With the support of the wireless electro-magnetic method(WEM)project, the control source extremely low frequency(CSELF)continuous observation network, which includes 30 electromagnetic stations in Beijing capital area(BCA)and the southern section of the North-South Seismic Belt in China, was built for recording the artificial and nature source singles. The natural source observation of the network was started in July 2013 and December 2013 in batches and the electromagnetic field was recorded continually with a sampling rate of 16Hz. Until now, the co-seismic electromagnetic signals have been recorded repeatedly in several stations. In this paper seven co-seismic electromagnetic signals recorded at Jinggu station and co-seismic electromagnetic signals associated with two strong earthquakes recorded at different stations surrounding the epicenter are studied.

It is found that the variation of the EM filed is similar to the seismogram, and the amplitude of the co-seismic EM signal is much larger than the background signal generated by earth induction, and the intensity of the vertical magnetic field is about ten times as big as the horizontal electromagnetic field. For co-seismic EM signals recorded at the same station, the relationship between the amplitude of electromagnetic field and the magnitude of the earthquake is basically linear in logarithmic domain. Meanwhile, the amplitude of electromagnetic field is also affected by focal depth of the earthquake and distance between the stations and the epicenter. When the epicenter distance is close, the amplitude of the co-seismic signal caused by the earthquake with shallow focal depth is higher. When the focal depth is similar, the amplitude of electromagnetic co-seismic signal caused by the earthquake closer to the station is larger.

For the co-seismic EM signals associated with a same earthquake recorded by different stations, the larger the epicenter distance is, the later the signal appears and the longer the duration is. However, the signal amplitude is not only affected by the epicenter distance, but also related to the near-surface medium at the observation point. The electromagnetic co-seismic signals observed at Dali station which is the farthest away from the epicenter of Jinggu earthquake show the characteristics of large amplitude, long duration, and low dominant frequency. This may be related to the electrical structure near the surface of Dali Platform. The electromagnetic field signals of the 5 components of Jinggu, Muding and Dali stations before and after the Jinggu earthquake of magnitude 5.9 were transformed by wavelet transform. Finally, the wavelet spectrum with the horizontal axis as time and the vertical axis as frequency was obtained to indicate the time-frequency changes of the abnormal electromagnetic signals of the same seismic wave. According to the wavelet analysis and combining with the time series before and after the Jinggu earthquake of M_S5.9, the energy enhancement mainly occurs in the shear wave and surface wave periods, while the P-wave is not obvious in the wavelet energy spectrum due to its small amplitude, and only some weak enhancement with scattered frequency can be observed. The main frequency of electromagnetic co-seismic signal is between 1Hz and 2Hz. At the beginning of the co-seismic signal, there are high frequency components, and the high frequency gradually decreases with the increase of epicenter distance. Moreover, compared with electric field, magnetic field can record more abundant high-frequency information. This may have to do with different dominant mechanisms for electric and magnetic field generation.

In this paper, several earthquakes recorded at Jinggu station and electromagnetic co-seismic phenomena caused by two strong earthquakes at Jinggu station are summarized and analyzed. The results show that the variation of co-seismic electromagnetic signal is very complicated, and its starting time, duration, amplitude, and frequency range have some rules, but some stations show their particularity under multiple seismic events, so it is difficult to discuss the mechanism of its generation. However, in terms of observation phenomena, the electromagnetic field variation data observed continuously by extremely low frequency stations give us a more comprehensive understanding of the Earth’s electromagnetic field itself and the electromagnetic signals related to earthquakes. The accumulation of more seismic-related electromagnetic phenomena and the support of theoretical simulation can deepen the understanding of electromagnetic field variation before, during and after the earthquake.

