanisms underlying the formation of the dominant dayside reconnection; the recently proposed sunward return flows, likely produced by the nightside reconnection or other processes; and the newly defined patch-polar cap hot patch and its dependencies on various factors (e.g., solar & geomagnetic activities, local plasma transport, and particle precipitation). We will also delve into patch occurrences, as they relate to spatiotemporal dynamics as well as the interplanetary magnetic field conditions. We will also comprehensively review the evolutionary process tied to the Dungey convection cycle, as it moves from the dayside to the nightside and finally as it exits the polar caps, initially modulated by the pulsed nightside reconnection and then sunward transportation by the return flow. Lastly, we will detail the effects of ion upflow and ionospheric scintillation associated with the polar cap patch. For each subject, we will provide a detailed account of progress that has been made and its corresponding prospects.
Advances in seismic imaging of mantle transition zone discontinuities
Yu Chunquan, Li Juan, Yang Fan, Zhang Yan
 doi: 10.19975/j.dqyxx.2022-034
[Abstract](71) [FullText HTML](38) PDF 3558KB(51)
Located between the 410-km and 660-km discontinuities, the mantle transition zone is the key region for understanding the thermal and chemical structure and the dynamic evolution of the Earth’s mantle. The top and bottom boundaries of the mantle transition zone correspond to mineral phase transitions from olivine to wadsleyite and ringwoodite to bridgmanite and ferropericlase, respectively. This paper summarizes the main seismological methods for studying and related research progress of the mantle transition zone discontinuities. These methods include SS and PP precursors, receiver functions, ScS reverberations, P'P' precursors, waveform modeling of seismic triplications, reflected body waves retrieved from ambient noise interferometry, etc. Overall, there is a positive correlation between the thickness of the mantle transition zone and velocity perturbations in the mantle transition zone on the large-scale structure, indicating that they are both mainly controlled by mantle temperature, consistent with the prediction of olivine phase transitions. However, the lack of negative correlation between 410-km and 660-km discontinuity topography, which is expected from olivine phase transitions, suggests that either the thermal structure is not coherent across the mantle transition zone vertically or there are lateral varitions in water content or mantle chemical composition. The strength (including velocity, density and impedance jumps) and width of the 410-km and 660-km discontinuities are mainly controlled by the chemical composition and water content of the mantle transition zone. Some studies also detected 520-km and 560-km discontinuities within the mantle transition zone, which might be caused by the phase transition from wadsleyite to ringwoodite and the exsolution of calcium-perovskite from majorite, respectively. The seismically detected low-velocity zones above and below the mantle transition zone may be related to the dehydration melting caused by hydrated mantle transition zone material entering the low water-solubility upper and lower mantle. Although great progresses have been made, many important scientific questions related with the mantle transition zone remain unsolved. Accurate and reliable seismic imaging of the mantle transition zone provides crucial information for understanding these questions. Multidisciplinary studies integrating seismology, mineral physics, geodynamics and geochemistry are also needed. Finally, this paper discusses some future seismological research directions of the mantle transition zone.
