Electromagnetic anomalies before the 2013 Lushan M_S7.0 earthquake

This paper studies the geoelectric field and electromagnetic disturbance observed at the eighteen stations laid out in the South-North Seismic Belt in China. Before the 20 April 2013 Lushan M_S7.0 earthquake, only at the five stations were abnormal electromagnetic disturbance signals recorded. The study shows that the electromagnetic signals are distributed heterogeneously in space-time, and they do not appear synchronously at different stations, which migrate toward near the epicenter with time. In spatial distribution, the abnormal signals before the earthquake were recorded only at several stations to the south of Chengdu station (including Chengdu station), but not recorded at the stations to the north of Chengdu. Before the Lushan M_S7.0 earthquake the earliest lead time of the signals is 21 days, and then 16 days. Immediately before the event the lead times are 5 days, 4 days and 1 day, respectively. By studying the signals originated from grounding electrode 4 500 A current and some earthquake examples, Xiannüshan station is considered as a new sensitive site for the electromagnetic observation.