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Fault interaction and earthquake triggering mechanisms: progress and prospects
Jia Ke, Zhou Shiyong
 doi: 10.19975/j.dqyxx.2022-071
[Abstract](11) [FullText HTML](9) PDF 2169KB(3)
Research on fault interaction and earthquake triggering, which is a hot issue in the field of source physics, can facilitate understanding of the underlying mechanisms of strong earthquakes and also has good application prospects in earthquake risk analysis and prediction research. Previous review articles provided detailed explanations from the perspectives of basic principles, methods, and applicability, as well as multiple earthquake case studies of stress triggering. However, the introduction to earthquake triggering from the perspective of seismicity analysis is not exhaustive, and the combination and complementarity of these two perspectives are not provided in detail. This paper summarizes the achievements and progress of research on fault interaction and earthquake triggering mechanism through the past few decades from the perspectives of physical and statistical models. The current challenges and possible future directions are reviewed and evaluated. From the perspective of the physical model, three important mechanisms of sources of fault interaction are analyzed: static stress triggering, dynamic stress triggering, and viscoelastic stress triggering, as well as the basic principles and methods of calculation. In the aspect of the statistical model, the basic principles and methods of seismicity analysis are introduced, and applications of the epidemic-type aftershock sequence (ETAS) model and b-value in fault interaction and earthquake triggering mechanism are analyzed. From the perspective of the combination of these two models, the unified connotation of mutual verification and the basic principle of the rate-and-state friction law are introduced. The analysis points out that the stress interaction between multiple faults or earthquakes can be comprehensively studied through the two different schools of Coulomb stress calculation and the ETAS model and that cross-validation can increase the reliability of the results. Retroactive application of rate-and-state friction law can provide a new perspective for understanding the earthquake triggering relationship and fault interaction.
State studies of Earth's plasmasphere
Lü Jingtian, Zhang Xiaoxin, He Fei, Huang Cong
 doi: 10.19975/j.dqyxx.2022-064
[Abstract](20) [FullText HTML](5) PDF 3410KB(6)
As an important part of the inner magnetosphere, the Earth's plasmasphere plays a vital role in linking the occurrence and development of space weather processes. The Earth's plasmasphere is a torus-shaped cold (< 10 eV) and dense (10~104 cm−3) plasma region of ionospheric origin co-rotating with the planet. The plasmasphere contains several populations of particles such as electrons, H+, He+, and O+ ions. The outer boundary of the plasmasphere is defined by a sharp gradient of density called the plasmapause, in which the density decreases by 1~ 2 orders of magnitude within 0.5 RE (the Earth radius). Additionally, large scale plasmaspheric features have been observed, including shoulders, plumes, notches, bulges, and refiling. The Earth's plasmasphere is dynamic, and the abundance of particles in it changes substantially with interplanetary and geomagnetic activity. The large-scale structure evolution of the plasmasphere during geomagnetic storms controls the generation and propagation of waves in the plasmasphere, thus affecting the wave-particles interaction, resulting in a change in the spatial distribution of electrons and ions in the plasmasphere, and then affecting other magnetospheric and ionospheric processes. During periods of geomagnetic storms, the plasmaspheric material eroded is transported sunward and observed near the dayside magnetopause regularly. This local plasmaspheric density enhancement greatly impacts global large-scale convection. Therefore, the plasmaspheric density is undoubtedly an important parameter in space weather. The structural dynamic change of the Earth's plasmasphere is an indicator of the disturbance state of the space weather environment. Its structural form and dynamic process are controlled by the geomagnetic activity, and the short-term changes in the geomagnetic field originate from the sun-terrestrial disturbance caused by solar activity. The high-speed plasma ejected from the solar corona impacts the Earth and induces large-scale convective motion in the magnetosphere. Part of the energy enters the magnetosphere, causing disturbances in its inner regions, including the plasmasphere. Further research on the Earth’s plasmasphere is of great significance to reveal the mass transport and energy transfer in the solar wind-magnetosphere-ionosphere coupling process, and space weather forecast. Here, we introduce the research progress in the response of the plasmasphere to geomagnetic activities, the waves in the plasmasphere, variation of the electron content in the topside ionosphere and plasmasphere, and plasmasphere models. Finally, we also list some important issues for future studies.