Hot plasma effects on the dispersion properties of plasmaspheric hiss and its electron diffusion
Ma Xin, Gu Xudong, Zhu Qi, Jiao Luhuai, Wang Jingzhi, Ni Binbin
 doi: 10.19975/j.dqyxx.2022-045
[Abstract](30) [FullText HTML](8) PDF 17516KB(3)
Electron diffusion caused by plasmaspheric hiss is an important mechanism for the loss of electrons in Earth's inner magnetosphere; it has also been considered responsible for the formation of the slot region between inner and outer radiation belts. The cold plasma dispersion relation of plasmaspheric hiss is widely used to quantify the scattering effect of energetic electrons. However, the existence of hot plasma in a realistic magnetospheric plasma environment modifies the dispersion relation of plasmaspheric hiss, thereby affecting wave-induced energetic electron scattering. This paper presents the results of some recent studies on the influence of hot plasma on the dispersion relation and electron scattering effects of plasmaspheric hiss. Using statistical analysis results based on satellite wave observations, the modification of the hiss dispersion relation under the effects of hot plasma was verified. Furthermore, based on typical case analyses and numerical calculations using the quasi-linear diffusion theory, we investigated the dependence of the hiss-driven electron scattering rates on the geomagnetic activities and hot plasma parameters (i.e., temperature anisotropy, hot electron temperature, and hot electron abundance). The results revealed that the cold plasma dispersion relation overestimates the scattering rate of energetic electrons below 100 keV. For electrons above 100 keV, the differences between the cold plasma and hot plasma dispersion relations in terms of the induced scattering rates are somewhat smaller; this indicates that using the cold plasma dispersion relation can lead to the underestimation of the hiss-driven rates of electron pitch angle diffusion at smaller pitch angles and the overestimation of the rates at higher pitch angles. In addition, the cold plasma assumption can cause resonant diffusion down to lower electron energies; however, when the observed hiss wave dispersion curves are used, the resonant electron diffusion tends to extend to smaller pitch angles with a broader range. Notably, the differences in the scattering rates between the cold plasma and hot plasma dispersion relations increase with the hot plasma parameters. Therefore, our results are important for future simulations of the hiss wave-induced electron diffusion processes in the actual magnetospheric plasma environment and the dynamic variability of radiation belt electrons.
Recent progress on the retrieval and modeling of thermosphere mass density
Lei Jiuhou, Li Ruoxi, Ren Dexin, Weng Libin, Ruan Haibing
 doi: 10.19975/j.dqyxx.2022-047
[Abstract](94) [FullText HTML](26) PDF 4582KB(78)
The thermosphere is the atmospheric layer extending from about 90 km to nearly 1000 kilometers, which is an important interreaction area between the Sun and the Earth. Under the effects of solar radiation flux changes, geomagnetic activities, and low atmospheric forcings, the thermosphere could undergo significant changes. On the other hand, the thermospheric molecule flow collides with space objects, leading to the drag effect, which impacts significantly on the trajectories of space objects. In this paper, we first survey multiple density retrieval methods. The space object tracking data has the advantage of a large amount of data and has long been used for density retrieval since the 1960s. However, the density from this method suffers from low accuracy and time resolution. With the development of the Global Navigation Satellite System (GNSS), satellite Precise Orbit Determination (POD) data was utilized to derive thermospheric density with higher accuracy and time resolution. The accelerometers of some geodesic satellites offer the highest accuracy of measurements. Subsequently, three widely-used empirical thermospheric models (Mass Spectrometer Incoherent Scatter MSIS, Jacchia, and Drag Temperature Model DTM) were summarized. The methodologies and data sources were further compared. Based on the derived neutral densities and thermospheric models, several new approaches in improving the previous atmospheric models were overviewed. Since the exospheric temperature is the crucial parameter for empirical models, one of the effective ways to improve models is to modify the exospheric temperature using accelerometer-based densities. The polynomial fitting as well as the Principal Component Analysis (PCA) techniques, were utilized to reconstruct the global density. Other methods such as assimilation and particle filter were also applied to improve atmospheric models. Finally, based on the derived neutral densities, the thermospheric responses to solar and astronomical events such as geomagnetic storms, solar flares, and solar eclipses were further reviewed.
Recent progress on full waveform inversion
Zhu Hejun, Liu Qinya, Yang Jidong
 doi: 10.19975/j.dqyxx.2022-031
[Abstract](168) [FullText HTML](96) PDF 9131KB(131)
Full waveform inversion is an acoustic/elastic/anelastic wave equation-based high accuracy seismic imaging method for studying the Earth's interior structure. To date, it has been widely used in exploration seismology, studies on crustal and mantle structures at both regional and global scales. With this approach, we are able to build a unified theory and algorithm platform to constrain multi-parameter seismic models for the Earth's interior, including P and S wave velocities, anisotropy, attenuation, density and reflectivity, etc. By jointly interpreting these seismic parameters, we hope to better constrain variations in temperature and composition, mantle convection and distribution of water and volatiles. Recent developments include selection of optimal misfit functions, multi-parameter inversion, model regularization, resolution and uncertainty quantification, as well as its applications to special types of datasets, such as ambient-noise recordings and teleseismic scattered waves recorded by dense linear arrays. Furthermore, in order to better interpret inverted multi-parameters and investigate related problems in Earth sciences, we need collaboration among different disciplines, such as synthesizing results from seismology, mineral physics and geodynamic modeling. These results enable us to better understand reservoirs, basin structures, fault distribution and mantle convection.
