Since October 1, 2010, a GPS receiver is put into operation at Tokai (Japan) in an experiment on Neutrino Physics (T2K). A significant variation of the altitude was detected from the beginning of March 2011, so that it has made worthwhile to investigate the possibility that such variations could be correlated to the Tohoku earthquake. In order to investigate in details this possibility, we analyzed the GPS data collected during 2011 by GEONet the GPS Earth Observation Network (GEONET). GEONET is the GPS network of Japan and consists of 1240 permanent stations. Preliminary results of the analysis seemed to show ten days before the earthquake, some possible anomalous behaviors of the stations. These anomalous behaviors were particularly relevant for stations of the network near the epicentral area. While co-seismic and post-seismic variations are widely expected, the anomalies recorded about ten days before the earthquake could be seriously considered among short-term precursors of the earthquake. In order to confirm this possibility, more detailed studies have been performed. In particular, GEONET currently makes available only daily solutions of the stations coordinates. On the contrary, it is very important to improve the time resolution just to understand the features of the anomalies till the last hours before the Earthquake. For this reason, we have performed an analysis to evaluate the coordinates and movement on hourly basis so improving the time resolution.
 Hayakawa, et al., “On the Correlation between Ionospheric Perturbations as Detected by Subionospheric VLF/LF Signals and Earthquakes as Characterized by Seismic Intensity,” Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 72, No. 13, 2010, pp. 982-987. http://dx.doi.org/10.1016/j.jastp.2010.05.009
 Kasahara, et al., “The Ionospheric Perturbations Associated with Asian Earthquakes as Seen from the Subionospheric Propagation from NWC to Japanese Stations,” Natural Hazards and Earth System Sciences, Vol. 10, No. 8, 2010, pp. 581-588. http://dx.doi.org/10.5194/nhess-10-581-2010
 Hayakawa, et al., “A Statistical Study on the Correlation between Lower Ionospheric Perturbations as Seen by Su-bionospheric VLF/LF Propagation and Earthquakes,” Journal of Geophysical Research, Vol. 115, No. A9, Article ID: A09305.