On July 12, 2020, an M_S5.1 occurred in Guye, Hebei Province, and as the largest earthquake in the capital circle in recent years, its unique geographical location has attracted more attention. During an earthquake, the electromagnetic properties of underground media will change, so dense electromagnetic observation stations were arranged in the capital circle. In this study, the data of geoelectric resistivity, geoelectric field, and extremely low frequency(ELF)observation within 400km of the Guye earthquake are analyzed using a combination of time-domain waveform analysis, sliding Fourier analysis with annual variation removed, normalized variation rate method(NVRM), and geo-electric azimuthal method. After eliminating the influencing factors such as operation status, observational environment, and the spatial electromagnetic effect, we analyzed the characteristics of electromagnetic phenomena that may be related to the Guye earthquake preliminarily and found that there was a variation process of “trend decrease—accelerated decrease—postseismic recovery” observed in 6 geoelectric resistivity stations and that the normalized variation rate exceeded the threshold value of ±2.4 in 7 stations within one year before the earthquake. In Luanxian station, the intensity of the geoelectric field in the north-south and north-western directions decreased and then rose back before the earthquake. In addition, the azimuth shifted to the direction of the Guye earthquake in the preseismic period, and then returned to the direction of the Luanxian-Laoting Fault. The ELF stations in Wen'an and Fengning precisely recorded the coseismic change of the 16Hz natural magnetic field, in which the variation of the vertical component is twice larger than that of the horizontal component. Under the condition of large subsurface structure difference beneath the stations, the observed electric values from the two stations are distinctively different; moreover, the coseismic disturbance is submerged by the background noise. The subsurface electric structure was obtained by interpolating and inversing the data collected from the ELF stations in the capital area, which indicates that the Guye earthquake occurs near the boundary of the electric property changes. Meanwhile, it shows high electric resistivity in the northern area, low electric resistivity in the southwestern area, and partially low electric resistivity in Baodi and Wen'an, which is consistent with the location of the abnormally stronger ground motion. Regarding the spatial selectivity of the anomalies, we believe it may be related to the direction of the two main conjugated structures in the capital area, which lie in NEE and NW direction, respectively. And the study also enlightens researchers that the investigation of the mechanism of seismic electromagnetic anomaly should start from the coseismic phenomenon, and then focus on the aspects of seismic signal source and propagation path, because the extremely low-frequency observation band is wide and the coseismic electromagnetic signals can be clearly recorded. There are many effective ways to extract electromagnetic signals related to earthquakes from strong interference background, such as making retrospective analysis of moderately strong earthquakes in time, summarizing the electromagnetic anomaly characteristics of different earthquake events, densifying the electromagnetic observation layout appropriately, so that the abnormal information can be mutually corroborated and a variety of means for fusion and comparative analysis can be developed.

The first control source extremely low frequency(CSELF)electromagnetic observation network through the world, consisting of 30 fixed stations located in the Beijing captical circle region(15 staions)and the sourthern secton of the north-south earthquake belt(15 stations), China, has been established under the support of the wireless electromagnetic method(WEM)project, one of the national science and technology infrastructure construction projects during the 11th Five-year Plan period. As a subsystem of the WEM project, the CSELF network is mainly to study the relationship between elctromagnetic anomalies and mechanisms of earthquake, and further improve our ability to monitor and predict earthquakes by monitoring real-time dynamic changes in both electromagnetic fields and subsurface electric structure. Carrying out the detection of the underground background electric structure in the CSELF network area/station is an important part of this project and of great significance to play its role in the study of earthquake prediction and forecast. In this paper, we elaborate how to acquire the subsurface electric structure of the CSELF network in the Beijing captical circle region and make a simple explanation for the structure. Firstly, a short magnetotelluric(MT)profile, almostly perpendicular to the regional geological strike, was deployed at each station of the CSELF network in the capital circle region during the 2016 and a total of 60 broadband MT sites was collected using ADU -07e systems. Then, all the time series data were processed carefully using the robust method with remote reference technique to MT transfer functions. MT data quality was assessed using the D+algorithm. In general, data at most sites are of high quality as shown by the good consistency in the apparent resistivity and phase curves. Different impedance tensor decomposition methods including the phase tensor analysis, Groom and Bailey(GB)tensor decompositon, and statistical image method based on multi-site, multi-frequency tensor decompositon were used to analyze data dimensionality and directionality. For data inversion, on the one hand, one-dimensional(1-D)subsurface electrical resistivity structures at each station and MT site were derived from 1-D adaptive regularized MT inversion algorithm. On the other hand, we also imaged the 2-D electric structures along the short MT profile by the nonlinear conjugate gradients inversion algorithm at each station. Robustness of all 2-D structures along each short profile were verified by sensitivity tests. Although fixed stations and MT sites are limited and distributed unevenly, the 3-D inversion of 15 stations was also performed to produce a 3-D crustal electrical resistivity model for the entire network using the modular system for 3-D MT inverson: ModEM based on the nonlinear conjugate gradients algorithem. Intergrating 1-D, 2-D and 3-D inversion results, the resistivity structure beneath the CSELF network in captical circle region revealed some significant features: The crustal electrical structures are mainly characterized by high resistivity beneath the Yinshan-Yanshan orogenic belt in the northern margin of North China, the Taihangshan area in the middle, the Jiao-Liao block in the east, while the North China Plain and Shanxi depression areas have relatively lower resistivity in the crust; There are obvious electrical resistivity difference on both sides of the gravity gradient of Taihang Mountains and the Tanlu fault zone, which indicates they could be manifested as an electric structure boundary zone, respectively. Overall, the electric structure characteristics of the entire network area shows high correspondence with the regional geological structure and earthquake activity to some extent. In summary, implementing the detection of underground electrical resistivity structure in the CSELF network of the capital circle region will provide important foundations for the researches on the regional seismogenic environment, the generation mechanism of seismic electromagnetic anomaly signals, and earthquake prediction and forecast.