Mid-lower mantle scatterers: Detection methods, research progress and prospect
Li Juan, Chen Sidan, He Xiaobo, Wang Wei, Yang Fan
 doi: 10.19975/j.dqyxx.2022-039
[Abstract](65) [FullText HTML](43) PDF 2105KB(36)
Thanks to the rapid development of seismic wave propagation theory, emerging of data analyzing methods, and the increasing coverage of the seismic, our ability to probe deep into the Earth has been increased from the scale of 100 kilometers to the scale of kilometers at present. The existence and wide-distributed heterogeneities at a scale of thousands of kilometers in the massive lower mantle have been revealed by seismic tomography for a long time, while the knowledge of smaller scales (~10~100 km) has come from high-frequency seismic scattered wave detection techniques that are mainly based on array analysis. A growing body of evidence shows that multiple-scale heterogeneities distribute through the Earth's entire lower mantle; the heterogeneities have been attributed to basaltic oceanic crust and its underlying lithospheric mantle. Therefore, detecting and characterizing of their distribution and formation mechanism will help understand the material composition, mineral phase transformation, and thermochemical structure of the Earth's interior. Furthermore, they shed light on the thermochemical and dynamic processes of the Earth's interior, such as mantle rheology, convection, and mixing efficiency. This review focuses on the small-scale heterogeneities/scatterers distributed in the mid-lower mantle at a depth of ~700~2000 km. Firstly, from the perspective of the definition of the scatterers and statistical description of small-scale heterogeneities, we introduce a variety of seismic waves, acting as the "probes" for the small-scale heterogeneities in the lower mantle, and then summarize the characteristics and limitations of available detection methods. Moreover, some representative studies are briefly reviewed. Based on the data of more than 200 lower mantle scatterers collected from previous publications, we characterize the distribution of scatterer's depths. Finally, we point out some unsolved problems in the detection method of lower mantle scatterers and propose the prospect for future research directions.
Solar energetic electrons events
Wang Wen, Wang Linghua
 doi: 10.19975/j.dqyxx.2022-040
[Abstract](29) [FullText HTML](17) PDF 3755KB(8)
Solar energetic electron events are one of the most common solar particle accelerations observed in the interplanetary medium (IPM). According to the different dominant species, solar particle events can be divided into proton-dominated large solar energetic particle events and electron-dominated 3He/electron-rich solar energetic particle events. The main difference is that in the proton-dominated large solar energetic particle event, the ratio of 3He /4He ~5×10-4 is the same as that of the corona, and the electron-dominated 3He/electron-rich solar energetic particle event, 3He /4He >0.01, much higher than that of the corona. The release time of solar energetic electron events on the sun can be divided into two groups: low-energy (below ~10 keV) electrons and high-energy (above ~15 keV) electrons. Compared with low-energy electrons, the release time of high-energy electrons is delayed by ~20 minutes, which corresponds to the coronal mass ejection height being about 2 solar radii away from the center of the sun. The release of 3He ions is delayed by about an hour compared to electron release, which corresponds to the coronal mass ejection height being about 5.7 solar radii away from the center of the sun. The energy spectrum of solar energetic electron events is generally a double power-law shape, with a low-energy spectral index of 1.9±0.3 and a high-energy spectral index of 3.6±0.7 with a break energy of ~60 keV. The low-energy spectral index and the high-energy spectral index show a significant positive correlation, while the low-energy and high-energy spectral index show no significant correlation with the break energy. Some events show a single power-law spectrum with an index of 3.5 ±1.2. Previous statistical studies have found that ~45% of the observed solar high-energy electron events above 15 keV are related to hard X-ray flares. By comparing energies above 50 keV, the high-energy hard X-ray spectral index in these events is positively correlated with the high-energy spectral index of electron