The resonant response of the ionosphere imaged after the 2011 off the Pacific coast of Tohoku Earthquake

We provide here a preliminary analysis of the ionospheric perturbations observed after the 11 March 2011 Tohoku Earthquake using a GPS-derived Total Electron Content (TEC) technique. Such anomalies are routinely observed after seismic events of magnitude M-w = 6 and more. Here, we use the high density and the wide coverage of the Japanese Global Positioning System (GPS) network GEONET to image the ionosphere just after the main shock. We describe ionospheric perturbations with exceptional extension in amplitude and duration. As already seen in earlier events, a first intense signal is observed about 10 minutes after the seismic rupture; the first response consists in two modes: one propagating beyond 3 km/s and the other at nearly 1 km/s. A further analysis of TEC time series of the latter mode near the source shows the typical frequencies of acoustic resonance. Beyond 400 km from the source, both the tsunami induced gravity wave and a third mode are imaged, the latter for the first time. We show that the pattern of this slow (225 m/s +/- 10 m/s) and long period gravity wave (1.8 +/- 0.2 mHz) is most visible in the North-West of the epicentral area. This description is corroborated by a computation of the normal modes of the solid Earth-atmosphere system.