The Tianchi volcano in Changbai Mountains is a giant active volcano with potential risk of eruption, so more detailed researches of deep magma chamber as well as its closely related three-dimensional electric structure model are being concerned by many scholars. Based on adaptive regularized three-dimensional quasi-Newton inversion program developed by the author, this paper carries out three-dimensional magnetotelluric inversion including topography using the existing data observed decades ago. Our three-dimensional magnetotelluric adaptive regularization quasi-Newton inversion method improves the conventional quasi-Newton method by approximating Hessian matrix of the data misfit using LBFGS formula instead of total Hessian matrix which is approximated in conventional quasi-Newton inversion. This not only guarantees the precise regularization-term Hessian matrix, but also allows the regularization parameter to be adaptively updated on every inversion iteration without destroying the descent of total objective function. Thus, the regularization parameter's adaptive updating strategy on every iteration can be established based on L2-norm ratio of the data misfit function and regularization function, and our AR-QN inversion algorithm was implemented. The model synthetic data inversion results indicate that AR-QN inversion has very stable iteration flow, and is weakly dependent on initial model selection, which tends to be more suitable for the three-dimensional inversion task when MT survey sites are sparsely distributed on the surface.
Besides the impedances data, the tipper data which is more sensitive to horizontal conductive discontinuity was fitted in this three-dimensional inversion as well. Through comparison of qualitative analysis to measured data and anomalous-body-erasing forward modeling test, our new three-dimensional electric structure was proved to be a reliable model under the constraints of current magnetotelluric data. The three-dimensional electric structure is more accurate and rational compared with the electric structure obtained by previous researches based on two-dimensional inversion, and clearly characterizes the distribution of crustal magma chamber and magma channels with high resolution, even inverted by the limited spare-sites-distributed magnetotelluric dataset. Our result locates the crustal magma chamber on the northeast of Tianchi within the depths of 10~30km, and the general magma chamber along with magma channels system is represented as “V” shape. The magma channel of Tianchi volcano can be clearly shown on the EW profile 10km away from the north of Tianchi volcano. The magma chamber and channels system can be divided into three layers from shallow to deep: Magma channel with a depth of less than 5km and connecting to the Tianchi crater; alkaline fluid magma layer with a depth of 5~10km; and trachytic magma layer with a depth of 10~30km. As the coverage of the existing MT survey sites is still incomplete on the whole Tianchi volcano area, adding more MT survey sites in the northwest, southwest directions and especially in North Korea will help us get more reliable, higher-resolution three-dimensional electrical structure model for Changbaishan Tianchi volcano area.