events, while the index relation disagrees with the prediction of classic bremsstrahlung theory; and by estimating the total number of electrons in high-energy electron events, it is found that the total number of electrons in high-energy electron events is only the total number of electrons that produce hard X-rays in the flare of ~0.1%~1%. In this paper, we further investigate 16 electron events with both good electron observations (with an energy coverage of 5~200 keV) and hard X-ray observations (with an energy coverage of 3~80 keV). The energy spectrum of Hard-X-ray-producing electrons can be deduced based on that the electrons generate X-rays through the relativistic thick-target bremsstrahlung mechanism, by comparing the electron spectrum index and the electron spectrum index of hard X-ray generation, it is found that the low-energy electron spectrum index is positively correlated with the electron spectrum index of hard X-ray generation, while the low-energy electron spectrum index of all events is obviously less than the Hard-X-ray-producing electron spectral index; The comparison of the high energy electron spectral index shows that in half of the events, the high energy electron spectral index is consistent with Hard-X-ray-producing electron spectral index, while in the other half of the events, the Hard-X-ray-producing electron spectral index is steeper than high energy electron spectral index observed in in-situ. These 16 events were also accompanied by strong 3He emissions, 13 were 3He-rich electron events with obvious 3He/4He>0.01, and the other 3 3He/4He<0.01. 15 out of 16 events have coronagraph observations and 14 of them are accompanied with coronal mass ejection. By comparing the simulated spectrum of electron events considering the energy loss during interplanetary transportation with in situ observations, it can be known that the source region of electron events should be located in the high corona (~1.3 solar radius). The location of source region of electron events should still be high in the corona (~1.1~1.3 solar radius) considering the variation of density model implemented, and based on these results, this paper proposes a new acceleration scenario for the acceleration of solar energetic electron events.
Empirical model of the Earth's cusp at low-altitudes
Liu Ziqian, Li Hui, Wang C, Han Jinpeng, Wang Jangyan
 doi: 10.19975/j.dqyxx.2022-044
[Abstract](48) [FullText HTML](27) PDF 1403KB(10)
The Earth's cusp is a critical "window" for the solar wind plasma to enterthe low-altitude magnetosphere and ionosphere. However, the overall configuration of cusp has not been established. Based on the simulation of two successive substorms on 8 March 2008, we propose a 2D model of low-altitude (1.1 RE) cusp modulated by the interplanetary magnetic field (IMF) BY and BZ. This model is constructed from curve fitting of low-altitude cusp with an elliptic function controlled by the cusp center and width, which is dependent on IMF BY and BZ. The plasma thermal pressure PT derived from the simulation data is used to extract the cusp center on the sphere surface with a radius of 6 RE. The cusp center is defined as where PT reaches the maximum, and the cusp boundary is identified as where PT decreases to 68% of that at the cusp center. After the high-altitude cusp has been determined, the low-altitude (0.1 RE altitude in the ionosphere) cusp can be obtained from mapping the high-altitude cusp along the magnetic field lines. The coordinate system of the low-altitude cusp used throughout this paper is the magnetic coordinate. The low-altitude cusp extracted from simulations is fitted with the elliptic function controlled by cusp location and width. The cusp geomagnetic latitude (MLAT) increases gradually with northward IMF BZ but decreases significantly with southward IMF BZ. The local magnetic time (MLT) is nearly 12 when BY = 0, corresponding to the observation results. When IMF BY is duskward (dawnward), the cusp center will locate at the post-noon (pre-noon) sector in the northern hemisphere. The MLAT width decreases as IMF BZ swings from north to south, and the MLT width is the opposite. This model is validated by comparing with observations from DMSP satellites during this concerning time interval. Based on this 2D model of low-altitude cusp, the 3D cusp could be obtained further, which would help to space weather prediction.
2023, 54(1): 1-2.  
