We processed the observational data of geo‐electric and geo‐magnetic fields around the epicenter of the Ms8.0 Wenchuan earthquake of 2008 in Sichuan Province using the maximum entropy spectrum and other methods, and studied the EM field changes related to the earthquake. The following understandings are got: (1) Significant EM phenomena including self‐potential changes, increased power spectrum density (PSD) of the geoelectric field, and waveform distortion of the geo‐magnetic field diurnal variation appeared along the northeastern edge of the Qinghai‐Tibet Plateau before the Ms8.0 Wenchuan earthquake. Especially, the PSD of the low frequency components with periods less than about 3 hours of the geo‐electric field were increased by more than 3 orders of magnitude. (2) Within about 500 km from the epicenter the PSD of the low frequency components of the geo‐electric and geo‐magnetic fields increased obviously. In particular, at Chengdu station which was 35km from the epicenter, the PSD of the geo‐electric field were increased by 4 orders of magnitude. (3) The spatial and temporal distributions of the EM phenomena and the great earthquakes occurred in the Qinghai‐Tibet Plateau, i.e. the Wenchuan and the Ms7.3 Yutian, Xinjiang, earthquakes have a tectonic relevance in a large spatial range. The mechanisms of the EM phenomena are discussed, as a result, it could be believed that in the processes of both the development of the numerous micro cracks in the underground media before earthquakes and the directional arrangement of the cracks, such influencing factors as the “electro‐mechanical transformation function” and the underground water migration and so on, induced the electric and magnetic changes. The EM phenomena before the Wenchuan earthquake were complex in temporal, spatial and intensity evolutions, and the possible reasons for the complex evolutions are the action of the local medium under the EM stations and the EM radiations excited by the focal preparation process.

Site‐selective phenomenon exists in the normal waveforms of the geoelectrical fields of Qinghai‐Tibetan Plateau and its adjacent areas: rock structure of the sites, cracks and fissure water, tectonic activity and other factors affect the normal waveforms of the geoelectrical field; a large lake will contribute to the occurrence of TGF‐A waveforms nearby, TGF‐B waveforms will mostly occur in the areas where there are thick Quaternary sediments, and high water content and good water permeability of rocks, while the geoelectrical fields with no diurnal variations are prone to appear in the bedrock mountains where there are intense tectonic activities. Before moderate or strong earthquakes in the region, short‐term and impending anomaly of the geoelectrical field has a cluster phenomenon, in which there is a temporal synchronization or a few days' difference, while spatially it can be distributed near several faults in the region, manifested as a discrete phenomenon. This phenomenon of temporal clustering and spatial scattering is the short‐term and impending precursor characteristics of the geoelectrical field in the region. The Σ – Δ summation method is introduced to analyze the usual stability of spontaneous field through the observational data of the geoelectrical field, which indicate that before a moderate or strong earthquake, background value jumps of part of geoelectrical fields are a transformation phenomenon of spontaneous field; the concept and calculation method of TA, which is the sum of the first 10 order tidal harmonic amplitudes divided by the sum of the first 100 order harmonic amplitudes, are established to show quantitatively that the tidal effect of waveforms of TGF‐A, TGF‐B and non‐diurnal variation is successively reduced; the first order difference method of ΔE is adopted to analyze the higher frequency electromagnetic components in the minute order geoelectrical field monitoring data, usually the high‐frequency jumps show a certain degree of randomness and limitedness. By using three different methods of analysis, the features of spontaneous field, the telluric field and high‐frequency electromagnetic components are analyzed from the composition of the geoelectrical field, which provides a theoretical and methodological basis for the physical analysis of the geoelectrical field.

We selected broadband teleseismic waveform data recorded by the Gansu Digital Seismic Network to study the crustal and upper mantle structure beneath the Northeastern Tibetan Plateau. The crustal thickness and VP/VS ratio are estimated by the grid-search method using crustal conversion and multiples, assuming an average P-wave velocity in the crust. Teleseismic P waveform modeling is carried out to investigate the S-wave velocity structure, assuming the fixed VP/VS value inferred from the grid-search method. Our results show that the crust is 50∼55km thick beneath Qilian terrane, and about 45 km beneath the Hexi Corridor and Qaidam terrane. The VP/VS ranges from 1.66∼1.85, and the Poisson's ratio is about 0.215∼0.294. Our results also show that the Moho interface is sharp beneath the Northeastern Tibetan Plateau. Both of the low-to-normal Poisson's ratio and the sharp Moho do not support the argument of significant partial melting in the upper-to-middle crust and underplating beneath the lower crust of the Northeastern Tibetan Plateau. The existence of anticorrelation between VP/VS and the crustal thickness strongly suggests that the crust is mainly composed of felsic mineral in the northeastern Tibetan crust. The seismic evidence may indicate that the thickened crust, mostly in its upper part, is probably due to the successive intracrustal thrusts.

We studied the changes of the electric field before the Wenchuan Ms8.0 earthquake of 2008 using both space‐based and ground‐based observational data and the method of marginal spectra. The evolution of marginal spectra of the electric field from the repeat orbits and continuous orbits reveals that the marginal spectrum of the electric field in the ionosphere had a remarkable increase (about 1~2 orders of magnitude) from 27 April, 2008 to the time when the Wenchuan earthquake happened. For the evolution of marginal spectra of the electric field from the ground‐based data, the marginal spectrum of the Songshan station of Sino‐French cooperation began to enhance (increased by 2 orders of magnitude) from 25~29 April 2008. Such a phenomenon appeared at Chengdu, Longnan and Tianshui stations (increased by 2 orders of magnitude) from 10~14 April 2008. Despite that the marginal spectrum enhancement was identical at Chengdu, Longnan and Tianshui stations, it ended at different times. The process ended earlier at the Songshan and Tianshui stations than the Chengdu and Longnan stations. It seems that the enhancement of marginal spectra of both the ground and space electric fields happened at a roughly same time.

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