Abnormal signals of geoelectric field related to Wenchuan M_S8.0 earthquake recorded at the Chengdu station

The short-term earthquake prediction must be based on reliable precursors, and identifying them from abundant anomalies is the primary problem of earthquake prediction in the world. Before the 12 May 2008 Wenchuan M_S8.0 earthquake the very strong abnormal signals of the geoelectric field were observed at Chengdu station, which is located in the range of the focal zone. The signals lasted from April 10 to 17, 2008 and twenty five days later the Wenchuan earthquake occurred. This paper demonstrates the geoelectric field precursors related to the shock based on the theorem of the multi-dipole geoelectric field observation and the retrospective check of decade scale data from 2007 to 2017. The study results show that the strong geoelectric anomalies recorded during the four periods, i.e. , from 23:00 on April 10 to 3:40 on April 11, from 4:00 on April 11 to 12:00 on April 16, 2:00−3:00 and 22:00−23:00 on April 17, originated respectively from four different sources, in which the second set of signals consists of forty pulses, which means that there were four stress concentration and release regions in the focal zone. The reasonable mechanism explanation to the abnormal signals is also given in the paper in which different stress concentration regions are considered as the sources of them. Because Chengdu station is located in the focal zone of the Wenchuan earthquake, the anomalies could be taken as one of the important reference indices for researches on the preparation processes of an earthquake.