[Abstract](4) [FullText HTML](4) PDF 236KB(1)
Advances and perspectives of flexibile inversion for earthquake rupture processes
Yue Han
2023, 54(1): 1-11.   doi: 10.19975/j.dqyxx.2021-054
[Abstract](157) [FullText HTML](95) PDF 2860KB(72)
To determine the rupture processes of big earthquakes is a fundamental problem of earthquake studies, which also supports earthquake physics research. Although the joint use of seismic, geodetic and seismicity data provides good constraints to determine the fault plane information of inland earthquakes, it is still challenging to unveil non-planar fault planes for oceanic earthquake ruptures. For complicated earthquakes, that ruptured more than one fault plane, a method to stably obtain focal mechanism changes is required to determine the fault plane information precisely. In this study, we analyze the stability and flexibility of multiple seismic methods, i.e. back-projections, multi-point-source inversion and finite fault inversion, to determine the rupture kinematics of big earthquakes, and find the stability spatial-temporal resolution and flexibility to resolve focal mechanisms that are not compatible in each single techniques. A sequential strategy is thus proposed to use back-projection results to prescribe the a priori information of multi-point-source inversion to achieve mechanism changing point source solutions with high spatial-temporal accuracy. Two strategies to connect these two techniques and other technique concerns are also discussed.
Advances in passive seismic analysis of sediment structure and applications in some typical basins
Zhang Ruiqing, Kuang Chunli, Zhang Xiaohan, Li Yonghua
2023, 54(1): 12-26.   doi: 10.19975/j.dqyxx.2021-063
[Abstract](134) [FullText HTML](54) PDF 4475KB(50)
Accurate constraints on sediment structure are of great importance for investigation and exploration of oil and gas resources, evaluation of site response, and imaging of deep crust and mantle structure. With the advance of seismic observation and the accumulation of large data from portable arrays, seismological methods using passive source to resolve sediment structures with high resolution have developed. In this paper, we review the advances in the seismological analyses and their basic principles, including receiver functions, transfer functions, H-β technique based on wavefield downward continuation, and P-wave particle motion using teleseismic data. Waveform fitting with high frequency is usually used for sediment structure constraints from local earthquakes. In addition, we also briefly review the analyses of spectral ratio, ambient noise tomography, Rayleigh wave Z/H amplitude ratio, and their joint inversion. Finally, we summarize the progress of research work on the structure of the shallow crust beneath the Songliao and North China basins based on these methods.
Research progress on seismic structures of crust and mantle beneath Tien Shan and their geodynamic implications
Zhang Bingfeng, Bao Xuewei
2023, 54(1): 27-43.   doi: 10.19975/j.dqyxx.2022-048
[Abstract](62) [FullText HTML](58) PDF 5887KB(48)
The mechanisms causing the uplifting of the Tien Shan, one of the largest and most active intracontinental orogenic belts on Earth, have been vigorously debated for decades. Seismic investigation is a fundamental tool used for deep structural exploration and is key to understanding continental geodynamics. As such, in this study, we reviewed the recent research progress on the crustal and upper-mantle structures of the Tien Shan and the remaining controversies. The results showed that the Tien Shan and adjacent basins exhibit contrasting structural and physical properties from the crust down to the upper mantle in various aspects, such as crustal thickness, Moho morphology, mantle transition-zone thickness, seismic velocity, and seismic attenuation. The mountainous areas have complex crustal seismic anisotropy patterns, whereas orogen-parallel anisotropic fabrics dominate at upper mantle depths. Low-velocity anomalies pervasively exist in the mid-lower crust and uppermost mantle of Tien Shan. Taken together, these observations provide evidence of the important roles played by intracontinental subduction and mantle upwelling in Cenozoic orogenesis of the Tien Shan. However, further development of our understanding of the geodynamics in Tien Shan has been hindered by the low imaging resolution of seismic anisotropy, lithosphere-asthenosphere boundary, and mantle transition zone in eastern Tien Shan. And some important geophysical parameters and their implications are still far from being well-understood. Future deployment of dense temporary seismic arrays in the eastern Tien Shan and joint inversion of multiple and complementary geophysical data will considerably increase the resolution of seismic models and ultimately enhance our knowledge of geodynamic evolution in compressional intracontinental orogens.