Interactions between solar wind and comets
Hou Chuanpeng, He Jiansen, Peng Jingyu
 doi: 10.19975/j.dqyxx.2022-056
[Abstract](20) [FullText HTML](9) PDF 3778KB(3)
Comets are a class of small, irregularly shaped objects in a solar system consisting mainly of volatile materials. During the approach of a comet to the sun, the neutral component of the volatile matter released by heating in the nucleus becomes charged after a process of photoionization, charge exchange, and electron impact, forming a ray-like plasma comet tail. This charged component results in pick-up ions, which are mostly water group ions (H2O+, O+) that are heavier than H+. Therefore, the conservation of momentum causes the solar wind to slow down during the pick-up or mass loading. As a result, the interplanetary magnetic field carried by the solar wind plasma accumulates around the comet nucleus. Simultaneously, owing to the angle between the direction of the solar wind velocity and direction of the interplanetary magnetic field, the velocity distribution of the pick-up ions has a ring-like distribution, providing free energy for wave excitation; thus, contributing to the development of turbulence. The neutral cometary component and solar wind plasma also radiate X-rays and extreme ultraviolet radiation during charge exchange, providing the possibility to determine the composition and state of the solar wind using spectral intensity analysis. The study of cometary activity and its interaction with solar wind is essential for understanding the composition and structural dynamics of cometary materials, evolution of solar wind in the heliosphere, and origin of organic matter and life in the solar system. In this study, we reviewed the research progress on solar wind-comet interactions from three perspectives: in situ detection, remote sensing observation, and theoretical simulation, and provided some prospects for future research trends.
Variation in the rotation rate of Earth and its role as a climate change indicator between 1972 and 2022
Xu Xueqing, Zhou Yonghong, Xu Cancan
 doi: 10.19975/j.dqyxx.2022-062
[Abstract](14) [FullText HTML](10) PDF 1532KB(4)
The universal time (UT1-UTC) and length of day change (ΔLOD) are two parameters that describe the Earth's rotation rate variations. UT1-UTC is typically used in space projects, whereas ΔLOD is used for scientific investigations. Therefore, studying the variations between the two parameter sequences is very important. In this study, we reviewed the Earth's rotation rate over the last five decades and revealed an abnormal phenomenon in UT1-UTC. Since May 2020, the Earth's rotation has reversed from a deceleration trend to an acceleration trend. Besides, we used ΔLOD to find the possible geophysical contributors and focused on the climate-related contributions. Results revealed that the interannual LOD exhibited a notable degree of consistency with ENSO indices after using the Difference+FDSR method to remove the internally and externally induced signals. Notably, two La Nina with intermediate strength and the beginning of a closely followed third trough were detected in the last three years. The contributions of these events to the latest rotation acceleration are approximately 9%.
Stress drops calculated from seismic Lg-waves and their applications for investigating the typical earthquake sequences in the eastern margin of the Tibetan Plateau
Shen Lin, Zhao Lianfeng, Xie Xiaobi, He Xi, Wang Weimin, Yao Zhenxing
 doi: 10.19975/j.dqyxx.2022-038
[Abstract](59) [FullText HTML](23) PDF 4401KB(13)
Stress drop measures the stress release level over a fault during an earthquake rupture. As one of the important parameters for characterizing source mechanisms and predicting strong ground motions, the stress drop is controlled by the tectonic environment, focal mechanism, and type of earthquake sequence. The stress drop measurements from the seismic data also depend on the observation frequency band. Therefore, the values obtained from various seismic phases, such as body and surface waves, may be different. Previous studies have often used indirect methods to remove the attenuation effects along the propagation path to obtain the source spectra and then estimate the stress drops. Using a broadband high-resolution Lg-wave attenuation model, the attenuation effect can be directly corrected to obtain the Lg-wave source excitation spectra. By fitting the observed spectra to the theoretical source model, we can calculate the seismic moments and corner frequencies from which the stress drops can be calculated. Taking a typical tectonic earthquake and a potentially induced earthquake, that is, the 2017 MS7.0 Jiuzhaigou earthquake sequence and the 2019 MS 6.0 Changning earthquake sequence, in the eastern margin of the Tibetan Plateau as examples, we explored the potential physical differences between tectonic and induced earthquakes. The stress drop in the 2017 Jiuzhaigou mainshock was approximately 27 MPa. The stress drops, and the magnitude of its aftershocks rapidly decay. However, for the 2019 Changning earthquake sequence, the temporal variation of stress drops declined slowly, with two aftershocks having stress drops comparable to the main shock. For an area with long-term water injection, it takes a long time to recover to its equilibrium status once a large earthquake disturbs the regional stress field. Given that no apparent difference in the absolute level of stress drops can be observed between the two types of earthquake sequences, the stress drops alone cannot be used to distinguish between induced and tectonic earthquakes in this area. The increasingly accumulated underground water may have created pathways linking multiple fault systems in the changing salt mining. Thus, the possibility of future induced earthquakes cannot be ruled out.
