Pattern and evolution of seismo-ionospheric disturbances following the 2011 Tohoku earthquakes from GPS observations

Global Positioning System (GPS) has been widely used to sense crustal deformation and ionospheric anomalies, particularly seismic ionospheric disturbances. In March 2011, the earthquakes with magnitude of up to Mw=9 occurred in Tohoku near the east coast of Honshu, Japan. The GPS Earth Observation Network (GEONET) in Japan with more than 1200 continuously operating stations provides a unique opportunity to study the detailed seismic ionospheric disturbances. In this paper, the pattern and evolution of seismic ionospheric disturbances following the Tohoku earthquakes are investigated by dense GEONET data, including amplitude, propagation pattern, direction, speed, and evolution. Maximal coseismic ionospheric disturbances are found with up to more than 4 TECU, and the disturbance period is around 10-20 min. The seismic ionospheric effects following the aftershocks are attenuated with the increase of the time and distance between the ionospheric pierce point and the epicenter of the main event, which last more than 2 h. Seismic ionospheric disturbance detected by GPS measurement is not only related to the main shock but also the giant aftershocks. Propagation velocities of the total electron content (TEC) disturbance show a decrease when it spreads 400-600 km away from the epicenter in the north-western direction, where it is just near the west coast of Japan. Furthermore, the TEC disturbance also has obvious directional features. In the first half hour, the TEC disturbance in the southeast direction has the biggest amplitude, while the dominant direction is changed to northwest tens of minutes later. In addition, signals with higher frequencies are existed in seismic TEC variation at the epicenter region but do not appear in the far field.