Source parameters, seismogenic structures of the 1950 Medog-Zayu MS8.6 earthquake and seismicity in the surrounding areas
Zhan Huili, Bai Ling, Chen Zhiwen
2023, 54(1): 44-55.   doi: 10.19975/j.dqyxx.2022-020
[Abstract](108) [FullText HTML](46) PDF 5926KB(35)
The collision between the Indian and Eurasian plates formed the 2500 km long Himalayan orogenic belt . At 22:09 on August 15, 1950, Beijing time, an MS8.6 earthquake occurred in Medog-Zayu area in the eastern Himalayan syntaxis, which is the largest continental earthquake in the world ever recorded since the historical earthquake records have become available. It has been felt by people across the entire Tibetan Plateau and the adjacent plains of India and caused extensive economic and property damages. Based on previous studies about the seismic hazards, earthquake locations and focal mechanisms, we systematically review the source parameters, deep structures of the earth beneath the source area and the features of the seismogenic faults.In addition, we collected earthquake catalogues provided by different agencies and summarized a relatively complete earthquake catalogue with magnitudes greater than or equal to 5.0 within 20 years before and 10 years after the great earthquake. We then analysed the seismicity in four different special and temporal stages. Using the modern earthquake catalogues and waveform data recorded by seismic stations we deployed in the nearby areas, we discussed the mechanisms of small and medium earthquakes occurring since 1964, the 2017 MS6.9 Mainling earthquake, and the 2019 MS6.3 Medog earthquake.The tectonic background of the eastern Himalayan syntaxis is complex. There are three secondary syntaxis which migrate from north to south in different directions, i.e., the Namche Barwa syntaxis, the Sang syntaxis and the Assam syntaxis. In the youngest Assam syntaxis, the Indian plate subducts not only toward NE along the Mishmi Thrust but also toward NW along the Main Frontal Thrust, resulted strong compressive crustal deformation in the Zayu island arc and in Sang syntaxis. Under the continuous high strength of tectonically compressional regime, the Main Frontal Thrust and the Mishmi Thrust ruptured simultaneously, led to the formation of the MS8.6 Medog-Zayu great earthquake. Large aftershocks and clear surface deformation are widely observed in the areas of the Mishmi hills and the Abor hills. The uncertainties of the mechanism of early great earthquakes need to be further analyzed. The integration of seismological, geological, remote sensing and field investigations may provide observations in more comprehensive ways.
Atmospheric circulation of exoplanets
Lian Yuchen, Hu Yongyun
2023, 54(1): 56-80.   doi: 10.19975/j.dqyxx.2022-027
[Abstract](107) [FullText HTML](52) PDF 8574KB(28)
To date, more than 5000 exoplanets and more than 2000 brown dwarfs have been confirmed, which shows rich diversities in many aspects. With the rapid growth of the planet family, both observational and theoretical research on exoplanet atmosphere is developing rapidly. This paper aims to review the progress of exoplanet atmospheric circulation research, and these atmospheric circulation characteristics are the basis for exoplanet and brown dwarf exploration in the future. First, this paper will review the main progress on exoplanet and brown dwarf detection in recent decades, and introduce the basic characteristics of the atmospheric circulation of hot Jupiters, terrestrial planets with atmosphere, and isolated and rapidly rotating brown dwarfs. Then, the simulation results of the atmospheric evolution and dynamic characteristics of exoplanets based on the atmospheric observations will be introduced, including the vertical temperature distribution of the atmosphere, the stability of the stratification, the radiation transfer, and the atmospheric composition, etc. Hot Jupiters are strongly irradiated and tidally locked. The general circulation models of hot Jupiters predict large day-night temperature differences and equatorial eastward jets. Warm Jupiters are generally not tidally locked, exhibiting a wide range of inclinations, orbital eccentricities and rotation rates, resulting in very different circulation patterns compared with hot Jupiters. Atmospheric circulation patterns of tidally locked terrestrial planets are similar to those of hot Jupiters, however, the existence of oceans and various chemical processes will largely change the climate of terrestrial planets. Besides, in the part of terrestrial planets, we will discuss the ultimate goal of exoplanet research—searching and characterizing habitable planets. As a transition state between planets and stars, brown dwarfs are rapidly rotating without external irradiation. Their interiors are fully convective, and their atmosphere exhibits clouds and temperature inhomogeneities. Finally, an outlook will be given to the key issues and challenges of exoplanet and brown dwarf atmosphere.
