Establishment of the ELF Network in Yunnan and Electromagnetic Precursory Monitoring Results of the Yangbi MS5.1 Earthquake on March 27,2017

This paper summarizes the layout of the Yunnan seismic ELF electromagnetic observation network,site selection,ELF electromagnetic instrument system,data processing and other construction. The principle and method of using the ELF electromagnetic wave technique to monitor and predict earthquakes are expounded. The long term monitoring of ELF electromagnetic fields is carried out in the Yunnan earthquake prone area,and at the same time,the changes in electrical parameters and spatial electromagnetic fields of the regional crustal medium structure are monitored. The functions such as automatic,quasi real time, remote monitoring, network monitoring, data processing specialization, data service,data sharing and industrialization of the ELF electromagnetic observation data have been realized. In order to capture the deep electromagnetic precursory information of the earthquakes,service for earthquake prediction research,which has broad application prospects and development potential. Through the research of the seismicity of Yunnan in the trial run period of the project,the preliminary results of the extreme low frequency electromagnetic observation of the Yunnan earthquake in recent years are given. The electromagnetic precursors and the electromagnetic effects of the Yangbi earthquake are recorded. In the 3-month period before the earthquake,the power spectrum of the electric and magnetic fields,the apparent resistivity and the impedance phase in the observed signals are all abnormal,and gradually increased with time. The maximum value is reached 20 days before the earthquake,and an earthquake occurs when the change is restored to normal.

Preliminary Study on the Relationship between the M_s8.0 Wenchuan Earthquake of 2008 and the Anomalies of Earth Resistivity Observed at Qingdao Seismic Station

Near-surface earth resistivity and underground water level anomalies were recorded at Qingdao seismic observatory of Shandong Province before and after the MS8.0 Wenchuan Earthquake of May 12,2008.The observed data of earth resistivity at the observatory revealed that the underground water level dropped and the resistivity increased.It is postulated that in the special tectonic setting at Qingdao observatory,the variation of stress and strain caused the change of water level beneath the station,thus,leading to the variation of earth resistivity.The relationship between the variation of stress field and the change of earth resistivity before earthquake is analyzed.

Study of the Structure of the Shenzhen Fault Zone with the Magnetotelluric Method

The magnetotelluric sounding method was used to study the active fault in Shenzhen city.Four magnetotelluric profiles with a dense station interval were laid out across the Shenzhen fault zone.The remote reference technique was used in both data observation and processing to eliminate the electromagnetic noise near the survey sites,and relatively smooth apparent resistivity curves were obtained.TM mode data and two-dimensional inversion method as NLCG were used to obtain the electrical structures underground.According to the surficial geology survey results on regional strata and distribution of magmatic bodies and faults,the electrical structures at depths less than 2000m of each profile were interpreted.Two regional faults,the Henggang-Luohu fault and the Liantang fault,and seven local faults consistent with the electrical boundaries were verified or discovered.The strata in the survey area were then related with the change of resistivity.Electrical horizontal slices of depths less than 5000m indicate that the Henggang-Luohu fault and the Liantang fault bifurcate at the shallow part but tend to merge in the deep part.

Deep structure beneath the southwestern section of the Longmenshan fault zone and seimogenetic context of the 4.20 Lushan M_S7.0 earthquake

Magnetotelluric measurements were carried out along two profiles across the middle and southwestern sections of the Longmenshan fault zone(LMSf)from 2009 to 2011,after the 2008 Wenchuan MW7.9 earthquake.The former profile crosses the Wenchuan event epicenter and the latter one crosses 2013 Lushan MS7.0 event epicenter.The data were analyzed using advanced processing techniques,including phase tensor and two-dimensional inversion methods,in order to obtain reliable 2-D profiles of the electrical structure in the vicinity of the two earthquakes.A comparison of the two profiles indicates both similarities and differences in the deep crustal structure of the LMSf.West of the southwestern section,a crustal high conductivity layer(HCL)is present at about 10 km depth below the Songpan-Garzêblock;this is about 10 km shallower than that under the middle section of the LMSf.A high resistivity body(HRB)is observed beneath the southwestern section,extending from the near surface to the top of upper mantle.It has a smaller size than the HRB observed below the middle section.In the middle section,there is a local area of decreased resistivity within the HRB but there is absence of this area.The 2013 Lushan earthquake occurred close to the eastern boundary of HRB and the Shuangshi-Dachuan fault,of which the seismogenic context has both common and different features in comparison with the 2008 Wenchuan event.On a large scale,the 2013 Lushan earthquake is associated with the HCL and deformation in the crust including HCL of the eastern Tibetan Plateau.In order to assess seismic risk,it is important to consider both the stress state and the detailed crustal structure in different parts of the LMSf.