Moreton waves and EUV waves in the solar atmosphere
Chen Pengfei
 doi: 10.19975/j.dqyxx.2022-066
[Abstract](86) [FullText HTML](26) PDF 3171KB(18)
Solar eruptions, which generate various types of wave phenomena in the solar atmosphere, are sources of space weather perturbations. These waves not only transport a large amount of energy, but also illuminate the properties of the magnetic field and plasma on the path of propagation. Therefore, it is of great interest to investigate wave phenomena in the solar atmosphere, among which Moreton waves in the solar chromosphere and extreme ultraviolet (EUV) waves in the corona have attracted much attention in the past few decades. Moreton waves are characterized by bright fronts in the Hα line center and blue wing (or dark fronts in the Hα red wing), which propagate at speeds ranging from ~500 km/s to more than 2000 km/s. They were also observed in He I 10830 Å. EUV waves are characterized by bright fronts in the EUV images, which propagate at speeds ranging from ~10 km/s to more than 2000 km/s. Whereas the understanding of Moreton waves is rather mature, the nature of EUV waves and their relationship with Moreton waves are controversial and have been debated for more than two decades. Initially, it was proposed that EUV waves are the coronal counterparts of chromospheric Moreton waves; that is, they are fast-mode MHD waves. However, many EUV waves have been found to have speeds that are less than the sound speed, which means that some EUV waves cannot be accounted for by the fast-mode MHD wave model. Therefore, several alternate models have been proposed, such as slow-mode soliton waves, echoes of fast-mode waves, successive magnetic reconnection models, as well as hybrid models, which predict the existence of two components of EUV waves—a fast-mode and a slower pseudo-wave. The pseudo-wave is explained by the magnetic field line stretching model, which predicts that the fast-mode wave should be ~3 times faster than the pseudo-wave. With later high-cadence observations, EUV waves with two components have been revealed in many events, confirming the validity of the hybrid model. However, recent observations have revealed many new features that deserve further elaboration of the existing models, such as secondary EUV waves, small-scale EUV waves, patchy EUV waves, quasi-periodic EUV waves, multiple EUV waves, homologous EUV waves, and stationary EUV waves. In particular, the very recent observations of stationary EUV waves may indicate that the fast component of the EUV wave might experience a mode conversion from fast to slow mode when the wave crosses a region of weak magnetic field. In this article, we review the progress made in the research of Moreton and EUV waves and discuss in detail the discovery of the Moreton wave, the early classical model of Moreton waves (as well as recent modifications), the discovery of EUV waves, various observational features, and various models of EUV waves. Lastly, we offer our perspectives on the current research in Moreton and EUV waves and highlight the importance of this research.
Research progress of aftershock forecasting in the early stage after the mainshock
Bi Jinmeng, Jiang Changsheng, Cao Fuyang
 doi: 10.19975/j.dqyxx.2022-058
[Abstract](97) [FullText HTML](61) PDF 2947KB(22)
Rapid, accurate, and nearly real-time aftershock forecasting has attracted increasing public and social attention in dealing with disaster risk and taking effective disposal measures after the mainshock. Many aftershock forecasting methods are seriously affected by catalogue incompleteness in the early stage after the mainshock, which makes it difficult to carry out aftershock forecasting with a disaster reduction effect in time. In recent years, with the development of technology and models, the forecasting of early aftershocks has become possible. In this study, aiming at the "bottleneck period" of aftershock forecasting in the early stage after the mainshock, we elaborated the matched filtering technology and deep learning technology from the perspective of improving aftershock detection rate, the bi-scale empirical transformation technology from the perspective of statistical seismology, and the research progress of the Omi and Lippiello models from the perspective of maximizing the use of aftershock information for real-time forecasting. We analyzed the advantages and disadvantages of various methods and proposed a technical route to comprehensively solve the "bottleneck period" of aftershock forecasting in the early stage after the mainshock. This study provides a scientific reference for researchers to engage in microearthquake detection, aftershock forecasting, and post-earthquake trend research.
