Analysis of ionospheric anomaly preceding the Mw7.3 Yutian earthquake

On February 12, 2014, a large Mw7.3 earthquake occurred in Yutian of Xijiang Province, China. We processed the global ionosphere maps provided by CODE (the Center for Orbit Determination in Europe) and the foF2 (the critical frequency of F2-layer) data of Chongqing ionosonde station to analyze the pre-earthquake ionospheric anomalies. Solar activities and magnetic storm were checked by the sliding inter quartile range method to remove their effects on the ionosphere. A positive ionospheric anomaly with the large amplitude of 20 TECU was observed near the epicenter on February 3 (10th day before the earthquake). In addition, the foF2 at Chongqing station had an unusual increase of more than 40% on the day, which was consistent with the TEC (Total Electron Content) anomaly. The global disturbance represents that the peak of TEC anomaly didn't coincide with the vertical projection of epicenter. The TEC anomalous area was closer to the equator, and it mainly occurred from local time 16 : 00 to 20 : 00. An enhancement of TEC with the small amplitude also appeared in the magnetically conjugated region.