The Electromagnetic Phenomena Before the Ms8.0 Wenchuan Earthquake

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AbstractWe 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.