Anomalous ionospheric disturbances over South Korea prior to the 2011 Tohoku earthquake

An anomalous disturbance in ionospheric total electron content (TEC) variations was observed by a GPS network in South Korea and three GPS stations in Japan before the 2011 great Tohoku earthquake on March 11, 2011. We investigate the standard deviation time series of the vertical TEC variations for all ground receivers and two satellite pairs. Ionospheric disturbances occurred approximately 40–50 min prior to the earthquake and lasted for over one hour. We also investigate the STD time series of the TEC variations on 1–15 March 2011. A pattern of ionospheric disturbance starting ∼45 min (05:00 UT) before the earthquake was relatively different from those of other days. It is noted that there were precursory features in the near-field close to the epicenter. However, no significant disturbance was observed in the regions that were relatively far from the epicenter. These precursory features may be associated with an anomalous electric field generated by complex processes.