Evidence of crustal ‘channel flow’ in the eastern margin of Tibetan Plateau from MT measurements

Magnetotelluric (MT) survey has been carried out in the eastern margin of the Tibetan Plateau and its neighboring Shimian-Leshan area, Sichuan Province. Analysis of this MT data reveals that the electric structure of the Tibetan Plateau differ much from that of the Sichuan block. In general, the electric re-sistivity of crust beneath the Sichuan block in the east is larger than that of the eastern margin of the Tibetan Plateau in the west. The crust of the plateau is divided into upper, middle, and lower layers. The middle crust is a low resistivity layer with minimum down to 3―10 Ωm about 10―15 km thick. It pre-sumably contains partial melt and/or salt-bearing fluids with low viscosity, prone to deform and flow, producing a "channel flow" under the southeastward squeeze of the eastern Tibetan Plateau. This low-resistivity layer makes the upper crust decoupled mechanically from the lower crust. In the brittle upper crust, faults are dominated by left-lateral strike-slip and thrust motions, leading to surface rising and shallow earthquakes. The low-resistivity layer also cut the Xianshuihe-Anninghe fault zone into two sections vertically. In this region, the thicknesses of upper, middle, and lower crust vary laterally, pro-ducing a transitional zone in the eastern margin of the Tibetan Plateau characterized by thicker crust and higher elevation in the west and thinner crust and lower elevation in the east.

Relation between electricity structure of the crust and deformation of crustal blocks on the northeastern margin of Qinghai-Tibet Plateau

Study on the electricity structure along a magnetotelluric(MT)sounding profile on the northeastern margin of Qinghai-Tibet Plateau indicates that four crustal blocks can be de-termined from southwest to northeast,namely Bayan Har block(BH),Qin-Qi block(QQ),Hai-yuan block(HY)or the North-South seismotectonic belt and Ordos block(OD).The BH,QQ and OD blocks display a similar electricity structure of the crust.The upper crust represents a high-resistivity layer and the upper part of lower crust represents a low-resistivity layer with the resis-tivity increasing gradually with depth from the lower part of lower crust to the upper mantle.The electricity structure of the crust in these three blocks is similar to that in the complete blocks on the southern and eastern margins of the Qinghai-Tibet Plateau and belongs to normal electricity layering of the crust in slightly deformed or complete intracontinental blocks.The crust in HY block as a boundary zone has been significantly deformed,hence its electricity layering was de-stroyed and the structure was complex and the block became a recent tectonically active and great seismo-active region.The contact belts between the blocks on the northeastern margin of Qinghai-Tibet Plateau exhibit both upthrusting outward and strike-slip movement different from those on the southern and eastern margins of the plateau.The genesis of the low-resistivity layer in the crust is analyzed and the thickness of the lithosphere is estimated in the paper.

Deep electrical structure of the Sulu orogen and neighboring areas

Because of the discovery of ultrahigh pressure metamorphic (UHPM) belt beneath the Sulu (Jiangsu Province-Shandong Province) orogen, this area has become a focused subject of current geoscience, as it has a close relationship with the evolution of the orogen and the neighboring North China craton. Probing the deep structure beneath this area would be of great significance for the geological interpretation of this issue. In this study, we make an analysis of magnetotelluric (MT) data along a profile across the Sulu orogen to provide evidence of deep structure below this region. The profile begins in west from the North China block, extending in S129°E, across the Tan-Lu fault, Sulu UHPM zone, and Sulu high pressure metamorphic (HPM) zone, and terminates in the Yangtze block in east. We use the nonlinear conjugate gradient method and TE-TM combined mode to perform inversion and interpretation of the MT data, and obtain an electrical structure image above depth of 150 km along the profile. It shows that the structure can be divided into seven sections in lateral direction, between which the electric boundaries coincide well with the major faults, such as the Tan-Lu, Haizhou-Siyang, and Jiashan-Xiangshui faults. In vertical direction the electrical structure can be subdivided into six layers of different resistivities. It is noted that there exist high-conductivity areas in crust below the North China block and Yangtze block, while such a feature is not present beneath the Sulu orogen, which is very different from the Dabie orogen. It is also observed that a fairly continuous zone of relatively low-resistivity exists at depths of 50–90 km of the electrical structure image, which is presumably a weak zone in the uppermost mantle. Just below this low-resistivity zone are the relatively high- resistivity layer of the North China block, relatively low-resistivity layer of the Sulu orogen, and relatively high-resistivity layer of the Yangtze block, all in the shallow upper mantle, respectively. From the whole 2D electrical structure image, there is no abnormally low-resistivity layer in the shallow upper mantle beneath the Sulu orogen and neighboring areas, indicating that no hot asthenoshperic material associated with lithospheric thinning exists at present.

Advances in alternating electromagnetic field data processing for earthquake monitoring in China

The alternating electromagnetic(EM) field is one of the most sensitive physical fields related to earthquakes. There have been a number of publications reporting EM anomalies associated with earthquakes. With increasing applications and research of artificial-source extremely low frequency EM and satellite EM technologies in earthquake studies, the amount of observed data from the alternating EM method increases rapidly and exponentially, so it is imperative to develop suitable and effective methods for processing and analyzing the influx of big data. This paper presents research on the self-adaptive filter and wavelet techniques and their applications to analyzing EM data obtained from ground measurements and satellite observations, respectively. Analysis results show that the self-adaptive filter method can identify both natural- and artificial-source EM signals, and enhance the ratio between signal and noise of EM field spectra, apparent resistivity, and others. The wavelet analysis is capable of detecting possible correlation between EM anomalies and seismic events. These techniques are effective in processing and analyzing massive data obtained from EM observations.