Many synthetic model studies suggested that the best way to obtain good 3D interpretation results is to distribute the MT sites at a 2D grid array with regular site spacing over the target area. However, MT 3D inversion was very difficult about 10 years ago. A lot of MT data were collected along one profile and then interpreted with 2D inversion. How to apply the state-of-the-art 3D inversion technique to interpret the accumulated mass MT profiles data is an important topic. Some studies on 3D inversion of measured MT profile data suggested that 2D inversions usually had higher resolution for the subsurface than 3D inversions. Meanwhile, they often made their interpretation based on 2D inversion results, and 3D inversion results were only used to evaluate whether the overall resistivity structures were correct. Some researchers thought that 3D inversions could not resolute the local structure well, while 2D inversion results could agree with the surface geologic features much well and interpret the geologic structures easily. But in the present paper, we find that the result of 3D inversion is better than that of 2D inversion in identifying the location of the two local faults, the Shade Fault(SDF)and the Yunongxi Fault(YNXF), and the deep structures.
In this paper, we first studied the electrical structure of SDF and YNXF based on a measured magnetotelluric(MT) profile data. Besides, from the point of identifying active faults, we compared the capacity of identifying deep existing faults between 2D inversion models and 3D models with different inversion parameters. The results show that both 2D and 3D inversion of the single-profile data could obtain reasonable and reliable electrical structures on a regional scale. Combining 2D and 3D models, and according to our present data, we find that both SDF and YNXF probably have cut completely the high resistivity layer in the upper crust and extended to the high conductivity layer in the middle crust. In terms of the deep geometry of the faults, at the profile's location, the SDF dips nearly vertically or dips southeast with high dip angle, and the YNXF dips southeast at depth. In addition, according to the results from our measured MT profile, we find that the 3D inversion of single-profile MT data has the capacity of identifying the location and deep geometry of local faults under present computing ability. Finally, this research suggests that appropriate cell size and reasonable smoothing parameters are important factors for the 3D inversion of single-profile MT data, more specifically, too coarse meshes or too large smoothing parameters on horizontal direction of 3D inversion may result in low resolution of 3D inversions that cannot identify the structure of faults. While, for vertical mesh size and data error thresholds, they have limited effect on identifying shallow tectonics as long as their changes are within a reasonable range. 3D inversion results also indicate that, to some extent, adding tippers to the 3D inversion of a MT profile can improve the model's constraint on the deep geometry of the outcropped faults.

The Yishu fault zone is one of the branch faults of the Tanlu fault zone in its central part. Moderate and strong earthquakes occurred in the Yishu fault zone repeatedly. Due to its complex structure, the Yishu fault zone attracts much attention from earthquake researches. The Anqiu and Juxian electromagnetic stations in Shandong Province locate near the Anqiu-Juxian Fault and Changyi-Dadian Fault, which are branches of the Yishu fault zone, respectively. Geoelectric field and geomagnetic field observation were carried out in these two stations. The Wudi electromagnetic station is in the west of Tanlu fault zone in the Jidong-Bohai block and 230km from Anqiu electromagnetic station. This paper firstly describes the crustal structure near the electromagnetic stations by using magnetotelluric(MT)method. By processing the data carefully, we obtain the MT data in good quality near the stations. The MT data of each electromagnetic station and its nearby area suggests that the electrical structure and geological structure of the station are comparable. This paper applied 1-D and 2-D inversion for MT data and obtained the crustal electrical structure model beneath the Anqiu and Juxian seismic station. The shallow electrical structure from the MT method was compared with the results of symmetrical quadrupole electrical sounding. The model suggests that the electrical structure beneath the Anqiu and Juxian electromagnetic stations is complex and shows the feature of block boundary. The Wudi electromagnetic station is located inside a basin, the crustal structure shows layered feature typical for the stable blocks. Beneath the Anqiu electromagnetic station, there is a 1km-thick relative low resistivity layer in the shallow crust and a high resistivity body beneath it with a depth of 13km. There is a high resistivity structure in the crust beneath the Juxian electromagnetic station. The crustal structures are divided into two different parts by Anqiu-Juxian Fault and Changyi-Dadian Fault, respectively. More conductive layers appear to the west of the two faults. Plenty of fluid possibly exists within the conductive body to the west of Changyi-Dadian Fault, which plays important role in the earthquake generation. There is a relative low resistivity layer in the crust within 1~2km beneath the Wudi electromagnetic station. Beneath the relatively low resistivity layer, a relatively high resistivity layer extends to a depth of around 15km, and the resistivity value decreases with the increase of depth. The electrical resistivity model suggests the seismic activity of the Yishu fault zone around the Anqiu and Juxian electromagnetic stations should be taken into account seriously, and monitoring and research on it need to be strengthened. The results of this paper provide a certain reference value for the crustal structure research to similar stations.