"Cat's eye" effect for satellite laser ranging based on the optical system of remote sensing satellite SiCH-2
Long Mingliang, Zhang Haifeng, Lin HaiSheng, Wu Zhibo, Deng huarong, Qin Si, Zhang Zhongping
 doi: 10.19975/j.dqyxx.2022-061
[Abstract](101) [FullText HTML](34) PDF 960KB(4)
Remote sensing satellite SiCH-2 from Ukraine was launched in 2011. At present, it has stopped working and is out of control, and it has been forecasted to become space debris by a two-line root (TLE) from the North American Air Defense Command (NORAD). Here, the "cat's eye" effect of the optical system is analyzed, and the satellite laser ranging (SLR) system at Shanghai Observatory is used to measure the echo intensity of satellite SiCH-2, which is very strong, and the ranging accuracy, which is better than 10 cm. The analysis of the measurement capability results shows that the laser reflection echo from satellite SiCH-2 reaches the laser ranging level of the satellite with the reflector, and is consistent with the detection ability of the photoelectric detection equipment "cat's eye" effect analysis. The measurement results also show that satellite SiCH-2 is in a state of rotation, with a period of 4.3 s. In this study, the laser ranging of on-orbit targets based on the "cat's eye" effect is realized for the first time, which provides a new method for analyzing long-distance space targets, promotes the application and development of high-precision laser ranging technology, and is beneficial to monitor the photoelectric detection series of satellites that fail or have abnormal orbits.
Empirical model of the Earth's cusp at low-altitudes
Liu Ziqian, Li Hui, Wang Chi, Han Jinpeng, Wang Jiangyan
 doi: 10.19975/j.dqyxx.2022-044
[Abstract](129) [FullText HTML](73) PDF 1433KB(13)
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.
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](180) [FullText HTML](89) PDF 4429KB(58)
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](134) [FullText HTML](88) PDF 6673KB(22)
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.
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](107) [FullText HTML](78) PDF 17516KB(6)
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](304) [FullText HTML](92) PDF 4582KB(117)
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.
Solar energetic electrons events
Wang Wen, Wang Linghua
 doi: 10.19975/j.dqyxx.2022-040
[Abstract](137) [FullText HTML](77) PDF 3755KB(22)
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.
Codefmap APP: A seismic deformation simulation application based on Android system
Ni Ruisheng, Xu Wenbin
 doi: 10.19975/j.dqyxx.2022-037
[Abstract](1176) [FullText HTML](168) PDF 5063KB(146)
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](178) [FullText HTML](80) PDF 7176KB(42)
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.
