The change characteristics of electromagnetic field before to after Wenchuan M_s8.0 earthquake

Before and after the 12 May M_s8.0 Wenchuan earthquake, we have observed anomalies associated with this event by several geophysical methods, such as DC resistivity, geoelectric field and electromagnetic (ELF), at the Hanwang station, Longnan, Gansu province 320 km away from the epicenter.In this paper, we use the analytical method to study the features and changing regularities on long-, medium-and short-term scales of the electromagnetic field prior to and after this quake based on these data.Since 2003, four earthquakes of M≥5 occurred in the southeastern Gansu and around the border between Gansu and Sichuan province.These events seemed to shift from the middle section of the North-south seismic zone in central China mainland toward southwest with increasing magnitude.Among them, the Wenxian M5.0 and other two M5.0 events at Minxian were likely the precursory quakes to the Wenchuan M_s8.0 in 2008. Corresponding to these quakes, the observations at the Hanwang station exhibit long-term tendency and short-time anomalous changes described as follows.During 2003～2008, the resistivity values measured by the instrument ZD8B in directions N54°W, N85°E and N73°W all showed long-and short-term variations.The annual change rates of the anisotropic degree S' of apparent resistivity in the years with M5.0 events (anomalous years) are two times that of the years without M5.0 quakes (normal years).The anomalies of the electric and magnetic fields on the frequency band (0.5～39 Hz) measured by the instrument ELF during the times of the Wenchuan M,8.0 and its aftershocks are greater than those in normal times by 1～5 orders of magnitude.Meanwhile this instrument has also recorded electromagnetic coseismic signal.And 17 hours prior to the main shock, the telluric electric field instrument (ZD9A-II) detected high-frequency anomalous information.