The October 7, 2014 M_S6.6 earthquake in southwest of Jinggu in the southwestern Yunnan Province occurred as the result of shallow strike-slip faulting within the crust of the Eurasia plate in the broad plate boundary region between the India and Eurasia plates. The strike of fault plane is 140°, and the aftershock distribution shows that the rupture plane is also NNW-trending. Tectonics of the region are controlled by the convergence of the India plate with Eurasia, which has driven the uplift of the Himalayas to the west of this earthquake, and has caused the formation of numerous intraplate continental transform structures in the surrounding region. The pattern of elastic-wave radiation from the earthquake is consistent with the shock occurring either as the result of right-lateral faulting on a northwest-trending fault or as the result of left-lateral faulting on a northeast trending fault. Faults of both types have been mapped in southwestern Yunnan, and it is unclear at this time which type of fault hosted this event. Magnetotelluric survey line is across Jinggu earthquake zone. The advanced data processing and analysis technology of MT is employed and the quantitative data from field surveys are analyzed to acquire the reliable electrical model. The MT data are inverted using nonlinear conjugate gradient (NLCG) inversion algorithm. At last, the interpretation of the electrical model is performed considering the geology and the other geophysical data. Based on the final inversion model of the target profile, it is found that:(1) Electrical structure of the source region can be divided into four layers:The surface is relatively low resistivity layer(0~5km), consisting mainly of Mesozoic and Cenozoic Basin sedimentary rocks, the value of resistivity is 100Ω·m; The high resistivity layer(5~10km) in upper crust mainly consists of Proterozoic metamorphic rocks, with resistivity higher than 1 000Ω·m; there are the upper crust high-conductivity layer(15~25km) and crust-mantle transition zone(blow 25km); (2) The focal depth of the Jinggu earthquake is about 10km, which locates in the interface between high resistivity layer and high-conductivity layer; (3) Most of the focal depths of the aftershocks are in the range of 5km and 10km, and the two depths(5km & 10km) are corresponding to the resistivity gradient belt.

Magnetotelluric data are collected along a NW-SE trending and about 900km long profile within northeastern boundary areas of the North China craton(NCC). This profile extends from the Hegenshan belt within the Central Asian orogenic belt(CAOB), across the Baolidao arc, Solonker-Linxi suture zone, Ondor Sum accretion complex, Bainaimiao arc, Inner Mongolia paleo-uplift, Yanshan belt, and ends on the Liaohe depression of the NCC. Impedance tensor decomposition methods are used to study the dimensionality and geo-electric strike of MT data of the region. Two-dimension (2D) analysis is appropriate for this profile. The 2-D subsurface electrical resistivity structure along profile is obtained using the non-linear conjugate gradient (NLCG) algorithm. The electrical resistivity structure is characterized by lateral segmentation, and divided into high resistive, low resistive, and high resistive areas; The lateral variation of electrical resistivity is significant within the CAOB, but it is smooth in the NCC; The extensive high conductive body(HRB)is observed in the mid-low crust beneath the Solonker-Linxi suture zone and Inner Mongolia paleo-uplift, respectively; The low resistivity could be due to the partial melts and crustal flows. Based on our electrical resistivity structure and other geological, geophysical observations, we speculate that (1)the final suturing of the Siberian craton to the NCC could be along the areas between Xilinhot Fault and Xar Moron Fault; (2)the relatively thick high resistive body beneath the Yanshan belt may serve as a tectonic barrier separating the on-craton and off-craton regions into different upper mantle convection system, and lower the effect of tectonic evolution of CAOB on the destruction to NCC.

There are multiple Quaternary buried active faults in the eastern Beijing plain region based on the results obtained from geological and geophysical studies.In order to investigate the condition of bedrock surface undulation,geometry of the buried faults and the extension of fault planes in the eastern Beijing plain area,two CSAMT profiles were completed in Miaojuan,Shunyi district and Sunhe,Chaoyan district in early 2010.The paper presents the advantages of CSAMT in exploration of buried active faults,the data collection procedure and the methods used for data processing.Combining with regional geological data,we make integrate interpretation to the geological structures of the study region.The results show that the CSAMT method can be used effectively in the exploration of buried faults.It can reveal the location,dip,displacement,and size of faults in the survey area,which provide reliable and elemental information for geological analysis.This method has become an important geophysical tool in buried fault detection and plays an increasingly important role in exploration of active faults in urban areas.

The measurement of electromagnetic field for monitoring aftershock series has been done by continuous observation after the M_S8.0 Wenchuan earthquake on May 12,2008 in Hanwang Observatory,Longnan,Gansu Province and vicinity for 22 days since May 22,2008.Two V5-2000 instruments for MT measurement made by Phoenix Company were set up 2.5km away from each other near Hanwang Observatory where a geophone for strong earthquake recording was set up.The coseismic signals of aftershocks exist in all components of the magnetic and electric fields of the two electromagnetic sites.Comparing with the seismic data at Hanwang station,the signals arrive simultaneously with the seismic waves and do not at the origin time of the earthquake.The main frequency of both seismic waves and electromagnetic signals is almost the same.The signal produced at the origin time of the earthquake seems apparent in the EM data,but the signal amplitude is much smaller than those of seismic wave arrival.