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Direct surface wave tomography for three dimensional structure based on surface wave traveltimes: Methodology review and applications
Yao Huajian, Luo Song, Li Cheng, Hu Shaoqian, Fang Hongjian
2023, 54(3): 231-251.   doi: 10.19975/j.dqyxx.2022-063
[Abstract](130) [FullText HTML](139) PDF 2481KB(98)
Surface wave tomography using dispersion data to obtain isotropic and anisotropic shear wave velocity structures is a very efficient approach to study regional and global tectonics and deformation and to probe high-resolution crustal, upper mantle and near surface structures. Conventional surface wave tomography based on dispersion data usually has two steps, that is, inverting for 2D phase/group velocity maps first and then conducting point-wise inversion to obtain a 1D shear wave velocity model at each geographical grid point, which are then combined to obtain a 3D shear wave velocity model. In this paper, we review in detail the direct surface wave tomography framework based on surface wave dispersion traveltimes, that is, the one-step surface wave tomography. This framework includes direct inversion of 3D isotropic shear wave velocity model using all dispersion measurements at different periods and from all paths (DSurfTomo), direct inversion of both isotropic and azimuthally anisotropic shear wave velocity model (DAzimSurfTomo), and direct inversion of radially anisotropic shear wave velocity model (DRadiSurfTomo).The new direct tomography method computes surface wave ray paths at different periods, thus better considering the ray path bending effect of surface waves in complex media on the precision of tomographic images. We then introduce some applications of the new direct tomography methods, including multi-scale isotropic and anisotropic shear wave velocity tomography in the crust and upper mantle as well as shallow crust. These tomographic studies provide important constraints on regional tectonic evolution, seismogenic structures, shallow fault zone structures, ore deposit structures, and urban subsurface structures. At last, we discuss the dispersion data and model parameterization problems in surface wave tomography, direct surface wave tomography based on finite frequency theory and full waveform inversion, and perspective research of joint tomography problems using surface wave dispersion data and other seismological or geophysical data in the framework of direct surface wave tomography.
Regional-scale joint seismic body- and surface-wave travel time tomography
Fang Hongjian, Liu Ying, Yao Huajian, Zhang Haijiang
2023, 54(3): 252-269.   doi: 10.19975/j.dqyxx.2022-055
[Abstract](225) [FullText HTML](134) PDF 1745KB(124)
To make full use of seismograms to put tight constraints on the structure of subsurface and earthquake sources has always been the research focus in seismology. With increasing computational power, full waveform based seismic tomography has been applied in some regions with promising results. However, the heavy demand for computational resources and strong nonlinearity still prohibit its wide applications. Additionally, most applications of full waveform tomography at regional or global scales can only fit relatively long-period waveforms; the highest frequency of waveform fitting in full waveform tomography is approximately 0.5 Hz on regional scales and even lower on global scales. An alternative way to take advantage of more information on seismograms is the joint inversion of body and surface waves. Instead of fitting low-frequency waveforms, as in full waveform tomography, the joint inversion method uses high-frequency body-wave arrival times and surface-wave dispersion measurements. The forward problem in joint inversion only involves ray tracing or solving the Eikonal equation numerically. Therefore, it is less demanding in terms of computational resources. Compared to separate inversion using either body or surface wave data, joint inversion can provide a unified VP and VS model, and thus more reasonable VP/VS ratio model, by taking advantage of the complementary strength of both data sets. These models could impose tighter constraints on lithology, porosity, and partial melting. Moreover, machine learning-based techniques to detect earthquakes and pick arrivals have obtained many high-frequency arrival times on regional scales with dense deployments, which could be used in joint inversion to improve regional wavespeed models in the crust and upper mantle. The improved models may benefit other seismological studies and provide better understanding of regional tectonics. In this paper, we review some widely used seismic tomography methods for constructing regional models, introduce the basics of joint inversion and its application in southwest China, and discuss potential improvements.
Recent progress on wave gradiometry method
Cao Feihuang, Liang Chuntao
2023, 54(3): 270-286.   doi: 10.19975/j.dqyxx.2022-054
[Abstract](120) [FullText HTML](95) PDF 10513KB(42)
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.
Recent progress on full waveform inversion
Zhu Hejun, Liu Qinya, Yang Jidong
2023, 54(3): 287-317.   doi: 10.19975/j.dqyxx.2022-031
[Abstract](507) [FullText HTML](243) PDF 8901KB(340)
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.
Advances in seismic imaging of mantle transition zone discontinuities
Yu Chunquan, Li Juan, Yang Fan, Zhang Yan
2023, 54(3): 318-338.   doi: 10.19975/j.dqyxx.2022-034
[Abstract](259) [FullText HTML](150) PDF 2613KB(130)
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.
Mid-lower mantle scatterers: Detection methods, research progress and prospect
Li Juan, Chen Sidan, He Xiaobo, Wang Wei, Yang Fan
2023, 54(3): 339-354.   doi: 10.19975/j.dqyxx.2022-039
[Abstract](184) [FullText HTML](147) PDF 2323KB(83)
Thanks to the rapid development of seismic wave propagation theory, emerging of data analyzing methods, and the increasing coverage of the seismic exploration, 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.