EikoNet traveltime calculation method and application based on deep neural network
Yao Shi, Hou Jue, Huang Yuepeng, Xu Tao, Bai Zhiming, Gao Zhenghui
2023, 54(1): 81-90.   doi: 10.19975/j.dqyxx.2021-049
[Abstract](1778) [FullText HTML](156) PDF 2308KB(85)
Seismic wave traveltime calculation plays an important role in many areas of seismology, such as seismic tomography, migration and microseismic location. Solving the eikonal equation with the finite difference method is an essential method for calculating traveltime. The conventional method of solving the eikonal equation needs to calculate the traveltime field of each source. As the number of grids increases, it will consume a lot of time and memory. We introduce the EikoNet based on a deep neural network. Its samples are generated by sampling in the three-dimensional space, using the given velocity model as labels to optimize the network. Furthermore, it can transmit information about seismic wavefield and velocity structure during calculation and is highly suited for GPU. The EikoNet can quickly determine the traveltime between any two points in a three-dimensional domain without meshes, significantly improving calculation efficiency and reducing memory consumption. Numerical experiments of the EikoNet and the fast marching method (FMM) on several velocity models show that the EikoNet has higher efficiency while maintaining high accuracy.
MAVEN-based investigation of Martian exobase temperatures: Diurnal and solar cycle variations
Gu Hao, Cao Yutian, Li Zichuan, Fu Menghao, Huang Xu, Sun Mingyang
2023, 54(1): 91-99.   doi: 10.19975/j.dqyxx.2022-051
[Abstract](60) [FullText HTML](41) PDF 9194KB(22)

The exobase is a key concept for understanding atmospheric escape and evolution on Mars. With the aid of neutral density data measured via the Neutral Gas and Ion Mass Spectrometer onboard the Mars Atmosphere and Volatile Evolution spacecraft, we calculated the exobase temperatures of four relatively abundant species on Mars: CO2, O, N2, and CO. Our calculations revealed that the exobase temperatures are highly variable, with median temperatures of 174, 152, 195, and 193 K for CO2, O, N2, and CO, respectively. Moreover, the calculated exobase temperatures for the four species increased systematically with an increase in the exobase altitude or a decrease in the exobase density. Further investigations indicated strong diurnal and solar cycle variations in the exobase temperature: (1) the exobase temperature on the day side was higher than that on the night side, revealing a maximum temperature near the local time of 14 h and a minimum temperature near the local time of 2 h; and (2) the exobase temperatures of the four species increased with the solar extreme ultraviolet (EUV) flux, a feature that was restricted to the day side and absent on the night side, where non-solar energy inputs are likely important. These variations originate from the varying degrees of upper atmospheric heating on Mars in response to the different solar EUV energy inputs.

Thermal transport property of the Earth's core and its convection
Zhang Youjun
2023, 54(1): 100-101.   doi: 10.19975/j.dqyxx.2022-035
[Abstract](50) [FullText HTML](52) PDF 408KB(15)
Seismic evidence reveals how continents grow episodically
Yang Xusong, Tian Xiaobo
2023, 54(1): 102-104.   doi: 10.19975/j.dqyxx.2022-060
[Abstract](27) [FullText HTML](14) PDF 758KB(18)
The new development of EEW: The application of new sensor networks
Sun Li
2023, 54(1): 105-107.   doi: 10.19975/j.dqyxx.2022-030
[Abstract](61) [FullText HTML](33) PDF 16933KB(24)
2023, 54(1): 108-108.  
[Abstract](5) [FullText HTML](5) PDF 101KB(0)

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